Deepwater Marine Sandstone Reservoirs

Deep Sea Sands

Studies of modern continental margins show that large deposits of sand and other terrigenous sediment occur below the level of the continental shelf (below ±200m). Sediment is carried from coastal and shelf settings to the deep sea environment through submarine canyons that dissect the continental slopes. Transportation is accomplished by a variety of gravity flow and occasional traction flow processes. At the foot of the continental slope sediment usually spreads out from the mouth of the canyon in a radiating complex of distributary channels to form a fan-like deposit termed a submarine fan. The geometry and facies distribution of these fans varies significantly depending on a number of controls involving sediment supply, tectonic setting and eustatic sea-level fluctuations.

Some sediment may accumulate in distinctly non-fan-type deposits. Broad bands of sediment, fed through multiple canyons, may form slope-apron deposits. Sediment may also remain within non-coalescing channels, often extending for long distances on the basin floor, to accumulate as elongate ribbon bodies of sand. Feeder canyons may also become filled with sand, gravel or muds to form lenticular bodies of sediment on the slope.

Submarine fans and associated deposits generally form during low sea-level stands. They are typically found off major river/delta systems and at the foot of submarine fault scarps.

The ancient analogs of these deep-sea systems have provided significant oil and gas production in many parts of the world, including the North Sea, southern California, and more recently, offshore Brazil. Numerous models have been proposed over the past two decades to describe these ancient systems. Many of these models are based on modern examples. However, many differences exist between modern fans and ancient fans as well as within each group. Furthermore, the models are based on one or, at best, only a few examples. Therefore, one must be very careful in applying one of these models to other areas (Bouma et al., 1985).

RESERVOIR ASPECTS: SUMMARY

Because of their ability to provide highly productive oil and gas reservoirs, deep sea sandstones constitute one of the most important depositional systems for the petroleum geologist.

Deep sea sands are defined as being deposited below the level of the continental shelf (below ± 200 m) These sands and associated facies are deposited by a variety of gravity flow mechanisms, including slides and slumps, debris flow, grain flow, fluidized flow and turbidity currents. Traction currents may also play a meaningful role, particularly in the deposition of sand in submarine channels. By far the most significant depositional mechanism, however, are turbidity currents, whose resulting deposit is defined as a

turbidite. The facies model for turbidites consists of an upward-fining sequence of textures and structures referred to as a Bouma sequence.

Three end-member types of depositional systems are identified. Elongate submarine fans develop on passive continental margins. They are usually fed by a single submarine canyon often with sediment from a major river/delta system, and are dominated by muds and fine sand. Radial submarine fans are generally smaller than elongate fans. They typically form in relatively short-lived basins along active continental margins, are often fed by a single canyon, and contain a high ratio of sand to mud. Slope-apron systems are usually fed by multiple or linear sediment sources that feed directly down the slope. They are typically coarse-grained deposits that form along active fault-bounded rift basins.

Important petroleum reservoirs are often formed in the feeder canyons, individual channels, channelized sand bodies and non-channelized sand lobes of these deep sea systems.

Three interdependent factors control deep sea sand deposition: sediment type and supply, tectonic setting and activity, and sea-level fluctuations. The major primary control in the deposition of submarine fans is probably the last, in that most fans are interpreted to have been deposited during relative lowstands of eustatic sea level. At such times, abundant sediment is fed to the fan deposits across exposed or relatively narrow shelves.

Recent studies of turbidites in Spain and Italy have emphasized the stratigraphic framework of submarine fans. Three end-member types of deposits or systems that compose turbidite sequences were identified. Each type, designated Types I, II and III, corresponds to a stage of growth in a complete turbidite system. Some systems express only one stage of growth, while others develop as composite features, displaying multiple stages of which one type usually dominates. Type I deposits develop during sea-level lowstand and are characterized by elongate non-channelized sandstone lobes. Type II deposits develop during the following stage as sand-rich fans on active margins. They are composed of channelized sand bodies that grade downcurrent into smooth sandy lobes. A turbidite system of this type corresponds to the sand-rich radial fans described above. Type III deposits are finer-grained, channel-levee complexes that form as the final stage of turbidite deposition during sea-level rise.

SEDIMENTATION PROCESSES OF DEEP WATER SYSTEMS

Sediment dispersal within submarine fans and associated systems is accomplished by several mechanisms, including traction flow and a variety of gravity flows.

For most submarine fans, sediment influx is high and gravity processes dominate. Because the resulting sediment often had a previous history as a fluvial, coastal or shelf deposit, submarine fan deposits are often referred to as "resedimented" beds.

Hampton and Middleton (1976) distinguished four types of sediment gravity flow: debris flow, grain flow, fluidized flow and turbidity currents. Submarine slumps and slides are added as a fifth type of gravity flow to Hampton and Middleton's scheme ( Figure 1 ).

Figure 1

The various flow processes described below follow the presentation of Klein (1985).

Slumps and Slides

Slumps and slides are coherent masses of sediment that move downslope along external shear planes. Cook et al. (1982) define transitional glide (slides) as shear failure taking place along discrete shear planes subparallel to underlying beds. They likewise define rotational slumps as taking place along concave-up shear planes accompanied by rotation of slide.

Slides and slumps may retain original sedimentary structures or they may show evidence of deformation in the form of folds or fractures ( Figure   1 , Slump fold in turbidites, Point San Pedro, California).

Figure 1

When undeformed, internal bedding may be subparallel to underlying beds in slides and at an angular discordance to enclosing strata in the case of rotational slumps. Mechanisms that appear to trigger slumps and slides include shocks (such as earthquakes), fluctuations in sea level, excess pore pressure in water-saturated fine-grained sediments, and oversteepened slopes.

Seismic reflection surveys indicate that slides and slumps are extremely common on present-day continental slopes. Large-scale slumping on the continental slope off south Texas displays characteristic patterns of hummocky surface relief and rotated bedding planes ( Figure   2 , Seismic section showing slumped sediment on continental slope off south Texas). Slumps and slides have a large preservation potential; therefore, such features should be common in the rock record.

Figure 2

Debris Flows

Debris flows are mixtures of granular solids, clay minerals and water that move sluggishly downslope in response to gravity. The granular solids, mostly sand and gravel, are transported and supported by interstitial mixtures of clay and water.

Debris flows can be initiated and move along very low-angle slopes and spread out as much as 800 km from their source (Embley, 1976; Kidd and Roberts, 1982).

Deposition takes place by a process of mass emplacement and the resulting sediments are of mixed grain sizes ranging from clay to coarse gravel clasts — essentially a fabric of coarse grains floating in finer material ( Figure   1 , Debris flow deposit, Cretaceous Pigeon Point formation, California). Figure   2 shows an idealized vertical sequence of a debris flow deposit as proposed by Middleton and Hampton (1973).

Figure 1

Figure 2

Grain Flow

Transportation of sediment within grain flows is proposed to take place by the upward supporting stresses acting on grains within flowing sediments caused by grain-to-grain collisions (Bagnold, 1954). A typical grain flow is represented by sand avalanching down the slip face of an eolian dune under the pull of gravity. Steep slopes are generally required for the initiation and sustained movement of grain flows, and the flows may be sufficiently strong to erode canyons. Grain flows have been reported from the upper ends of submarine canyons, where they have been termed "rivers of sand" (Dill, 1964).

Grain flow deposits are thick and massive, with a fabric consisting of dispersed clasts of pebbles floating in sand. Grains are often parallel to flow, and inverse grading may occur near the base. Boundaries are sharp, and the base of the beds may include scour markings and load structures. A hypothetical vertical sequence produced by a grain flow deposit is shown in Figure   1 .

Figure 1

Fluidized Flow

In fluidized flow, excess pore pressure keeps the sedimentary particles afloat in interstitial fluid. Large concentrations of sediment, with pore pressure significantly greater than hydrostatic pressure, are required to induce this type of flow. Gravity moves the fluidized sediment downslope and rapid deposition occurs as pore pressure quickly dissipates from the base upward, a mechanism referred to as "freezing-upward."

Sediment ranging in grain size from clay to sand may be transported by this process. Only partial sorting and grading occurs (coarse-tail grading). Owing to the rapid upward escape of fluid, dish structures, water escape pipes and sand volcanoes are common sedimentary structures in this type of deposit. The base of deposits may contain flame and load structures, and possibly grooves and striations. A suggested idealized vertical sequence is shown in Figure   1 .

Figure 1

Turbidity Currents

Turbidity currents are a type of density current. Such density currents arise from differences in density between the current and surrounding water. Gravitational acceleration causes the denser current to flow down slopes on the bottoms of oceans and lakes. Density currents may be due to colder temperatures, higher salinities or suspended sediment. Turbidity currents are caused by suspended sediment and the resulting deposit is defined as a "turbidite."

The concept of turbidites was first introduced by Kuenen and Migliorini (1950) to explain the origin of "flysch" sediments. Flysch, a term less often used nowadays, refers to thick sequences of interbedded sand and shale of deep sea origin. The sands usually have erosional bases and are internally graded (of upward-fining grain size), and the interbedded shales contain deep marine fauna.

Turbidity currents move as surges with the transported sediment suspended by fluid turbulence ( Figure 1 , Experimental turbidity current in water depth of 26 cm in a flume at Cal Tech). Flow may be initiated by earthquakes, by the failure of over-steepened

slopes, or by gradation from one of the other previously mentioned types of gravity flow.

Figure 1

Turbidity currents flow like rivers in submarine canyons, initiating active erosion and cutting new canyons or modifying existing ones. Sediment is deposited in the canyon and at the base of the canyon where the currents spread out onto the basin floor to form submarine fans ( Figure 2 , Depositional model of Shale Grit and associated formations).

Figure 2

The fans grow by channel extension and bifurcation, with coarser sediments deposited in the channels and finer sediment spread out onto the ocean floor at the toe of the fan.

Sediment is deposited in interchannel regions by overbank spill, and levees are often developed on the flanks of channels.

Relatively little is known about the velocity of turbidity currents. The classic data came from an earthquake near the Grand Banks of Newfoundland that occurred in 1929. The quake triggered a large-scale regional slump and turbidity current. The area was traversed by transatlantic telephone cables, each of which was broken by aftershocks of the earthquake and by slides and turbidity currents. The times of the cable breaks are known and, using the distance of transport between cable breaks, it is possible to estimate the velocity of the turbidity current that severed the cables ( Figure 3 , Profile showing velocity of turbidity current and position of cable breaks, Grand Banks, Newfoundland).

Figure 3

Heezen (1963) calculated a velocity of 55 nautical miles per hour (28.3 m/sec) at the base of the continental slope. At a cable break located about 300 nautical miles further seaward on the relatively flat abyssal plain, the turbidity flow was traveling at a velocity of 12 nautical miles per hour (6.2 m/sec). It has been estimated that low concentrations of quartz pebbles up to 3 cm in diameter could be suspended by fluid turbulence alone at a velocity of only 22 nautical miles per hour (11.4 m/sec) (Walker, 1984). Note that Figure 3 shows the Grand Banks turbidity flow was traveling at this velocity (± 22 knots/hr) at a distance of about 200 nautical miles seaward of the continental slope.

Deposition by turbidity currents can be very rapid, particularly at the base of relatively steep continental slopes, where most submarine canyons terminate. Deposition is accomplished by particle-by-particle settling, with the coarsest material settling out first.

Based on studies of thousands of individual beds, Bouma (1962) developed a facies model for turbidites consisting of an upward-fining sequence of textures and structures, referred to now as a Bouma sequence ( Figure 4 ).

Figure 4

Capital letters (A through E) are often used to designate the five divisions of the Bouma Sequence. These may be easily confused, however, with several schemes for classifying submarine fan facies that also employ capital letters. One such widely used classification is the Mutti and Ricci Lucchi system (1972) that designates fan facies A through G. We follow the method of several authors — for example, Howell and Normark (1982) — by using the symbols Ta through Te to designate the five main Bouma divisions ( Figure 4 ):

Ta: A massive, graded, upward-fining sand or gravelly sand characterized by a sharp scour base containing flute marks from turbulent scour and various tool markings produced by obstacles and traveling objects.

Tb: A lower interval of parallel laminated sand; some grading may be present and the contact with the underlying graded interval is gradual.

Tc: An interval of fine sand and silt showing small current-ripple bedding, often with wavy or convoluted laminations. The contact with the underlying interval of parallel laminations is rather sharp.

Td: An upper interval of parallel laminations made up of very fine sand to silty clay with a sharp lower contact.

Te: An interval of clay tops the sequence. This interval may be divided into a Tet interval, representing terrigenous muds that settle out in the waning stages of turbidity flow deposition, and a Tep interval that is not a turbidite, but rather

pelagic debris of the ocean basin floor (Howell and Normark, 1982). This Te interval does not show any distinct sedimentary structures and the contact with the underlying parallel laminated unit is gradual.

The complete Bouma sequence is seldom developed in any single turbidite, and the topmost part, bottom part or both may be missing. In the proximal portions of a turbidite deposit, closer to the sediment source, all intervals are normally deposited. As velocities progressively wane in the down-current direction, the turbidity flow gives up its coarser fractions leaving only the finer intervals to be deposited ( Figure 5 , Idealized Bouma sequences in terms of waning current flow and distance across basin). Thus turbidites show a decrease in both grain size and bed thickness in a distal direction.

Figure 5

Part a of Figure 6 illustrates the hypothetical shape of the depositional lobe of a single turbidite consisting of a full Bouma sequence (Ta - Tet) at the mouth of a channel and only the uppermost unit of the sequence (Tet) farthest from the source. Part b of Figure 6 shows how succeeding stages of turbidite deposition may form a complex of partially overlapping depositional lobes.

Figure 6

The uppermost portions of a Bouma sequence are frequently scoured away by the channeling action of a subsequent depositional sequence. This may leave only the lowermost units to be present in the proximal portion of a deposit. In more distal portions, where the lower Bouma units are often missing due to non-deposition, the upper may also be eroded leaving only the middle units.

Walker (1984) presented a list of sedimentary features by which classical turbidites may be recognized in the geologic record:

1. A monotonous interbedding of sandstones and shales through many tens or hundreds of meters of section ( Figure 7 , Turbidites of Cretaceous Chatsworth formation, Simi Hills, California). Beds are characterized by flat tops and bottoms with no channeling or scouring greater than a few centimeters.

Figure 7

2. Sandstone beds grade upward from sharp and abrupt bases into finer sand, silt and mud. Muds may contain transported shallow-water faunal assemblages with the uppermost very fine clays often containing bathyl or abyssal benthonic fauna.

3. The undersurfaces (soles) of sandstones often contain abundant markings that may be classified into three types: tool marks carved into the underlying mud by rigid objects like sticks and stones ( Figure 8 )

Figure 8

; flute casts cut into underlying muds by fluid scour ( Figure 9 ); and organic markings from trails and burrows filled in by the turbidity currents ( Figure 10 ). The tool and scour marks provide an accurate indication of paleoflow direction.

Figure 9

Figure 10

4. Within sandstone beds, combinations of parallel lamination ( Figure 11 ),

Figure 11

ripple cross-lamination ( Figure 12 ),

Figure 12

convolute lamination ( Figure 13 )

Figure 13

and graded bedding ( Figure 14 ) may be observed.

Figure 14

Flow Relationships

It is possible that one type of gravity flow may or may not be a precursor for one of the other classes. For example, turbidity flows may evolve from one of the other sediment gravity processes. Dzulynski et al. (1959) applied the term fluxoturbidite to sandy submarine canyon deposits that showed characteristics intermediate to turbidites and sliding or slumping. Such deposits are described as coarse-grained, thick-bedded deposits with poorly developed current markings and absent to poorly developed grading.

Turbidity currents probably represent a continuum of flow concentrations with both high-density (50-250 g/l) and low density (0.025-8.5 g/l) currents having been identified (Middleton and Hampton, 1976). High-density turbidity currents generally transport a spectrum of grain sizes down slopes at velocities of up to 28 meters per second (Grand Banks flow). Low-density turbidity currents carry mostly clay and silt-sized particles at relatively low velocities averaging 10 to 50 centimeters per second over almost no slope. Low-density flows may develop directly from: (1) high-density flows as current velocity wanes and the turbidity current gives up its coarser fractions, (2) shelf and slope mud deposits, and (3) the discharge of mud-charged rivers or melting glaciers.

According to Howell and Norwark (1982), most deepwater redeposited sediment probably goes through two or more mass transport mechanisms before ultimate deposition in a submarine fan ( Figure 1 , Inferred relations of flow processes).

Figure 1

Importance of Traction Flow

Much of the literature dealing with deepwater sand deposits restricts discussion to the importance of turbidity currents and possibly other gravity flow mechanisms as depositional agents. Evidence indicates, however, that deposition from traction flow may play a meaningful role, particularly in the deposition of sand within submarine channels.

Currents of 50 cm/sec or greater, sufficient to transport medium sand, have been measured in submarine canyons (Keller and Shepard, 1978; Shepard and Marshal, 1978).

In a series of dives with research submersibles, Valentine et al. (1984) observed and sampled large dune-like sand waves filling the floor of a submarine canyon, 100 m to 300 m wide, located off Georges Bank, offshore southern New England. The sand waves, composed of coarse to medium, moderately to poorly sorted sand, are asymmetric in form and attain heights of 3 m and lengths of 15 m between crests ( Figure 2 , Steep face of deepwater sand dune at depth of 621 m with ripples oblique to crest on steep face and parallel to crest on gentle face). The sand waves were observed to a depth of about 620 m (deepest dive).

Figure 2

Valentine et al. consider the source of the canyon fill sand to be the shelf sediments found on Georges Bank that were derived from glacial outwash during the Pleistocene. Tidal currents, probably assisted by internal waves and storm-generated currents, are thought to be responsible for transporting the shelf sediments into the canyon and reworking the sediments on the canyon floor.

Large-scale cross-stratification, observed in several ancient deep sea sandstones including those of the Permian Delaware Mountain Group in the Delaware Basin of Texas and New

Mexico (Harms and Williamson, 1988) ( Figure 3 ,

Figure 3

Figure 4 and Figure 5

Figure 5

, Cores showing cross-stratification in deepwater sandstones) and the Upper Cretaceous Chatsworth Formation of southern California (Link et al., 1984), attests to the importance of traction transportation as a deep sea depositional agent.

Figure 4

Depositional Systems

Submarine Fans

Submarine fans form at the base of continental slopes where channels dump their sediment load in a manner somewhat analogous to a subaerial alluvial fan. A system of radiating channels, often flanked by levees, spreads out from the fan apex, with smooth lobate bodies forming seaward from the fan-channel mouths ( Figure   1 ). Generally, the coarsest sediment is deposited in the main feeder channel, fan distributary channels and the proximal portion of the fan lobes, while finer-grained sediment occurs in interchannel regions and in the distal part of fan lobes.

Figure 1

Several methods have been proposed to classify submarine fans. One such scheme identifies two end-member fan types: elongate fans and radial fans (Nelson, 1983; Stow, Howell and Nelson, 1985). The two opposing types form in response to variations in sediment type and rate of sediment supply. Hybrid fan types, gradational between the end-member models, are common.

Another method of classification is to simply designate fans with respect to their tectonic setting as either passive margin fans or active margin fans. Generally, elongate fans form in passive margin settings and radial-type fans develop in active margin settings. There are, however, many exceptions. For example, the modern Astoria Fan, located off Oregon on the U.S. west coast, displays many characteristics of an elongate fan. Yet it is situated in an active margin setting (Nelson, 1985).

Elongate Submarine Fans

Elongate submarine fans are the larger of the two types of fans, being generally over 100 km across. Other designations for this type include high-efficiency fans (Mutti, 1979) and large deepwater ocean-basin fans (Howell and Normark, 1982). Some geologists loosely refer to this type as a muddy fan. Elongate fans develop mostly on passive continental margins where basins are large and lower sea floor gradients exist. The fan is fed primarily from a single sediment source, usually from a major river system and its associated delta or sometimes from an ice channel (Laurentian Fan). Sediment has generally traveled long distances from its source, and muds and very fine sands dominate. Sediment is often released into the feeder canyon from a deltaic shelf-upper slope setting by such mechanisms as slope failure.

A main feeder channel within a canyon breaks up into a number of prominent distributary channels. These channels, often with levees, have long valleys and extend over much of the fan. An elongate shape is developed as coarser sediment is effectively funneled to terminal lobes. Downfan direction is commonly seaward. The sandy lobes developed at the terminus of the distributary channels are essentially smooth and unchanneled. These depositional lobes have been likened to the stream mouth bars formed at the mouths of delta distributary channels. However, the submarine fan depositional lobes are deposited primarily by turbidity currents as opposed to the traction current deposition of deltaic sands.

Mutti and Ricci Lucchi (1975) proposed a variation of the elongate fan model wherein the lobes are detached from their feeder channels as a result of sediment bypassing ( Figure 1 , Model of an ancient submarine fan with detached lobes).

Figure 1

Such bypassing could result in a zone of thick shale separating the sandy depositional lobes from the channel deposits. This model, however, is based on a single type locality, the Eocene Hecho Group in Spain. Its effectiveness, therefore, as a guide in predicting facies distributions has been seriously questioned by Walker (1980), and later by Shanmugam and Moiola (1985), who concluded that nondeposition within the "bypass zone" of the Hecho Group was probably caused by local structural control.

Modern Elongate Fans Modern examples of the elongate fan type include the Astoria, Bengal, Indus, Mississippi, Amazon, Laurentian and Rhone fans. One of these, the Mississippi Fan, has recently been subjected to extensive study.

The Mississippi Fan, located in the east-central Gulf of Mexico, is fed by the Mississippi Canyon whose head occurs about 50 km south of the Mississippi delta ( Figure 2 , Morphometric map of the Mississippi Fan.)The fan contains a large volume of Pleistocene sediment that was transported through the feeder canyon by a combination of slumping and sliding, probably in response to a lowering of sea level.

Figure 2

(Bouma et al., 1985). Seismic stratigraphic analysis combined with core information reveal the fan to be made up of eight major depositional lobes, each representing major growth phases in the development of the Mississippi Fan complex (Feeley et al., 1985). Each sequence (lobe) migrated generally east and seaward through time, their position being controlled by relief of older fan lobes and accumulation in topographic lows.

Analysis of seismic facies indicates turbidity currents and mass transport to be the two dominant transport/deposition mechanisms of the Mississippi Fan.

Table 1 (Summary of the changes in dominant transport/deposition mechanisms downfan) summarizes the changes in these depositional mechanisms in a downfan direction that take place within each of the seismic sequences.

Table 1

The turbidites change from thick channel fill accumulations in the upper fan to essentially unchannelized lobes in the lower fan, where they constitute the majority of sediments. Mass transport sediments, in the form of slump deposits, are the dominant facies in the upper fan. In a downfan direction, the mass transport facies become, increasingly, debris flows.

Petrographic analyses of core samples (Roberts and Thayer, 1985) show high porosity clean sands with good reservoir potential occurring in the middle and lower fan channel fills, as well as in sand-sheets of the lower fan lobes. Cores taken in a 3 to 4 km wide mid-fan channel (Stelting et al., 1985) recovered about 100 m of interbedded mud and sands which generally fine upward from 13 to 15 m thick intervals of basal gravels and coarse sand ( Figure 3 and Figure 4 , Lithostratigraphic summary from cores of mid-fan channel sites, Mississippi Fan).

Figure 3

Cores of the lower fan sand-sheet penetrated two fan lobes that contained up to 500 m of interbedded sand, silt and mud, with sand constituting about 50% of the cored interval (O'Connell et al., 1985).

Figure 4

Sand within these lower fan lobes frequently occurs in massive beds up to 10 m thick ( Figure 5 , Lithostratigraphic summary from cores of lower fan sand sheet lobes, Mississippi Fan).

Figure 5

Ancient Elongate Fans Although numerous examples of elongate submarine fans exist in present day oceans, few have been reported from the rock record. This is not surprising, since ancient turbidites deposited on oceanic crust in passive margin settings (elongate fans) are likely to remain buried under overlying sediment or to be dismembered by tectonic accretion onto continental margins. Turbidites deposited in tectonically active margin settings (radial fans), on the other hand, are more likely to be uplifted and exposed. One ancient analog to the elongate type of submarine fan is probably represented by the Precambrian Kongsfjord Formation located on the northern Norway coast (Pickering, 1985). Based on outcrop studies, a depositional model for the Kongsfjord fan was developed ( Figure 6 , Depositional model for lobe sequences in the Kongsfjord formation, Norway). Following is a summary description of the Kongsfjord Formation lithofacies and their relationship to various fan divisions:

Figure 6

Inner Fan

· Channel Axis/Channel Margin Facies: Small pebble to medium-grained sandstone occuring as very thick to very thin turbidites and other mass flow (debris flow common) deposits. Deposits display traction cross-bedding, stratification and graded bedding of Bouma Tabc divisions; local scour and fills and small-scale channeling.

· Interchannel/Levee: Siltstone and mudstone occuring as sheets of thin- to very thin-bedded turbidites of Bouma Tde divisions.

Middle Fan · Channel Fill: Coarse to medium-grained sandstone occuring as very thick to medium-bedded turbidites of the Bouma Tabc divisions with internal traction cross-bedding; often as thinning and fining-upward sequences on a scale of meters; more than 50% sand ( Figure 7 , Channel fill course-to medium-grained sandstone of middle fan, Kongsfjord formation, Norway.).

Figure 7

· Channel Margin Levee: Medium to fine-grained sandstone and siltstone occuring as irregular wedges of thick to thin-bedded turbidites of the Bouma Tbcd divisions with soft sediment deformation features; less than 10% sand. ( Figure 8 , Channel margin levee medium-to fine-grained sandstone of middle fan, Kongsfjord formation, Norway).

Figure 8

· Interchannel: Very fine-grained sandstone to mudstone occuring as sheets of thin to very thin-bedded turbidites of the Bouma Tcde divisions; less than 20% sand.

Outer Fan · Lobe: Very coarse to medium-grained sandstone occuring as sheet-like beds of very thick- to medium-bedded Bouma Tabc turbidites that form topographic highs immediately downfan from a channel mouth; local scour and fill; contains sequences that thicken and/or fine upward; more than 80% sand ( Figure 9 , Sheet-like coarse-to medium-grained sandstone of the outer fan, Kongsfjord formation, Norway).

Figure 9

· Lobe/Fan Fringe: Fine- to very fine-grained, sheet-like deposits occurring as medium- to thin-bedded turbidites of Bouma Tbcde divisions; sand constitutes from 40 to 80% of lobe fringe, less than 40% of fan fringe.

From the above descriptions of interpreted facies, it can be concluded that the Kongsfjord turbidite system fits the model of an elongate fan as generally conceived. Sand has been effectively transported through a system of distributary channels on the mid-fan to the smooth, sheet-like, mostly unchanneled depositional lobes of the outer fan. Clearly, such turbidite systems, with potential reservoir sands in both channel facies and unchanneled depositional lobes, present attractive exploratory targets.Radial Submarine

Fans typically form along active margins in restricted basins floored by continental crust. Sediment is commonly coarse-grained and well-sorted, with sand in equal or greater abundance than mud. The sediment is usually derived from local rivers or from littoral drift on narrow shelves and is transported into the basin by a single feeder canyon or channel. The main channel generally divides into a limited distributary system to form a typical radial-shape fan. Although downfan directions are often in a direction normal to the basin margin, they may be parallel to the basin margin if the basin axis dips in one direction.

This type of fan has been variously termed a low-efficiency fan (Mutti, 1979), restricted-basin fan (Nelson and Kulm, 1973), and a canyon-fed fan (Stow et al., 1984). The term sandy fan is also used by some geologists.

Modern Radial Fans Many modern examples of radial fans are described from the western margin of North America. These include the La Jolla, Navy, San Lucas, and Redondo fans from which the Normark (1978) model was derived ( Figure 1 , Model for submarine fan.).

Figure 1

An elemental feature of Normark's model is the suprafan, defined as the area of active sand deposition on a fan that forms downfan from the termination of a leveed valley. The suprafan has an approximately lobate shape and is characterized by distributary channels that gradually die out on the lower, smooth portion of the suprafan.

Normark found that most of the radial-type modern submarine fans have three recognizable morphologic subdivisions:

1. An upper fan containing a large leveed valley that produces very coarse (1 to 5 km wide) deposits in meandering or braided shallow channels. The coarse deposits grade laterally into finer-grained, more regularly bedded levee sands and silts.

2. The middle fan is characterized by the suprafan described above. It is recognized as a convex-upward depositional bulge in a radial profile that contains a coarsening- and thickening-upward sequence of sandy turbidites. On the upper suprafan this sequence is cut by numerous channels and isolated depressions,

while such channels and depressions are absent on the relatively smooth lower suprafan.

3. The lower fan division is free of channels and coarse turbidites. It is nearly flat, and often indistinguishable morphologically from the basin plain.

The Navy Fan, a radial-type, sand-rich fan, located in the South San Clemente Basin, offshore California, is one of the most studied modern fans in the world (Normark, 1969, 1978; Normark and Piper, 1972, 1985; Piper and Normark, 1983). An extensive network of seismic data was collected and over 100 piston cores were taken. However, neither the Navy Fan nor any other modern radial fan has been drilled, as was the Mississippi Fan. Thus, many questions remain concerning the internal structure and lithologic make-up of the fan.

The Navy Fan lacks a classic radial shape because of the irregular shape of the tectonically active basin . The fan displays, however, the upper-, middle-(suprafan), and lower-fan morphologic divisions common to sand-rich submarine fans. This relatively young fan began to form in the Late Pleistocene and continued growing until turbidity current deposition ceased after Holocene sea-level rise. Coarse sediment (up to gravel size) was funneled into the Navy feeder channel from the Tijuana River, via the submarine Coronado Canyon during periods of low sea level. Sand is concentrated in one active channel and two abandoned distributary channels, as well as on the suprafan lobes of the mid-fan region. Evidence for channel migration or braiding is absent. Instead, as shown in Figure 2 ,

Figure 2

Figure 3 ,

Figure 3

and Figure 4 (Sequence of lobe and distributary channel development on Navy Fan), the apparent aggradation of a channel and the mid-fan lobe it feeds causes the channel to abruptly shift position into a low area around the aggrading lobe (Normark, Piper and Hess, 1979).

Figure 4

These depositional lobes that make up the mid-fan region are without channels on their surface, but are bordered by small channels that begin along the edges of the lobes. They are several kilometers wide, and although the longest piston cores on the fan are only about 5 m, the lobes are inferred to be about 10 m thick. Overall, the Holocene sediments of Navy Fan show a decrease in grain size and thickness, and an increase in mica content, in a seaward direction. Likewise, muds increase distally in thickness.

Ancient Radial Fans Ancient analogs of radial (restricted-basin, sand-rich) fans are well preserved in the rock record. These fans provide for major hydrocarbon reserves in many regions of the world, such as southern California and the North Sea. A model that best describes ancient, as well as modern, radial fans is Walker's (1984) model shown in Figure 5 (Simplified fan model with simplest possible terminology on the right ).

Figure 5

On the left side of the model Walker incorporated the salient features of Normark's (1978) model for modern fans ( Figure 1 ) with the characteristics proposed by Mutti and others for many ancient fans. The terminology used by Walker to describe the left side of Figure 5 is similar to that used by Normark (1978):

· an inner fan with a single feeder channel

· a mid-fan made up of branching channels that feed a depositional lobe (suprafan), the upper portion of the mid-fan being channeled and the lower portion being smooth

· a smooth outer fan that grades into

· a basin plain.

The right side of Figure 5 contains the simplest possible terminology to describe the basics of almost all ancient and modern fans: a channeled fan, a non-channeled fan, and a basin plain.

Channeled and non-channeled regions are common characteristics of almost all modern and ancient submarine fans, and the use of such simple terminology is often preferred. Furthermore, channelized sequences, with their thinning- and fining-upward cycles, and non-channelized sequences, with their thickening-and coarsening-upward cycles, can often be easily recognized not only in outcrops but also in the subsurface by cores and well logs. Also, channeling can often be revealed by seismic data.

Several examples of ancient, sand-rich radial-type fans have been reported from outcrop studies in southern California. One of these is the Upper Cretaceous Chatsworth formation, located in the Simi Hills on the Los Angeles/Ventura County line ( Figure   1 , Map of Simi Hills, Southern California.). The exposure has been extensively studied by Link, Squires and Colburn (1984), and the following discussion partially summarizes their report.

Figure 1

The Chatsworth formation is envisioned to have formed as part of an Upper Cretaceous deep sea fan complex in a west-facing forearc clastic wedge. The Simi Hills block has

subsequently been rotated from 45° to 90º in a clockwise direction by strike-slip faulting. Thus paleocurrent patterns, determined primarily from flute casts and cross-laminations, now indicate a north to northwest sediment transport direction ( Figure   2 , Interpretation of paleocurrent data from the Chatsworth formation).

Figure 2

Approximately 90% of the Chatsworth formation consists of medium to coarse-grained, often pebbly, arkosic sandstone that is moderately to poorly sorted, subangular and immature. Thus, rapid sedimentation in a high-relief, tectonically active setting is indicated. Figure   3 (Depositional model for the Chatsworth formation) illustrates a depositional model for the Chatsworth formation that shows a sand-rich system composed mostly of channelized sands with minimal slope and outer-fan facies.

Figure 3

Slope facies consist mainly of mudstone with thin interbeds of calcareous sandstone and limestone ( Figure   4 , Slope facies of the Chatsworth formation). Slump folding, wedge-outs and broken beds are common. Sandy limestone beds 1-2m thick and marine mollusks, shark teeth and rip-up clasts are channelized into the underlying mudstone.

Figure 4

Although these assemblages suggest deposition at neritic depths, benthic foraminifera from interbedded mudstones indicate bathyl water depths (Almgren, 1973, 1981).

Middle fan facies comprise more than 90% of the Chatsworth formation. They consist of thick-bedded channel sandstones interbedded with thinner sequences of interchannel mudstone, siltstone and sandstone ( Figure 5 .

Figure 5

, Middle-fan facies of the Chatsworth formation) and ( Figure 6 , Massive channel sandstone, Chatsworth formation, California).

Figure 6

Difficulty was experienced in differentiating inner- and middle-fan facies and the entire sand-rich channelized complex has been designated as mid-fan. The channels are composed of medium to coarse-grained pebbly sandstone that is poor to moderately sorted, subangular, indurated and locally micaceous. The beds are laterally discontinuous and display thinning- and fining-upward megasequences that grade upward into mudstone.

Most sequences are aggradational, and downcutting is minimal. Sedimentary structures include groove casts, wavy laminae and dish structures typical of gravity flow. Large-scale trough and planar cross-bedding occuring in some channels is interpreted as reflecting both lateral accretion within the channels and prograding sand waves down the channel ( Figure   7 , Sketch of large-scale cross-bedding in the Chatsworth formation).

Figure 7

Individual channelized megasequences are up to 3 m thick, and multistory composite sequences are up to 200 m thick and 1 km wide. Sequences are commonly superimposed on one another, and the overall sandstone to mudstone ratio is very high (10:1 to 12:1).

Alternating thin, laterally discontinuous beds of mudstone, siltstone and sandstone occur between the channel sequences ( Figure   8 , Interchannel turbidites, Chatsworth formation, California).

Figure 8

These interchannel beds consist of Bouma turbidite divisions Ta-e, Tb-e, and Te divisions. They are locally inclined and slump-folded, and commonly contain parallel laminae, small-scale cross-bedding, graded beds, climbing ripples, deformed beds and sole marks. The mudstones are commonly bioturbated and contain microfossils suggestive of bathyl depths (Almgren, 1981). Some of these interchannel turbidites are arranged in small-scale thinning- and fining-upward cycles interpreted as crevasse splay channels. Others display thickening- and coarsening-upward sequences interpreted as crevasse-splay lobes. Locally, these alternating midchannel sequences contain other sandy units interpreted to be overbank deposits that form levees adjacent to major distributary channels ( Figure   9 , Levee deposits, Chatsworth formation, California).

Figure 9

The mid-fan association of facies described above for the Chatsworth formation is interpreted to represent a complex of braided suprafan lobes that corresponds to the terminology proposed by Normark (1978) and Walker (1978 and 1984).

Outer fan facies consist of sandstone and shale sequences that thicken and coarsen upward ( Figure   10 , Depositional lobe facies of the Chatsworth formation).

Figure 10

Sandstones are generally non-channelized, laterally continuous and flat-based with directional sole marks. The sandstone beds average about 1 m in thickness, and the sandstone-mudstone ratio is 1:1. Lithologically, the sandstone is similar to mid-fan sandstone but is somewhat finer-grained. The sandstones contain a variety of sedimentary structures typically associated with turbidites, including rip-up clasts, soft sediment deformation, dish structures, flute and groove casts, wavy laminae, small-scale cross-bedding and load features. These thickening- and coarsening-upward sequences are interpreted as representing outer-fan depositional lobes deposited by turbidity currents.

Primary porosity of the Chatsworth formation sands is low (5%) but secondary dissolution and fracture porosity is well developed locally. The Chatsworth is the oil reservoir at the nearby, abandoned Horse Meadows and Mission oil fields that produced a combined 259,000 BBL of oil. Reservoir porosities averaged 20-21% and permeabilities averaged 60-65 md (Hall, 1981). Figure   11 (SP and resistivity log of the Horse Meadows oil field), an SP/resistivity log from the Horse Meadows field, indicates that the oil was obtained from mid-fan, fining-upward channel facies.

Figure 11

Slope-Apron Systems

Many ancient deepwater sandstone deposits possess characteristics that do not fit the standard models for canyon-fed submarine fans. Basically, these non-canyon-sourced systems are characterized by multiple or linear sediment sources that feed directly down a generally non-channelized or straight-gullied slope. Both slope and base of slope facies associations are included. The base of slope deposits generally lack the characteristic channelized upper and mid-fan, and non-channelized lower-fan subdivisions. Such non-fan systems have been termed slope-apron systems by Stow, Howell and Nelson (1985).

The Ebro System Modern examples of non-fan systems are found along some margins of the Mediterranean Sea (Wezel et al., 1981). Once such example is the Ebro deep sea "fan" located off the shelf of eastern Spain, along a young, passive, rifted margin (Nelson et al., 1985). Figure 1 and Figure 2

Figure 2

(Map of the modern Ebro Fan, offshore Mediterranean Sea, Eastern Spain ) shows the Ebro system to consist of several lenticular channel-levee complexes developed at the foot of the continental slope.

Figure 1

During lowstands of sea level in the Pleistocene, each channel was fed abundant amounts of coarse-grained sediment through a separate slope canyon from a migrating deltaic source. The continental rise is steeper than usual, which caused each isolated leveed valley to traverse the entire rise without converging or breaking into distributaries. Large amounts of sand were deposited in a series of channelized, lenticular sediment bodies that developed sequentially from north to south. Each lenticular unit of the Ebro system is approximately 15 km wide, 40 to 50 km long, and 150 to 200 m thick.

Brae Turbidite System Figure 3 and Figure 4

Figure 4

(Location of Brae field, North Sea (upper) and cross section across Brae field (lower)) shows the location of the Upper Jurassic Brae slope-apron system that developed adjacent to a major fault escarpment on the western margin of the South Viking Graben of the North Sea (Stow, 1985).

Figure 3

Rapid erosion of the adjacent uplifted lock caused deposition of a thick sequence of generally coarse, clastic sediments in a narrow (± 5 km) zone extending up to 20 km along the fault margin. Sandstones and conglomerates, which form the Brae reservoir, interfinger basinward with organic shales of the Kimmeridge formation. These shales provide the hydrocarbon source for the Brae field.

Stow (1985) divided sediment of the Brae system into four main facies groups ( Figure 5 , Photographs of cores from Brae system showing the four main facies groups. From left to right: mudstone group, sandstone group, conglomerate group and slump units).

Figure 5

The mudstone group consists of laminated mudstone and siltstones or fine-grained sandstone that grade from dominantly mudstone to dominantly sandstone facies. A variety of sedimentary structures, including basal scouring, grading and climbing ripple laminations, indicate turbidity current deposition. The mudstones contain marine microfossils together with a mixed assemblage of marine and terrestrial flora and fauna.

The sandstone group includes turbidity and associated flow deposits that range from thin (1 to 10 cm) beds of sandstone with laminations and internal grading to massive (over 40 cm) beds of sandstones and pebbly sandstones.

The conglomerate group consists of a variety of deposits, including breccias, pebbly sandstones, and pebbly mudstones that probably resulted from debris-flow and other mass flow processes.

Slump units, indicative of slope deposition in a tectonically active setting, show a variety of deformation features including convoluted, contorted and overturned laminae, small-scale faulting, steeply inclined lamination and chaotic mudstone-sandstone mixes.

Facies groups occur in an irregular slope-parallel arrangement, with the base of fault scarp conglomerates interfingering downslope with sandstones and pebbly sandstones that, in turn, interfinger basinward with progressively finer-grained mudstone assemblages ( Figure 6 and Figure 7 , Facies distribution for Brae field slope-apron system).

Figure 6

Some channeling is indicated in coarser facies by marked lateral and vertical facies changes.

Figure 7

Up to six fining-upward megasequences are observed ( Figure 8 , Vertical sequences in southern Brae field). As interpreted by Stow, each megacycle probably represents an episode of rapid uplift of the source area followed by relative dormancy during which progressively finer sediment was deposited.

Figure 8

Delta-Fed Submarine Ramp Heller and Dickinson (1985) presented a depositional model for a type of sandy, slope-apron system they termed a deltafed submarine ramp ( Figure 9 ,

Figure 9

Model for canyon-fed submarine ramp) and Figure 10 (model for delta-fed submarine ramp).

Figure 10

The system typically develops at the base of the slope of a rapidly prograding sand-rich delta. Heller and Dickinson developed their model to explain the facies relationships of the Eocene Tyee formation of Oregon and the Eocene Matilija turbidites of southern California ( Figure 11 , Paleography of Oregon Coast Range during deposition of Eocene Tyee formation).

Figure 11

The basic elements of the submarine ramp system as quoted from Heller and Dickinson (1985) consist of the following:

· The delta platform of a sandy deltaic system that consists of a subaerial delta plain and a narrow marine shelf along the submerged delta front.

· A prodelta slope, which is also the flanking slope of the adjacent basin, that lacks a dominant feeder channel or submarine canyon, but is, instead, traversed by multiple shallow gullies or delta-slope troughs.

· Proximal ramp deposits that bank up against the prograding delta slope, and are deposited from high-density turbidity currents (Lowe, 1982). These deposits are locally channeled at the base of the slope, but are predominantly sheet flows spreading out onto the ramp surface.

· Distal ramp deposits that merge outward with the basin plain, and are deposited as turbidites by sheet flows of lower density and gradually waning energy similar to those that operate on the outer parts of other types of submarine fans (Lowe, 1982).

· The basin plain, bordered by ramp-fringe deposits similar to other fan-fringe deposits.

In addition to having a multiple sediment source, the model can be distinguished from the standard submarine fan models by the absence of channelized inner-fan and middle-fan environments. Proximal ramp deposits occur as sheet flows of massive sandstones, and asymmetrical cycles are absent. Instead, an overall thickening- and coarsening-upward progradational type of sequence exists. A comparison of the delta-fed submarine ramp type of stratigraphic succession with that of a standard canyon-fed submarine fan is shown in Figure 12 (Canyon-fed submarine fan sequence (left) and delta-fan submarine ramp sequence (right)).

Figure 12

Note the absence of fining-upward channelized sequences in the ramp succession.

Fault-Controlled Submarine Ramp

Based on a study of the Upper Jurassic Hareelv formation of Greenland, Surlyk (1987), developed a model for deepwater sandstones that occur mainly in deep eroded gully-fills ( Figure 13 ,

Figure 13

Depositional model for canyon-fed submarine fan (based on Mutti & Ricci Lucchi, 1972), Figure 14 (delta-fed submarine ramp), and Figure 15 ( fault-controlled submarine ramp).

Figure 14

The dominent facies of the Hareelv formation is a thick, structureless, well-sorted, fine- to medium-grained sandstone.

Figure 15

This facies occurs primarily as scours and gullies deeply incised into black, organic mudstone, with shallow shelf sands providing the sediment source ( Figure 16 , Paleographic map during deposition of Hareelv formation).

Figure 16

Earthquakes generated by periodic fault movements probably triggered strongly erosive, high-density gravity currents that cut deep gullies into the slope mud. Some additional gully-fill deposition was probably provided by a short-lived, rapidly prograding delta on the northeast flank of the basin.

Large volumes of sand were deposited en masse in the scours and gullies, and liquifaction of the sand was probably responsible for some intrusion into the unconsolidated mudstone to form dikes and sills ( Figure 17 , Geometry of sandstone bodies, Hareelv formation, Greenland).

Figure 17

According to Surlyk, the terms "channel" and "channelized" should be avoided in this case. A channel refers to an open conduit through which water and sediment are funneled for relatively long periods. In this example, however, Surlyk regards erosion and gully filling to be closely related processes and he considers the terms erosive gullies and scours to be more appropriate.

The characteristics that describe the Hareelv formation as quoted from Surlyk are as follows:

· Sandstone bodies do not show any trends in volume or abundance over distances of tens of kilometers away from the fault zone or along the length of the fault zone.

· Sandstone bodies do not show any marked trends in grain size over distances of tens of kilometers away from the fault zone or along the length of the fault zone.

· Deep scouring is ubiquitous. The larger erosive gullies are more than 50 m deep and hundreds of meters wide.

· The larger gully sandstone bodies are up to 50 m thick, several hundred meters wide, and more than 5 km long.

· The larger sandstone bodies have volumes of about 0.05 km3.

Sandstone bodies are completely surrounded by impermeable mudstone, but individual bodies may be erosively truncated by later sandstones. Continuous mudstone beds are uncommon within the major sandstone bodies. Thus, permeability barriers are unlikely to occur within these sandstones.

The thick, extensive sand bodies of the Hareelv type have the potential to become excellent petroleum reservoirs. Furthermore, the lenticular nature of the sand bodies and the fact that they are encased in impermeable, organic-rich shale offers prospects for stratigraphic entrapment.

CONTROLS ON DEEP SEA SEDIMENTATION

Stow, Howell and Nelson (1985) list three primary controls on submarine fan and associated deep sea sand systems. These are: (1) sediment-type and supply, (2) tectonic setting and (3) sea level variations. These controls are not independent of one another. For example, the amount and type of sediment supply as well as local sea level fluctuations are often determined by tectonic factors.

Sediment Type and Supply A large and rapid supply of sediment can be furnished by major river delta systems.

However, sea level and shelf width will determine the amount of this material available for downslope resedimentation. In high latitudes, glaciers may supply sediment to the shelf margin. Secondary factors include: (1) original source rock type, (2) climate and vegetation, (3) relief and tectonic activity of the source area, and (4) local marine conditions that affect the physical reworking and suspension of sediment as well as final sediment distribution.

Elongate fans are often developed from a single major source, such as a river with a large continental drainage basin and its associated delta, that supplies abundant amounts of sediment of mixed-size grades. Coarser sediment, often with smaller input and sourced from the coastal plain, or small drainage commonly develops radial-type fans.

Tectonic Setting and Activity On mature passive margins where rates of fan deposition exceed tectonic uplift, large, mature, elongate-type fans generally develop.

On transform and convergent margins where rapid uplift, subsidence or horizontal movement along fault zones takes place, short-term fans of the radial type usually form.

Sea-Level Fluctuations On immature riffed margins, and portions of marginal seas and transform margins, vertical tectonics dominate. Sediment is often dumped across steepened fault-scarp slopes, and a slope-apron system will tend to develop.

Changes in global and local sea level have a major effect on marine sedimentation (Vail et al., 1977; Vail and Hardenbol, 1979; Vail and Todd, 1981; Vail et al. 1984; Vail and Haq, 1987). During highstands of sea level most sediment remains trapped in near-shore environments. Wider shelves predominate, and the amount of sediment transported across the shelf into canyons is reduced. The development of deepwater fans and associated systems ceases, or they become relatively inactive. However, large elongate fans or slope-apron systems off major rivers may continue to grow at a reduced rate.

Lowstands of sea level result in narrow shelves and more rapid sedimentation, and sediments are often funneled directly through canyons and fan valleys to deeper basins. Shanmugam and Moiola (1982a) found a close relationship between the ages of Tertiary and Quaternary hydrocarbon-bearing submarine canyon and fan deposits and the ages of global sea-level lowstands ( Figure 1 , Submarine Canyon and fan reservoirs and their relation to Vail and Hardenbol's (1979) lowstands of sea level).

Figure 1

They also theorized that bottom currents which tend to flow parallel to bathymetric contours (commonly known as contour currents) would be stronger during periods of low sea level and cause major winnowing of turbidites. To test this hypothesis, they plotted the ages of winnowed turbidites on a global sea level curve. This plot revealed a strong correlation between winnowed turbidites and low sea-level stands ( Figure 2 , Winnowed turbidites and their correlation to lowstands of sea level).

Figure 2

LITHOFACIES OF SUBMARINE FANS AND ASSOCIATED SYSTEMS

A lithofacies is formed in response to the depositional processes inherent to a specific environment or subenvironment. An active depositional system comprises a number of depositional subenvironments. For example, a submarine fan may be composed of slope, feeder channel, channel, interchannel and outer fan lobe subenvironments, each producing a characteristic lithofacies. Lateral and vertical relationships between individual facies units may be sharp or gradational.

Facies Classification

A widely used scheme for shorthand descriptions and facies designations of submarine fan deposits was developed by Mutti and Ricci Lucchi (1972), Ricci Lucchi (1975a, b) and by Walker and Mutti (1973). Fan sediments were classified into seven lithofacies (A through G). These facies should not be confused with the Ta through Te turbidite sequence of Bouma .

The following summary descriptions of Mutti and Ricci Lucchi's facies are taken directly from the above-mentioned authors, as presented by Howell and Normark (1982):

Facies A consists of conglomerate and coarse-grained to pebbly sandstone. Bed thicknesses are generally greater than one meter, but lateral variations in thickness are frequent. Scour and channeling is typical; however, in distal parts of a fan, facies A conglomerate beds often have planar bases. Most facies A outcrops

display a succession of composite beds. A given bed may be nongraded or show normal, reverse, coarse-tail, or distribution grading.

Clasts may be either framework or matrix-supported, and a succession of flow units may include both massive and stratified units.

For beds within facies A, the Bouma sequence is generally not applicable, though for some coarse sandstone beds Ta or Tae is appropriate. Facies A beds result principally from slurry or debris-flow processes or from turbulent and traction fallout or grain-flow processes.

Facies B is generally composed of coarse- to medium-grained sandstone in thick, massive and often composite bed sequences.

Some scour and channel features occur, but lateral bed continuity is greater than in facies A.

A typical bed includes granules or mud chips along a basal scour surface, with faint parallel laminations, dish structures and elutriation scars in the remainder of the bed ( Figure 1 , Idealized structures within a facies B sandstone). Individual flow units are often difficult to determine due to uniformity of grain size in an outcrop.

Figure 1

The Bouma sequence is not applicable to beds of this facies. Facies B generally occurs in a channelized setting, particularly but not exclusively on the inner and middle fan. Facies B beds reflect hydraulic processes of grain flow, and when they are transitional to facies C in character, fluxoturbidity currents are suggested.

Facies C comprises coarse- to fine-grained sandstone commonly interbedded with thin layers of mudstone. The sandstone beds are the classical turbidites of Bouma (1962). The sixfold subdivisions of an ideal turbidite are not always fully developed.

Sandstone beds of facies C are generally 0.25 to 2.5 m thick, though thinner beds with a complete Bouma sequence are common. Facies C beds are of uniform thickness for great lateral distances. Mud chips or pebbles may lie along the basal surface, normal grading is common, and sole markings are well developed.

Facies C is generally associated with the upper parts of channel-fill sequences and with such nonchannelized settings such as middle fan fringe, outer fan or even basin plain. These sandstone beds are primarily deposited by turbidity currents.

Facies D consists of thin interbeds of sandstone and mudstone with sandstone beds being tabular and persisting laterally for great distances. Each sandstone bed is typically graded and displays the upper part of the Bouma sequence Tcde or Tce. Bed thickness is generally 0.05 to 0.25 m, and sole marks are commonly well developed. Thick facies D strata are transitional with thin facies C beds, and the two are often interbedded. Facies D strata are traditional 'distal turbidites' though such thin-bedded turbidites actually occur in nearly all parts of a submarine fan as well as on the basin plain.

Facies D sandstone beds in general represent deposition by low-density turbulent flows.

Facies E consists of thin interbedded sandstone and mudstone with a variety of internal-bedding characteristics including flaser bedding, massive, sand, graded sand, and climbing ripples. It can be differentiated from D beds by the following characteristics: (1) the sandstone is coarser grained than in facies D beds of equivalent thickness, (2) E has a higher overall sandstone-to-shale ratio, (3) E has thinner but more numerous sandstone beds, and (4) E commonly contains wavy and discontinuous sandstone bedding. The Bouma sequence may not always apply to facies E; however a Tce cycle, with a pronounced grain-size discontinuity, is a likely representation.

Facies E beds are characteristically associated with channelized environments of a submarine fan. The facies B/E couplet is typical of inner to middle fan associations, and facies E along with D, G and F compose most overbank or levee deposits. Facies E beds represent high-concentration gravity and traction flow processes near channel margins.

Facies F consists of remobilized deposits exhibiting mass slumping and localized resedimentation processes. Typical examples of facies F are localized zones of slump folds, sequences of pebbly mudstone where the matrix shows flow and deformation features rather than stratification, and zones of isolated and enclosed slump blocks.

Facies F forms by sediment failure, gravity slumping and sliding, and is typically found near the lower slope or along channel margins of inner and middle fan environments.

Facies G material comprises pelagic and hemipelagic detritus that tends to blanket all areas of a submarine fan. Bedding, where discernible, is generally thin and parallel. Facies G is best developed in slope and interchannel settings and, less commonly, as fill in abandoned channels.

Facies Associations Figure 2

Figure 2

and Figure 3 (Distribution of the Mutti and Ricci Lucchi (1972) facies in a submarine fan) shows the distribution of the Ricci Lucchi (1972) facies in a submarine complex (Shanmugam and Moiola, 1985).

Figure 3

As discussed earlier, all ancient submarine fans can easily be described according to the presence or absence of channeling. A channelized sequence, with its thinning- and fining-upward cycles, is usually composed of A, B, C and E facies. A non-channelized sequence, with its thickening- and coarsening-upward cycles, contains facies C, D and G. Facies F and G may occur in all regions, but facies F is more characteristic of the slope and facies G of the basin plain.

Walker (1978) developed a fan model that included the inferred position of various facies on the fan ( Figure 4 , Model of a submarine fan showing inferred position of various facies).

Figure 4

The model contains features that Walker (1984) admits may not occur on all fans, for example, terraces, inner-fan meandering channel levees, etc. Thus, the model in Figure 4 may not be as widely applicable as Walker's later (1984), simplified models, shown in Figure 5 (Simplified fan model with simplest possible terminology on the right).

Figure 5

However, it remains an excellent presentation that can be used to predict facies distributions in many ancient radial (sand-rich, restricted basin) types of submarine fans.

Classical turbidites are parallel-bedded and unchannelized sequences that occur on the smooth outer parts of suprafan lobes, lower fans and basin plains. These classical turbidites may also occur on channel margins or levees or in interchannel basins. Classical turbidite sequences are represented by Mutti and Ricci Lucchi's facies C and D and contain all or a portion of the Bouma Ta through Te divisions.

Massive and pebbly sandstones occur in channelized fan environments, with the finer facies in the distributary channels and coarse facies higher on the fan towards the inner-fan channel. These deposits, along with conglomerates, if available from a source area, are often found in the inner-fan channel or as lag in some distributary channels and are represented by Mutti and Ricci Lucchi's facies A and B.

In Figure 6 (Hypothetical sequence produced by the progradation of a submarine fan),

Figure 6

Walker (1978) presents a generalized hypothetical sequence produced by the overall progradation of a submarine fan of the type described by the model in Figure 4 . Note the thickening- and coarsening-upward turbide sequences that characterize the smooth, unchanneled portion of the fan, and the thinning- and fining-upward sequences of massive sandstone that occur in the channelized portion of the fan.

The photo in Figure 7 (Thickening-upward sequence (arrowed) in overturned turbidites, Ordovician Cloridorne formation, Quebec) shows an example of a thickening-upward turbidite sequence that abruptly returns to thin-bedded turbides at the top (beds are slightly overturned).

Figure 7

The photo in Figure 8 (Thinning-upward sequence (arrowed) in Cambrian St.

Figure 8

Roch formation, Quebec) shows a thinning- and fining-upward sequence of massive sandstone which is interpreted as a channel fill.

Subsurface Diagnosis & Detection

Paleontology

As discussed by Howell and Normark (1982), in situ assemblages of macrofossils are rarely found in submarine fans and associated systems. This is due to two factors: first, most fans in the rock record typically form in restricted basins where anoxic bottom conditions hostile to animal life are common. Secondly, large elongate-type fans generally form in deep ocean basins at depths below the calcium carbonate compensation depth (4,000 to 5,000 m below sea level). When isolated specimens of body fossils are found, transportation from shallow-water environments is suggested.

Larger assemblages of macrofossils are often found in the thick resedimented sandstones and conglomerates, having been possibly transported from several shallow marine, or even non-marine, environments. An example is provided by the neritic marine mollusks occurring in the slope channel sandstone of the Chatsworth formation of Southern California (Link, Squires and Colburn, 1984).

Microfossils are found mainly in the finer-grained facies, particularly in the top-interval turbidites. These microfossils may be good paleobathymetric and chronostratigraphic

indicators. Some microfossils, however, in the Tet interval may have been reworked from older strata. In general, the key paleontologic criterion for recognition of deepwater turbidite systems is the presence of reworked neritic microfossils with bathyal-pelagic microfossils.

Trace fossils are well developed in middle- and outer-fan environments. However, certain form-genera, indicative of shallow-water conditions, may occur along the soles of sandy turbidites in middle-fan settings (Crimes, 1977). This apparant anomaly can be explained by the fast-flowing depositional currents that result in episodes of high oxygenation and slow buildup of organic detritus, both conditions of high-energy neritic environments. Typically, "deepwater grazing" forms increase in abundance in outer fan settings where relatively tranquil conditions provide an organic-rich environment for sediment eaters and grazers (Ksiazkiewicz, 1970; Crimes, 1977).

Associated Fades

A block diagram restoration of the Upper Carboniferous Shale Grit and associated formations of northern England is shown in Figure 1 (Depositional model of Shale Grit and associated formations).

Figure 1

Although based on outcrop data, this diagram displays the typical facies associations of a prograding fan that can be used as a guide in making subsurface interpretations. Table 1, below, summarizes the lithology and environmental interpretations of the Shale Grit sequence.

Table 1. Deep-Water Sandstone Facies and Submarine Fans

Stratigraphy of Shale Grit Fan and Associated Rocks  

Formation Approximate Thickness (m)

Lithology and Interpretation

Kinderscout Grit 150 Mainly coarse sandstones, some shales. Shallow water deltaic complex. (Collinson, 1969)

Grindslow Shales

100-120 Massive and laminated mudstones and shales. Mainly prograding slope deposits(Walker, 1966a,b; Collinson, 1969). Upper fan channels at base.

Shale Grit 130-240 Sandstones and shales. The upper part was mainly deposited on the braided suprafan; the lower part was deposited on smooth suprafan slopes.

Mam Tor Sandstones

250 Black basinal mudstones. 

The classical turbidites of the Lower Fan (Mam Tor Sandstones) overlay and interfinger laterally with black pelagic muds of the basin plain (Edale Shales). Switching of depositional lobes can cause the top of an abandoned lobe to be covered by pelagic mud ( Figure 1 and Figure 2 (Hypothetical cross section across a prograding lower- and middle-fan system).

Figure 2

The massive sandstones of the upper fan (Shale Grit) pass abruptly into slope muds (Grindlow Shales) in a proximal direction. In prograding sequences, slope facies tend to abruptly overlie the upper fan sandstones. The slope facies, in turn, usually coarsen upward into deltaic or shelf facies. Slope facies may also lie directly below fan deposits as shown in Figure 3 (Onlap of sandstone lobe onto slope mudstone, tertiary Cengio sandstone, Italy).

Figure 3

In this example a sandy depositional lobe deposited in a small, restricted basin was probably diverted by submarine topography to abruptly onlap slope facies (Cazzola et al., 1985). Slope facies may be mudstones or pebbly mudstones, or contain laminations of silt or very fine sand. Slide, slump and other soft sediment deformation features are common. Locally, channels of coarse sand or gravel may incise into the slope deposits.

Cores

Cores and cuttings of deep sea sands and associated facies should reveal diagnostic evidence of turbidity flow and other gravity flow processes. The following features may be found in conventional cores:

· Thin beds of sandstones with flat tops and bottoms, alternating with thin beds of shale.

· Sandstone beds with abrupt bases that tend to grade upwards into finer sandstone, siltstone and shale.

· Within sandstone beds, combinations of parallel lamination, ripple cross-laminations, climbing ripple cross-lamination, convolute lamination and graded bedding.

· Possible sole marks, slide marks and load structures at the base of beds.

· Pebbles or rip-up clasts dispersed in a matrix of mud resulting from grain flow or debris flow.

· Dish structures and fluid escape pipes characteristic of fluidized flow.

· Small scale slump features and other soft sediment deformation features.

The photographs in Figure 1 (Gravel),

Figure 1

Figure 2 (Pebbly muds ),

Figure 2

Figure 3 (Sandy silts),

Figure 3

Figure 4 and Figure 5 (Laminated muds with silts)

Figure 4

show cores of five selected sedimentary facies taken from the mid-fan region of the modern Mississippi Fan.

Figure 5

Core A shows clean gravels that appear to be clast-supported and normally graded. Core B shows poorly sorted pebbly muds that form structureless beds in which pebbles and sand gravels are matrix-supported. Core C consists of sandy silts, interbedded muds or silty muds. Cores D and E show muds thinly laminated with silt.

Figure 6 (Massive medium-grained sand ),

Figure 6

Figure 7 (Laminated silt ),

Figure 7

Figure 8 (Silt-laminated mud ),

Figure 8

Figure 9 (Interbedded mud,

Figure 9

sand and silt) and Figure 10 (Laminated silt and mud ) show photographs of cores taken from the area of a sandy, sheet-flow depositional lobe on the lower Mississippi Fan.

Figure 10

Core A consists of structureless, massive medium-grained sand. Core B shows finely laminated silt and massive fine-grained sand. Core C shows silt-laminated muds. Core D consists of interbedded mud, sand and silt-laminated mud; and core E consists of interbedded mud, silt-laminated mud and finely laminated silt. Contacts between sandstones and finer-grained sediment in cores A, B, D and E commonly show convolute bedding and discontinuous laminations.

Cores and cuttings of turbidite sequences may contain glauconite, shell debris, mica or carbonaceous detritus. All four ingredients may be found together in turbides sourced from both deltaic and marine environments (Selley, 1985). Colors of turbidite sediments range from black to gray and green as the result of deposition in oxygen-poor marine environments. Where turbidites have been exposed by uplifting, secondary leaching by meteoric, oxygen-rich groundwaters may take place, turning the sediments reddish, brown or orange. Such situations may possibly occur where turbidites are situated under unconformities.

Well Logs

Deep sea sediments may display diagnostic profiles on gamma ray and SP logs indicative of location on a particular fan. Channelized sequences display typical fining-upward or blocky motifs, and unchannelized depositional lobes show characteristic coarsening-upward profiles ( Figure   1 ,

Figure 1

Figure   2 and Figure   3 ,

Figure 2

Log patterns from the Forties formation, North Sea).

Figure 3

Caution must be exercised, however, because deltaic distributary channels (fining-upward), as well as crevasse subdeltas and deltaic distributary mouth bars (coarsening-upward) display similar profiles. Turbidites, however, tend to show a very nervous' pattern, with the gamma ray/SP curve swinging back and forth erratically, reflecting thin interbeds of sand and shale ( Figure   4 and Figure   5 ,

Figure 4

Log patterns from the Forties formation, North Sea).

Figure 5

Figure   6 (SP/resistivity log of a turbidite formation with environmental interpretations ) shows an SP/resistivity log of a turbidite formation with environmental interpretations by Walker (1984).

Figure 6

Figure   7 (SP/gamma-ray log profiles of upper fan turbidite channel from Lower Miocene Louisiana Gulf Coast ) shows the blocky SP/gamma-ray log profiles of three Lower Miocene "upper fan"-type turbidite channels from the Louisiana Gulf Coast.

Figure 7

Coarsening-upward serrated profiles typical of turbidite suprafan lobes are displayed on two SP logs from the offshore U.S. Gulf Coast ( Figure   8 , SP logs of turbidite depositional lobes from offshore U.S.

Figure 8

Gulf Coast). Note that the logs in Figure   8 each show three cycles of lobe progradation.

Idealized dipmeter motifs for deep sea sands of the North Sea, along with characteristic gamma-ray profiles, were presented by Selley (1979). Base-of-slope and inner-fan feeder channels consisting of conglomerates and grain flow channel sands should ideally show a random dipmeter pattern ( Figure   9 , Idealized gamma ray and dipmeter motifs for slope or conglomerate channels).

Figure 9

Fining-upward channels, typically occurring in inner- to mid-fan regions (proximal fan), ideally show upward decreasing dips (red patterns) reflecting lateral accretion surfaces. These red motifs will tend to show dips pointing towards the center of channels and thus perpendicular to channel axes ( Figure   10 , Idealized gamma ray and dipmeter motifs for proximal fan channel turbidites).

Figure 10

Note that the lowest channel in Figure   10 incises into the underlying depositional lobe, reflecting progradation of the channelized region of the fan over the unchannelized fan lobes.

Figure   11 (Idealized gamma ray and dipmeter motifs for distal fan turbidites) shows the upward-coarsening deposition lobes of the outer (distal) fan environment.

Figure 11

In these facies, upward-increasing dips form blue patterns ideally pointing in the direction of lobe progradation.

Seismic

Figure 1 (Seismic facies of an idealized canyon-fed submarine fan system) is an idealized seismic facies diagram presented by Mitchum (1985) that illustrates seismic reflection patterns of a canyon-fan system.

Figure 1

It consists of the canyon and the submarine fan, which is divisible into upper and lower fan segments.

Canyon One of the most important aspects of canyons is the possibility of finding submarine fans below them. Thus, they should be mapped carefully for any clues to fan location. However, many fans have been found for which no associated canyons have been recognized.

In longitudinal section, canyon-fill deposits commonly show a reflection pattern characterized by shale-prone events prograding down-canyon ( Figure 2 , Longitudinal seismic section of a canyon-fill deposit).

Figure 2

In transverse section, canyon-fill deposits appear as an onlap-fill reflection configuration onlapping the canyon walls ( Figure 3 , Transverse seismic cross section across a canyon-fill deposit).

Figure 3

Upper Fan The distinctive sedimentary feature of upper fan seismic facies, as shown by the model in Figure 1 , is the development of one or more large channels flanked by levees with concave-upward surfaces.

An example of an upper-fan seismic facies, in transverse section, is shown in Figure 4 (Transverse seismic section across the upper-fan portion of a modern submarine fan, offshore California (Sparker survey)).

Figure 4

Note the pronounced notch marking the central channel along with the concave-upward upper surface, an important feature in the recognition of the upper fan.

Lower Fan The typical seismic reflection configuration of the lower fan includes (1) overall mound shape, (2) convex-upward reflection patterns with bidirectional downlap, and (3) commonly high-amplitude reflections. Due to a generally higher sand content in the fan, velocity "pull-up" anomalies under fans are also common.

Note that all of the above features can be found on the seismic sections across two North Sea fans, shown in Figure 5

Figure 5

(Middle Paleocene "Halibut" Fan)and Figure 6 (Eocene Frigg Fan ).

Figure 6

 

Sequence StratigraphySEQUENCE STRATIGRAPHY OF DEEP SEA SANDS

Sequence stratigraphy is the study of rock relationships within a chronostratigraphic framework of repetitive, genetically related strata bounded by surfaces of erosion or non-deposition, or their correlative conformities (Van Wagoner et al., 1988).

A sequence ( Figure 1 , Model for the depositional sequence), the fundamental unit of sequence stratigraphy, is a relatively conformable succession of genetically related strata bounded by unconformities and their correlative conformities (Mitchum, 1977).

Figure 1

A depositional system is a body of genetically related sediments deposited in stratigraphic continuity and as such is part of a depositional sequence. One or more depositional systems may make up a depositional sequence

A systems tract is a linkage of contemporaneous depositional systems (Brown and Fisher, 1977) and is used by Van Wagoner et al. (1988) to designate four possible subdivisions within a sequence, each systems tract being controlled by eustatic sea level. The four systems tracts are designated: lowstand, transgressive, highstand and shelf-margin systems tracts ( Figure 2 , The four possible system tracts: Lowstand System Tact (LST), Transgressive Systems Tract (TST), Highstand Systems Tract (HST), and Shelf Margin Systems Tract (SMST) ).

Figure 2

Deepwater reservoir-quality sands are usually deposited during intervals of relatively low sea level in lowstand systems tracts.

Models of Posamentier and Vail

Posamentier and Vail (1988) divide lowstand systems tracts, deposited in basins with a discrete shelf edge, into two end members: a lowstand fan (or basin floor fan) followed by a lowstand wedge.

Lowstand fans occur when the rate of eustatic sea-level fall exceeds the rate of subsidence at the depositional shoreline break and some or all of the shelf is subaerially exposed ( Figure 1 , Model of Lowstand Systems Tract with basin floor submarine fan).

Figure 1

Streams downcut into the shelf, and sediment is deposited on the slope and on the basin floor as point-sourced submarine fans. A portion of the slope may also be exposed causing the downcutting of canyons. When eustatic sea-level fall is again less than subsidence at the depositional shoreline break, lowstand fan formation generally terminates. Posamentier and Vail emphasize that although submarine fans can be deposited at any time, they are most likely to be deposited and have the highest sand: mud ratio during lowstand fan time.

Lowstand wedges are deposited as regressive progradational sequences after the rate of seafloor subsidence at the depositional shoreline break once again exceeds the rate of eustatic sea-level fall ( Figure 2 , Model of a Lowstand Systems Tract with a lowstand wedge containing leveed channel deposits).

Figure 2

As relative sea level slowly rises after deposition of the lowstand fan, the sand:mud ratio, as well as total sediment load, decreases. Downcutting of the canyon slows and leveed-channel deposits, along with overbank-turbidite deposits, form beyond the canyon mouth. These deposits have a lower sand:mud ratio than the underlying lowstand fan. Sands are usually restricted to the channels but can occur as thin units in turbidite overbank deposits and, in sandy systems, as sheet sands beyond the levees.

These leveed-channel and overbank deposits are point-sourced from the canyon and are sometimes referred to as a slope fan. Thus the lowstand fan and the overlying slope fan of the lowstand wedge deposit can each be considered to represent one stage in the development of a complete deep sea fan system.

Models of Mutti Based on studies of ancient turbidites in Spain and Italy, Mutti (1985) distinguished three types of turbidity deposits that can be used as a reference in describing most ancient systems ( Figure 3 ,

Figure 3

The three main types of turbidite depositional systems), ( Figure 4 ,

Figure 4

Figure 5 and Figure 6 , Plain view models of the three main types of turbidite depositional systems).

Figure 5

The three main types, systems I, II and III are the end-members of a continuum distinguished by a progressive decrease in the volume of turbidity currents and by a corresponding increase in mud content.

Figure 6

These features are controlled by sea-level variation, local and regional tectonic setting, and the amount and type of sediment available for resedimentation.

Type I deposits are characterized by the bulk of the sandstone occurring as nonchannelized elongate outer-fan lobes deposited by large-volume turbidity currents during sea-level lowstands. The sandstone bodies display lateral continuity over distances of up to several tens of kilometers in a direction parallel to the current. Lobes are commonly 3-15 m thick and grade in a downcurrent direction into thinner-bedded and finer-grained lobe-fringe deposits. This type of deposit essentially fits Mutti's (1979) model of a "highly efficient" fan. Type I systems typically develop in narrow, elongate foreland basins that enhance distance of sand transport (Mutti, 1985). They may also form in relatively unconfined basins provided sufficient fine-grained sediment is involved to enhance the mobility of the turbidity currents. According to Mutti and Normark (1987), well-documented examples of modern Type I deposits are presently unavailable.

Type II deposits, typically coarser-grained and smaller than Type I, are characterized by the deposition of channelized sand bodies in the upper fan. The channelized sand grades downcurrent into smooth, sandy lobes resembling the modern suprafan lobes of Normark (1978), . Type II deposits fit Mutti's (1979) model for a "poorly-efficient" fan and

correspond to the sand-rich radial fans . They form in actively deforming, short-lived basins that develop on active margins, for example, during early rifting phases, along active transform margins, and during the final stages of continental collision. Ancient examples of Type II systems include the sand-rich "radial"-type fans of southern California, such as those described from the Upper Cretaceous Chatsworth formation . The Upper Jurassic Brae "slope-apron" system on the rifted margin of the South Viking Graben in the North Sea may also be an ancient analog of a Type II system. Modern examples of Type II systems include some of the California Borderland Fan, such as the Navy Fan

Type III deposits essentially record channel-levee complexes. They are mostly fine-grained, thin-bedded sediments that are characterized by small sandstone-filled channels surrounded by and grading downcurrent into mostly muddy facies. A conspicuous feature of the Type III deposit is the lack of associated sandstone lobes. Type III systems are deposited during periods of relative sea-level highstand with sediment supplied by "normal" conditions of shelf-edge instability such as those produced by deltaic progradation, major storms and earthquakes. Ancient examples can be found in the late stages of turbidite systems in the Eocene Hecho Group of Spain (Mutti, 1985). Possible modern analogs are the Delgada Fan levee-valley complex of offshore northern California (Normark and Gutmacher, 1985) and the inner portion of the Mississippi Fan (Freely et al., 1985).

Stages of Growth Each type of deposit, Types I, II and III, corresponds to a stage of growth. Some turbidite systems apparently express only one stage of growth. This can take place, for example, in areas of strong tectonic uplift, where fan-deltas or other types of coarse-grained alluvial and near-shore deposits feed sediment across narrow shelves to form sand-rich, small-volume, Type II turbidite systems ( Figure 7 , Turbidite depositional sequences in basins with strong marginal uplift).

Figure 7

Other turbidite systems may develop as composite features displaying multiple stages of which one stage predominates. An example is the Eocene Hecho Group of Spain, where turbidite systems are characterized by three stages of growth-Stages I, II and III — corresponding to the three types of turbidite deposits — Types I, II and III — and shown diagrammatically in Figure 8 (Different stages of growth of turbidite systems with relative changes of sea level).

Figure 8

The three different stages of growth are interpreted as resulting from a reduction in the volume of mass flows associated with a progressive rise in sea level (Mutti, 1985). As shown in Figure 9 (Inferred depositional model for the Eocene Hecko Group turbidites, Spain ), unchannelized sandstone lobes predominate within the turbidite systems of each sequence in the Hecho Group.

Figure 9

These turbidites are thus referred to as Type I deposits or Type I systems.

In conclusion, the sand-rich Type I and Type II deposits of Mutti (1985) that are characteristic of sea-level lowstands most likely correspond to Posamentier and Vail's (1988) lowstand fan deposits. However, as mentioned above, some Type II deposits can probably also develop during sea-level highstands in areas where fan-deltas actively prograde across narrow shelves. The finer-grained Type III deposits of Mutti that develop during relative sea-level rise probably correspond to the leveed-channel and overbank deposits (slope fan) of Posamentier and Vail's lowstand wedge.

 

PETROLEUM GEOLOGY OF DEEP SEA SANDS

Deep sea sands are often highly productive oil and gas reservoirs. Deep sea systems usually form at the foot of prograding deltas and in fault-bounded troughs. Such environments favor the development of source rocks in associated muds. Systems formed in open seas probably have lower potential for petroleum due to poorer source rock

conditions. The thin, laterally extensive turbidite beds of outer-fan facies tend to interfinger with surrounding organic-rich muds. Ideal conduits are thus provided through which hydrocarbons can migrate updip into sandy depositional lobes and channels of the fan or slope-apron deposit. Migration directly from source to reservoir may take place where source beds blanket the sandy depositional lobes, as well as where sand-filled channels or channel-levee complexes are encased in source muds.

In general, channel and channelized lobe sands have better porosities and permeabilities than the turbidite sands occuring in interchannel areas or in the more distal regions of depositional lobes. Consequently, they tend to form the best petroleum reservoirs. Impermeable seals are provided by fine-grained sediments of the lobe fringe, interchannel and, possibly, slope deposits. Productive reservoirs may occur in a variety of structural and stratigraphic traps, as illustrated by the examples discussed below.

Channel Reservoirs

These are defined as reservoirs occuring as isolated, elongate, shoestring sand bodies that are encased in fine-grained muds and silts. The sand bodies may form as fill in slope canyons or in channels on the fan itself, where they are confined laterally by levee silts and muds and overlain by more levee deposits or pelagic muds. These channels may or may not feed depositional lobes of submarine fans.

Rosedale Channel An early account of an oil-productive submarine channel sand was given by Martin (1963) for the Late Miocene Rosedale Sandstone of the South San Joaquin Basin, California ( Figure 1 , Location map of Rosedale Channel). The Rosedale Sandstone occurs in a channel about 1 mile (0.6 km) wide and 400 to 1,200 ft.

Figure 1

(130 to 400 m) thick ( Figure 2 , Isopach of Rosedale Sandstone) and Figure 3 (Cross section across line 2-2' in Figure 2).

Figure 2

Figure 3

Microfossil studies indicate that erosion of the underlying shale took place to form a canyon that was later filled with sand. These studies also show that both cut and fill took place in water depths greater than 1,300 ft. (2,000 m). Cores taken within the channel fill show many characteristics of turbidites, such as graded bedding, interbedded sandstone and shales, poor sorting and displaced faunas. Martin interpreted the Rosedale Channel to be comparable, in most respects, to present day submarine canyons. Although only minor oil production has been obtained from this reservoir, the channel represents a remnant of a much more extensive Late Miocene feature — one that is associated with major oil production.

Ramsey Channel Rocks of the Permian Delaware Mountain Group of the Delaware Basin in New Mexico and Texas contain numerous sandstone-filled channels that form important subsurface hydrocarbon reservoirs (Payne, 1976; Berg, 1979; Harms and Williamson, 1988). The channels are developed primarily in the Ramsey Sandstone of the Upper Bell Canyon formation. The Ramsey was formed as a basinward-thining "sheet" of interbedded thin sandstones and siltstones. Sediment was derived from a northeasterly source and

deposition took place in the deep waters of a density-stratified basin ( Figure 4 , Proposed direction of transport for Upper Bell Canyon sandstones).

Figure 4

Harms and Williamson (1988) postulate that density currents from an evaporitic shelf moved along the basin floor, cutting channels or depositing sand and silt in existing channels. Most channels were filled in a complex manner, with sandstone restricted to channel floors abutting the steep channel walls.

A regional isolith map of the Ramsey Sandstone shows trends of thick sandstone marking the axes of the erosional channels ( Figure 5 , Sandstone isolith map of the Ramsey Sandstone, Upper Bell Canyon formation). These essentially straight channels are aligned normal to the basin margin and extend southwestward into the basin for at least 70 km.

Figure 5

Oil accumulations form at locations where the channel fill sandstones abut the updip (northwest) erosional margins of the channels ( Figure 6 , Structure map on top of Ramsey Sandstone).

Figure 6

The relatively impermeable laminated siltstone into which the channels are cut provides lateral and top seal for the trap. Dark organic shales are described from interbeds within the Upper Bell Canyon formation. These black shales, which are rich in planktonic organic material, may have provided the oil source.

Over 150 fields have produced about 140 million BBL of oil from the Delaware Mountain Group (Weinmeister, 1978), with most of the production comming from the Upper Bell Canyon formation (Ramsey Sandstone channels). As shown by the sandstone isolith maps of the El Mar and Grice fields ( Figure 7 ) and the Paduca field ( Figure 8

Figure 8

), productive sandstone channels range from 1.5 km to 6.0 km in width and are generally 10 to 25 m thick.

Figure 7

Stratigraphic cross sections across the El Mar and Paduca fields illustrate how the Ramsey channels have incised into the underlying siltstones and how shale markers have served to delineate the channel ( Figure 9 , Stratigraphic cross sections showing erosional channels, Upper Bell Canyon formation).

Figure 9

A complete channel sequence from a Paduca field well is shown in Figure 10 (Complete channel sequence from a well in the Paduca field, New Mexico).

Figure 10

Very well-sorted, fine-grained sand can be seen to fine upward from a scour base.

North Sabine Lake Field The North Sabine Lake field, located in southwestern Louisiana, produces from a package of Oligocene, lower Hackberry sands that were deposited in a preexisting submarine canyon (Eubanks, 1987). The canyon is part of a system of canyons containing gas-productive Hackberry sands located in the vicinity of the Texas/Louisiana border ( Figure 11 , Net sand isopach of Oligocene lower Hackberry sand, Louisiana/Texas border).

Figure 11

The canyons extend down the paleoslope, and incise into underlying Lower Frio and Vicksburg shales. The SP of well logs in the field indicates an overall fining-upward sequence.

The development and production history of the field indicate that many small lenses have coalesced to form a single large reservoir. Other small sand lenses, not connected to the main reservoir complex, have produced only small amounts of hydrocarbons.

Shale-Filled Canyons Many deepwater canyons are filled with shale. In the Texas Gulf Coast, the Yoakum "channel" and other smaller, shale-filled canyons in the area are responsible for trapping hydrocarbons at the canyon's edge. Figure 12 (Diagram of shale-filled canyons providing seal for oil accumulation at Valentine and Menking fields Texas) illustrates how two such shale-filled canyons,

Figure 12

the Smothers and Lavaca "channels," provide lateral seal for oil in the Technik and Kubena turbidite channel sandstones (Chuber, 1979).

Submarine Fan Reservoirs

Important oil and gas production has been obtained, world-wide, from ancient deepwater sandstones deposited in submarine fans and related slope-apron systems. Basically, this group of reservoirs includes all deepwater sandstones not included in the canyon-filled and lenticular channel-fill type of reservoirs covered in the previous section.

Stevens Sandstones Over 400 million BBL of oil have been produced in 15 fields from the Upper Miocene "Stevens Sandstone" located on the Bakersfield arch of the southern San Joaquin Valley, California. As described by MacPherson (1978), the Stevens Sandstone encompasses more than 1,200 m of interbedded sand and shale. Two regional morphological features comprise the Stevens complex — the Rosedale "fan" and the Fruitdale "fan" ( Figure 1 , Major turbidite fans of southern San Joaquin Valley, California).

Figure 1

MacPherson recognized four cycles of turbidite deposition on the Bakersfield arch.

During each cycle, fans issued simultaneously from the Rosedale and Fruitdale canyons, comingling along their lateral margins. The overall Stevens sequence, therefore, consists of four fan complexes named, in ascending order, the Rosedale, the Coulter, the Gosford and the Bellevue ( Figure 2 , Cross section showing Upper Miocene fans on Bakersfield arch, southern San Joaquin Valley, California).

Figure 2

The sequence is characterized by continued basinward offlap, with each fan system occupying successively deeper parts of the basin. Widespread mud (shale) deposition took place during periodic cessation of sand influx. Regional correlation of these shales allowed recognition of the individual turbidite fans.

Each turbidite fan has four distinct facies originating in response to discrete sedimentary processes. These facies as described by MacPherson are:

1. Proximal: Deposited within or adjacent to submarine canyons and consisting of facies A and B (Walker and Mutti, 1973).

2. Mid-fan: Deposited within a delta-shaped apron on the sea floor and consisting of facies C through E of Walker and Mutti, (1973), the suprafan of Normark, (1970, 1976), and the middle-fan association of Mutti and Ricci Lucchi (1972).

3. Lateral margin (basin margin): Facies developed on the wedging, peripheral edge of the midfan; not generally emphasized or recognized elsewhere but highlighted here because of their importance in California oil exploration.

4. Distal margin: Arcuate toe of the submarine fan; possibly equivalent to facies F and G, referred to by Walker and Mutti (1973) as the rhythmically bedded, laterally extensive laminae common in outcrops of deep sea sediments.

Some oil production comes from the proximal canyon-fill sands of Rosedale Ranch and the Fruitdale fields. Most of the oil production on the Bakersfield arch comes from the sandy, channelized mid-fan facies, primarily on the upper two sandstone fan systems, the Gosford and Bellevue ( Figure 2 ). Oil accumulations in mid-fan facies occur as the result of two distinct mechanisms: growth faulting and differential compaction.

Growth faulting on the Bakersfield arch most likely originated in a manner similar to U.S. Gulf Coast growth faulting, where rapid deposition of sand occurs over underlying thick prisms of low-density, undercompacted shale. Syndepositional down-to-the-basin faulting takes place, accompanied by expansion of section and rollover on the downthrown block. In the Stevens sand, accumulations occur in the downthrown blocks of nearly vertical growth faults ( Figure 3 , Stratigraphic section across Bellevue field showing oil entrapment on downthrown side of growth fault).

Figure 3

The primary mechanism for entrapment is the stratigraphic discontinuity created between the upthrown and downthrown blocks. Another type of trap is associated with the downthrown rollover anticlines.

The second major trapping mechanism in mid-fan facies is created by differential compaction ( Figure 4 , Depositional model of Stevens turbidites showing compaction of interchannel shales leaving sandy channel areas as structural highs).

Figure 4

Compaction of interchannel shales creates shallow synclines in interchannel regions, leaving the sandy distributary channel areas as structural highs. Because of alternate stacking of distributary channels, a broad productive anticline often occurs over an underlying nonproductive syncline. Conversely, a nonproductive syncline may occur over a productive anticline. Figure 5 is a stratigraphic cross section through the Strand

field, illustrating vertical offset of productive compactional anticlines.

Figure 5

Oil also occurs in the lateral-margin facies of the Stevens sand as the result of wedging-out of the sheet-like turbidite facies against the basin margin. Figure 6 a cross section through the Greely field, illustrates wedging-out of lateral-margin facies as the Rosedale fan encroaches on a paleobathymetric high.

Figure 6

Prolific oil production from distal margin facies is obtained from fractured silicious shales. Origin of the silicious shales is attributed to large concentrations of diatom tests, similar to that described for the Monterey formation of California by Bramlette (1946).

Figure 7

Figure 7 (Map showing fan complex developed at end of Early Eocene, North Sea) shows where local flexures have created open, oil-filled fracture systems directly updip from Stevens Sandstone reservoirs.

Frigg Field The Eocene Frigg field, containing over 7 trillion cubic feet of recoverable gas, is located in the Viking Graben of the North Sea between the Norwegian shield on the east and the Shetland platform on the west. According to a study by Heritier et al. (1979), from which this brief summary is taken, the location of the Frigg structure in the deepest part of the basin, not far from the edge of the Shetland platform, along with its particular shape all suggests a deep sea fan origin. Further evidence suggesting deep sea deposition includes: (1) the monotonous character of the facies, (2) shales are gray to green, in places very pyritic, (3) sands are massive or thin-layered, generally fine to medium, coarser in proximal areas and finer in distal areas, (4) sands commonly show a bimodal distribution-a few coarser grains supported in a finer homogenous sand, (5) the sands generally contain shale clasts, and (6) the sands contain glauconite and carbonaceous detritus.

Figure 8 (Cross section from the Shetland platform to axis of Viking Basin showing location of Frigg fan)

Figure 8

shows the Frigg feature forming in the final stage of fan sedimentation that occurred in the Viking Graben from early Paleocene (Danian) to lower Eocene (Ypresian). Sediment originated on the Shetlands platform to the west and was transported by turbidity currents flowing in a generally northeastward direction to form the Frigg fan complex ( Figure 8 ).

A vertical stacking of sandy channel lobes occurred in the Frigg Fan similar to that of the Stephans sand in California described above. Differential compaction caused a channel lobe to be deposited above the compacted shaly section on the flank of the underlying thick sandy deposit. Figure 9 (Compaction of fine-grained levee and interchannel deposits leaving sandy deep sea channel deposit as topographic high) illustrates how compaction takes place on the flank of a deep sea fan channel.

Figure 9

Three cross sections shown in Figure 10 ,

Figure 10

Figure 11

Figure 11

and Figure 12 (Cross sections through Frigg field showing how compaction anticlines from traps for hydrocarbon accumulation.

Figure 12

Letter designation of sandstone refer to palynological units) illustrate how the compaction anticlines form traps for hydrocarbon accumulation in the Frigg field. Note that the chronostratigraphy, and thus the differentiation of individual depositional lobes, is based on palynologic assemblages. Both marine microplankton (dinoflagellates) and continental microflora (pollens and spores) were used.

Figure 13 (Sedimentological interpretation of a Frigg seismic section) shows a sedimentologic interpretation of a typical Frigg seismicsection.

Figure 13

Note the well-defined "flat spot" at about 2.0 sec caused by the density contrast at the gas-liquid contact.