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Fractures, faults, and hydrocarbon entrapment, migration and flow Atilla Aydin* Rock Fracture Project and Shale Smear Project, Department of Geological and Environmental Sciences, Stanford University, Stanford, CA, 94305-2115, USA Received 27 April 1999; received in revised form 2 April 2000 Abstract This paper presents an overview of the role of structural heterogeneities in hydrocarbon entrapment, migration and flow. Three common structural heterogeneity types are considered: (1) dilatant fractures (joints, veins, and dikes); (2) contraction/ compaction structures (solution seams and compaction bands); and (3) shear fractures (faults). Each class of structures has a dierent geometry, pattern, and fluid flow property, which are described by using analog outcrop studies, conceptual models, and, in some cases, actual subsurface data. Permeability of these structures may, on average, be a few orders of magnitude higher or lower than those of the corresponding matrix rocks. Based on these dierences and the widespread occurrence of fractures and faults in rocks, it is concluded that structural heterogeneities should be essential elements of hydrocarbon migration and flow as well as entrapment and that they should be included in large-scale basin models and reservoir-scale simulation models. This proposition is supported by a number of case studies of various reservoirs presented in this paper. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Fractures; Faults; Permeability; Fluid flow; Hydrocarbon seal, migration and flow 1. Introduction Fractures and faults are the most ubiquitous and ecient avenue for hydrocarbon migration and flow as well as entrapment. Since Nelson’s (1985) pioneering book on fractured reservoirs, several volumes have been published on this topic (Jones & Preston, 1987; Ameen, 1995; Long et al., 1996; Moller-Pederson & Koestler, 1997; Jones, Fisher & Knipe, 1998; Coward, Daltaban & Johnson, 1998; Parnell, 1998; Haneberg, Mozley, Moore & Goodwin, 1999). However, as the dates of these publications indicate, the importance of fractures and faults in hydrocarbon entrapment, mi- gration and flow has just been recently recognized. Recent advances in borehole and seismic imaging tech- nologies played a crucial role in this recognition. It is now impossible to take the previously common atti- tude that most basins and reservoirs have no fractures or faults. Even though recognition of the existence of fractures and faults in reservoirs is an important forward step, understanding the impact of these structures on fluid flow is far from satisfactory. One reason for the di- culty in handling fractures and faults is their complex- ity; another is the fact that the nature of the fracture/ fault contribution to hydrocarbon entrapment, mi- gration and flow varies widely (Smith, 1980; Aydin, Myers & Younes, 1998; Walsh, Walterson, Heath & Child, 1998; Knipe, Jones & Fisher, 1998). Each class of structure occurs in a specific geological and geome- chanical environment and has a specific genesis. Each class has its own geometry (orientation, dimensional properties), spacing, distribution, connectivity, and hydraulic properties, which result in limitations or ad- vantages for hydrocarbon transport and entrapment in Marine and Petroleum Geology 17 (2000) 797–814 0264-8172/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S0264-8172(00)00020-9 www.elsevier.com/locate/marpetgeo * Tel.: +1-650-725-8708; fax: +1-650-725-0979. E-mail address: [email protected] (A. Aydin).
Transcript
Page 1: Fractures Faults and Hydrocarbon Entrapment

Fractures, faults, and hydrocarbon entrapment,migration and ¯ow

Atilla Aydin*

Rock Fracture Project and Shale Smear Project, Department of Geological and Environmental Sciences, Stanford University, Stanford,

CA, 94305-2115, USA

Received 27 April 1999; received in revised form 2 April 2000

Abstract

This paper presents an overview of the role of structural heterogeneities in hydrocarbon entrapment, migration and ¯ow.Three common structural heterogeneity types are considered: (1) dilatant fractures (joints, veins, and dikes); (2) contraction/

compaction structures (solution seams and compaction bands); and (3) shear fractures (faults). Each class of structures has adi�erent geometry, pattern, and ¯uid ¯ow property, which are described by using analog outcrop studies, conceptual models,and, in some cases, actual subsurface data. Permeability of these structures may, on average, be a few orders of magnitudehigher or lower than those of the corresponding matrix rocks. Based on these di�erences and the widespread occurrence of

fractures and faults in rocks, it is concluded that structural heterogeneities should be essential elements of hydrocarbonmigration and ¯ow as well as entrapment and that they should be included in large-scale basin models and reservoir-scalesimulation models. This proposition is supported by a number of case studies of various reservoirs presented in this

paper. 7 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Fractures; Faults; Permeability; Fluid ¯ow; Hydrocarbon seal, migration and ¯ow

1. Introduction

Fractures and faults are the most ubiquitous ande�cient avenue for hydrocarbon migration and ¯ow aswell as entrapment. Since Nelson's (1985) pioneeringbook on fractured reservoirs, several volumes havebeen published on this topic (Jones & Preston, 1987;Ameen, 1995; Long et al., 1996; Moller-Pederson &Koestler, 1997; Jones, Fisher & Knipe, 1998; Coward,Daltaban & Johnson, 1998; Parnell, 1998; Haneberg,Mozley, Moore & Goodwin, 1999). However, as thedates of these publications indicate, the importance offractures and faults in hydrocarbon entrapment, mi-gration and ¯ow has just been recently recognized.Recent advances in borehole and seismic imaging tech-nologies played a crucial role in this recognition. It is

now impossible to take the previously common atti-

tude that most basins and reservoirs have no fractures

or faults.

Even though recognition of the existence of fractures

and faults in reservoirs is an important forward step,

understanding the impact of these structures on ¯uid

¯ow is far from satisfactory. One reason for the di�-

culty in handling fractures and faults is their complex-

ity; another is the fact that the nature of the fracture/

fault contribution to hydrocarbon entrapment, mi-

gration and ¯ow varies widely (Smith, 1980; Aydin,

Myers & Younes, 1998; Walsh, Walterson, Heath &

Child, 1998; Knipe, Jones & Fisher, 1998). Each class

of structure occurs in a speci®c geological and geome-

chanical environment and has a speci®c genesis. Each

class has its own geometry (orientation, dimensional

properties), spacing, distribution, connectivity, and

hydraulic properties, which result in limitations or ad-

vantages for hydrocarbon transport and entrapment in

Marine and Petroleum Geology 17 (2000) 797±814

0264-8172/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.

PII: S0264-8172(00 )00020 -9

www.elsevier.com/locate/marpetgeo

* Tel.: +1-650-725-8708; fax: +1-650-725-0979.

E-mail address: [email protected] (A. Aydin).

Page 2: Fractures Faults and Hydrocarbon Entrapment

a given environment. This paper deals with the classi®-cation of structural discontinuities in rocks, their geo-logical, geomechanical, and hydraulic characteristics,and reviews a few selected case studies from both out-crop analog and subsurface cases to demonstrating theimpact of fractures and faults on the migration, ¯ow,and entrapment of hydrocarbon.

2. Classi®cation and characterization of fracture/faultsystems

The term `fracture' is used in this paper to refer to astructure de®ned by two surfaces or a zone (Fig. 1(a))across which a displacement discontinuity occurs(Fig. 1(b)). The following classi®cation of such a struc-ture is based on its mode of formation and representsa simpli®cation of a more rigorous geomechanicalclassi®cation of rock fracturing (Pollard & Aydin,1988) based on a broader range of possible displace-ment con®gurations at a fracture's tip line:

1. Dilatant-mode fractures/joints, veins, dikes2. Contraction/compaction-mode fractures/pressure

solution seams and compaction bands3. Shear-mode fractures/faults

Structural and hydraulic characteristics of fundamentalelements constituting each of these classes will be dis-cussed in the order in which they are listed.

2.1. Dilatant fractures

Dilatant fractures are characterized by a displace-ment discontinuity that results from fracture wallsmoving away from, and relative to, each other (seearrows in Fig. 1(b)). This displacement discontinuity isa measure of the mechanical aperture (hm in Fig. 2)and is related to the hydraulic aperture (ha in Fig. 2),which is one of the most important petrophysical par-ameters characterizing ¯uid ¯ow through a single dila-tant fracture. According to the so-called parallel platemodel (see, for example, Chapter 3 in Long et al.,1996), the ¯ow rate along a fracture is related to thecube of the aperture; and thus, permeability kj, of asingle fracture in an impermeable medium is given by(Fig. 2):

kj � �ha �2=12: �1�Note that here ha is the hydraulic aperture. If the per-meability of the matrix rock is signi®cant, then a dualpermeability model taking into account the matrix per-meability as well as the fracture permeability is appro-priate (see Taylor, Pollard & Aydin, 1999, for a tableof fracture permeability formulations in terms of awide range of fracture geometry, shape, and matrixpermeability).

Fig. 2. Parallel plate model of a fracture with smooth fracture walls.

hm is the mechanical aperture of the idealized fracture. Permeability

of such an idealized system driven by an hydraulic head gradient is

given by kj � �ha� 2=12, where ha is the hydraulic aperture. Matrix

rock permeability (kr) is assumed to be negligible.

Fig. 1. (a) A fracture and (b) various fracture types classi®ed based

on the prominent displacement of the fracture walls as dilatant, con-

traction/compaction, and shear. (Modi®ed from Pollard and Aydin,

1988).

A. Aydin /Marine and Petroleum Geology 17 (2000) 797±814798

Page 3: Fractures Faults and Hydrocarbon Entrapment

The fracture permeability de®ned earlier is con-trolled solely by fracture aperture which is a dynamicparameter most di�cult to obtain and to verify fromnatural fractures. Measured or determined aperturesfrom natural fractures at outcrops (Barton & Hsieh,1989), in boreholes (Luthi & Souhaite, 1990), in in-situwell tests (Paillet, Hsieh & Cheng, 1987), and tracertests (Novakowski & Lapcevic, 1994) range from a fewhundred micrometers to several millimeters.

Among the dilatant fractures, three subgroups,based on genesis, geometry, and contribution to hy-drocarbon transport, are considered.

2.1.1. HydrofracturesHydrofractures form in an environment of high ¯uid

pressure (Hubbert & Willis, 1957; Secor, 1968), whichis the major driving force for their formation. Hydro-fractures may be vertical (dikes) or horizontal (sills) ora combination of the two depending on the interplaybetween the state of stress and the abnormal ¯uidpressure leading to fracturing (Mandl & Harkness,1987).

Field observations (Verbeek & Grout, 1983) fromthe Uinta basin in the western United States (for lo-cation see the inset in Fig. 3(a)) show that solid hydro-carbon exists in dikes several meters wide and severalkilometers long (Figs. 3(a) and 4(a)). The geometry ofsome of these hydrocarbon dikes is well knownbecause they have been mined for Gilsonite (Fig. 4(a)),a commercial name for a type of immature hydro-carbon used for various industrial purposes. In theabsence of a signi®cant deformation within the solidGilsonite, it follows that the liquid hydrocarbon ®lled

the dikes and that the fractures were conduits for hy-drocarbon migration and ¯ow. Those dikes wallsexposed by extensive mining show plumose structure(Pollard & Aydin, 1988), typical for dilatant fractures(Fig. 4(b)). In addition, there is evidence that hydro-carbon-®lled fractures exist in the reservoirs within theUinta basin and that dikes originated from an environ-ment of high ¯uid pressure as demonstrated by pres-sure distribution data from wells (Fig. 3(b) and (c)).The source of this high pressure in the Uinta basin,however, cannot be unequivocally determined. It hasbeen suggested that a 25- to 30-percent volumeincrease associated with kerogen/oil transformationwas su�cient to hydrofracture the Green River shale(Spencer, 1987; Fouch, Nuccio, Anders, Rice, Pitman& Mast, 1994; Bredehoeft et al., 1994), which is thesource rock for most hydrocarbons in the Uinta basin.Model studies also show that, once initiated, suchhydrofractures are capable of cutting through theimpermeable cap rock (Mandl & Harkness, 1987). Evi-dently, in the Uinta basin hydrofractures cut acrossnot only the source rock and cap rock but also theoverlying high-permeability detrital rocks (see the geo-logic column and map in Fig. 3(a) and (b). It is there-fore inferred that the Gilsonite dikes transportedhydrocarbons a few kilometers vertically and tens ofkilometers laterally. Given the observed aperture ofthe dikes (on the order of several meters, Fig. 4(a)),their lengths and density (Fig. 3(a)), and the expressionfor permeability derived from the parallel plate model(Eq. (1); Fig. 2), the contribution of this type of per-meability fracture to hydrocarbon migration and ¯owin the vertical and lateral directions is enormous. A

Fig. 3. (a) Geologic map showing a set of northwest-trending Gilsonite dikes in the Uinta Basin, northeastern Utah, and location of the basin

(inset) within the state of Utah in western USA. (b) Lithologic column within the basin. (c) Pressure distribution from a well at Altamont ®eld.

(a) and (b) are from Verbeek and Grout (1993) and (c) is simpli®ed from Bredehoeft, Wesley and Fouch, 1994.

A. Aydin /Marine and Petroleum Geology 17 (2000) 797±814 799

Page 4: Fractures Faults and Hydrocarbon Entrapment

similar process has recently been proposed indepen-dently for primary hydrocarbon migration and ¯ow inthe Austin Chalk, Texas (Berg & Gangi, 1999, whichwas published during the review process of this manu-script).

2.1.2. Tectonic joint networksMost joint networks in the upper crust form due to

crustal and local tectonic driving forces (Pollard &Aydin, 1988). They commonly occur in a set, a largenumber of subparallel joints in brittle rock units(Fig. 5(a)). Because of the strata-bound nature of thesejoints as shown in Fig. 5(a) and (b) (Helgeson &Aydin, 1991; Gross, Fischer, Engelder & Green®eld,1995), their contribution to vertical hydrocarbon ¯owfrom one mechanical unit to another in a multi-litho-logical medium is rather limited. However, if the jointsmaking up a fracture network have su�cient aperture,length, spacing, connectivity, and distribution, theycan contribute to the permeability of reservoirs andhydrocarbon production. For example, it has beendemonstrated that a set of joints (Fig. 6(a)) observedat surface and detected in a tight sandstone reservoirin the Piceance basin, Colorado, USA (see Fig. 3(a)for location) increases the permeability of detritalreservoir rocks by 10±104 times that of the correspond-ing matrix permeability (see Fig. 6(b) for permeabilitytable) determined from in situ tests and cores, respect-ively (Lorenz et al., 1988).

It is possible to exploit joint sets if the density and

height of the joints are su�ciently large by horizontalwells (Fig. 5(b)) and speci®c production technologysuch as water and CO2 ¯ooding and gravity drainage.The Midale ®eld in Canada (inset, Fig. 6(c)) providesan excellent case for the impact of a fracture set on hy-drocarbon production in a carbonate reservoir (Beli-veau & Payne, 1991; Beliveau, Payne & Mundry,1991). A watercut map (Fig. 6(c)) from this reservoirdemonstrates the impact of a northwest-trending frac-ture system on the permeability anisotropy in the ®eld.In this reservoir, a gravity drainage-based recoverymethod has apparently been successfully utilized. Inthis e�ort, aside from the length and spacing of thefractures, the heights of the fractures are also import-ant.

Joints frequently occur in clusters or zones whichare weaker and, therefore, prone to shearing. Outcropobservations con®rm this conclusion and reveal that asmall magnitude of shear is capable of bridging pre-viously separate fractures within a fracture zonethereby increasing not only the aperture of the shearedfracture and the connectivity between neighboringfractures, but also length and height of e�ective ¯owpathways (Taylor et al., 1999; Myers & Aydin, 2000).Shearing may also o�set thin impermeable shale layersbetween brittle units with fracture zones, again increas-ing the connectivity between fracture zones in di�erentlayers.

Field tests and borehole images from shallow aqui-fers show that ¯uid ¯ow occurs more e�ectively

Fig. 4. (a) Photograph of one of the dikes mined to several tens of meters depth and several kilometers length. (b) An enlargement of the dike

surface showing plumose structure that is evidence for the dilation origin of the original fracture and its southward (towards the viewer) propa-

gation.

A. Aydin /Marine and Petroleum Geology 17 (2000) 797±814800

Page 5: Fractures Faults and Hydrocarbon Entrapment

through joint clusters or joint zones (Paillet et al.,1987; Martel & Peterson, 1991; Long et al., 1996).This important concept appears to be applicable alsoto hydrocarbon reservoirs: The photographs inFig. 7(a) and (b) illustrate a joint zone with hydro-carbon stain in a limestone core from the TempaRossa oil ®eld in the southern Apennines, Italy(Fig. 7(c) and (d)). The photographs show that echelonjoints forming a zone are connected by a through-going and zigzagging fracture (Fig. 7(a)) that is prob-ably related to a minute degree of shearing and hasthe highest degree of hydrocarbon staining (Fig. 7(b)).

Some tectonic joint systems are ®lled by vein ma-terials that re¯ect on the role of fractures in focusing¯uids in the past time. Even though the ®lled fracturesare not as e�ective as un®lled fractures, they may stillhave higher permeability than the surrounding rocks(Lorenz et al., 1988; Tamanyu, 1999). Contrary to a

common misconception about the existence of openfractures at depth, it has been shown that open frac-tures with large apertures occur as deep as 30 km inthe earth (Ague, 1995).

2.1.3. Thermal or desiccation jointsDilatant structures associated with di�erential

volume decrease due to either cooling or drying occurin certain environments. It is straightforward to ident-ify these joint networks by their orientation (perpen-dicular to bedding or lava top and bottom) and theircommonly polygonal pattern. These joints are import-ant in hydrogeology and waste management (Lore,Aydin & Goodson, 2000a), but they may also contrib-ute to storage and ¯ow of oil and gas in a few rarecases in which the reservoir rock happens to be eithervolcanic rock or thermally fractured sedimentary rock.

Fig. 5. (a) Photograph showing strata-bound nature of tectonic joint system in the Wingate sandstone overlying apparently unjointed Chinle

shale and mudstone, Capitol Reef National Park, Utah, USA. (b) Schematic diagram showing how joints con®ned within isolated brittle units,

which can be utilized by an inclined well.

A. Aydin /Marine and Petroleum Geology 17 (2000) 797±814 801

Page 6: Fractures Faults and Hydrocarbon Entrapment

2.2. Contraction/compaction fractures

Localization of contractional strain within tabularzones is here referred to as contractional or compac-tional structure. The predominant displacement discon-tinuity is such that the fracture walls move towardseach other (Fig. 1(b)), which may be characterized asanti-crack (Fletcher & Pollard, 1981). This class ofstructures includes pressure solution surfaces whichusually contain clayey seams that are commonly lesspermeable in the direction normal to the solution sur-face (Nelson, 1985; Peacock, Fisher, Willemse &

Aydin, 1998). However, if they are subjected to high¯uid pressure or shearing in certain environments, theymay contribute to in-plane hydrocarbon ¯ow. Theimpact of these structures on large-scale hydrocarbonmigration is not well known, but they may be import-ant in carbonate source rocks and reservoirs.

Contraction or compaction bands (Olsson, 1999;Mollema & Antonellini, 1996; Cakir & Aydin, 1994)are another common type of structure in porous rocksand may be characterized by a kinematical con®gur-ation similar to that of the pressure solution structuresdescribed earlier. Bands re¯ect a loss of porosity and

Fig. 6. (a) Map showing a joint set in a Mesaverde sandstone outcrop in the Piceance basin. Subsurface data indicate that this NW-trending set

also occurs in the reservoir (Lorenz, Warpinski, Branagan & Sattler, 1988). (b) Permeability of major reservoir units measured from core and

corrected for stress and water saturation versus those determined by in situ tests from Multiwell experiments. Note that the permeability in the

direction of the NW-trending joint system presented in (a) is two to four orders of magnitudes higher than those measured from core. From Lor-

enz et al., 1988. (c) The impact of fractures on watercut map in the Midale ®eld, a carbonate reservoir in Williston Basin, Canada (see inset for

location). Higher permeability direction (NE) determined from this map coincides with the dominant strike of a closely spaced natural fracture

system in the reservoir. From Beliveau and Payne (1991).

A. Aydin /Marine and Petroleum Geology 17 (2000) 797±814802

Page 7: Fractures Faults and Hydrocarbon Entrapment

have lower permeability than that of the host rock.Therefore, they retard hydrocarbon migration and¯ow in the direction perpendicular to the bands. Themagnitude of this retardation depends on the grainsize or pore throat of the band material and theirthickness and density of the grains.

2.3. Shear fractures/faults

Faults are de®ned as structures across which ap-preciable shear displacement discontinuities occur.Fault blocks predominantly move along the plane orzone of the discontinuity (Fig. 1(b)). However, displa-cement discontinuity varies from pure fault plane par-allel orientation to pure fault normal orientation whichresults either in dilation or contraction within the faultzone. Based on the sense of discontinuity, three majortypes of faults are commonly referred to in geology:strike-slip, normal and reverse, each of which hasdi�erent geometric attributes.

Faults have three fundamental elements that impacton hydrocarbon ¯ow: (1) juxtaposition, (2) fault rock,

and (3) the surrounding damage zone (Antonellini &Aydin, 1994; Scholz & Anders, 1994; Caine, Evans &Forster, 1996). Among these, (1) the juxtaposition ofdi�erent horizons and (2) the occurrence of fault rockdistinguish between faults and joints. The threeelements of faults and their hydraulics are brie¯ydescribed later.

2.3.1. Juxtaposition of layers across faultsAllan diagrams (Allan, 1989) are routinely con-

structed to visualize juxtaposition geometry across afault surface in industrial applications. In this process,the most important parameters are fault geometry, slipdistribution, and detailed stratigraphy. However, thenature of slip distribution along faults below the resol-ution of seismic technology and along complex faultzone architectures remain to be problematic in the pro-cess of constructing a complete and accurate juxtaposi-tion diagram. Once a juxtaposition geometry withpossible highest resolution is constructed, hydrocarbon¯ow from one permeable unit to another or fault seal-ing due to juxtaposition of reservoir units against low-

Fig. 7. Photographs showing (a) a fracture zone and (b) hydrocarbon stain on a through-going fracture surface in a limestone core, Tempa

Rossa ®eld, southern Apennines, Italy, which is a reservoir controlled by faults and fractures associated with contractional tectonics of the Italian

peninsula. (c) is a cross section and (d) is the location map.

A. Aydin /Marine and Petroleum Geology 17 (2000) 797±814 803

Page 8: Fractures Faults and Hydrocarbon Entrapment

permeability units may be possible (see, for example,Mathaii & Roberts, 1996; Allan, 1989) provided thatthe a�ect of fault rock is negligible or otherwise quan-ti®able.

2.3.2. Fault rockFault rock forms the core of a fault and is usually

composed of ®ne grain material generated by frictionand wear and has lower porosity and permeabilitythan the parent rock. Fault rock may also be made upof shale, salt, or sheared precipitation materials. Theauthor proposed a process based methodology to beused in dealing with fault rock architecture andhydraulics. Here, he presents a few distinct per-meability models for fault rock and the surroundingdamage zone associated with major faulting processesin common reservoir environments.

Deformation bands or shear bands in porous rockform by localization of shearing and volumetric strainalong tabular zones. Fig. 8(a) illustrates the per-meability structure of an idealized fault zone formedby this mechanism. Although the exact nature of thepermeability of such a structure varies based largelyupon the nature of the parent rock (Caine et al., 1996;Knipe et al., 1998), it is common to determine about atwo to four order of magnitudes reduction in per-meability across zones of deformation bands in poroussandstone (Antonellini & Aydin, 1994; Matthai,

Aydin, Pollard & Roberts, 1998; Antonellini, Aydin &Orr, 1999). The plot in the upper part of Fig. 8(a) rep-resents an idealized case of fault-normal permeability,kf normalized by that of the parent rock (kf ) of closelyspaced deformation bands (fault rock) and a damagezone made up less prominent deformation bands (kd).The fault-normal e�ective permeability of such a blockwith a thickness, T, having a simple fault rock with athickness, t, and damage zone with a total thickness,d, is the harmonic average (Eq. 6 in Antonellini &Aydin, 1994):

kef � kfkdkrT=��kdkr �t� �kfkr �d� �kfkd��Tÿ tÿ d�

�Thus, the density of deformation band faults in a

rock volume of interest and their permeability controlthe e�ective permeability of the volume. These par-ameters are generally related to the lithology of rockand the magnitude of slip across the fault.

These faults would seal hydrocarbons depending onthe capillary pressure characteristics (Smith, 1980;Weber, Mandl, Pilaar, Lehner & Precious, 1978;Knipe et al., 1998), the quanti®cation of which is anon-going challenge. An outcrop photograph (Fig. 9)from the Arroyo Grande oil ®eld in California, USA(Antonellini et al., 1999) shows tar-impregnateddomains in sand and conglomerate bounded by defor-mation band zones and demonstrates the impact of

Fig. 8. Idealized fault architectures and corresponding permeability structures. (a) A deformation-band fault zone with reduced permeability (kf )

in a direction perpendicular to the fault. The degree of permeability reduction depends on the lithology of the rock but on average the reduction

is two to four orders of magnitude with respect to that of the rock matrix. (b) A fault developed by shearing across a joint zone. Fault rock

formed by this process is similar to that of the deformation band process but it is surrounded by a damage zone, more permeable than the parent

rock. (c) A brecciated fault zone ®lled with hydrocarbon. The permeability (kf ) depends on the porosity of the zone and the ratio of the fault

thickness to particle radius.

A. Aydin /Marine and Petroleum Geology 17 (2000) 797±814804

Page 9: Fractures Faults and Hydrocarbon Entrapment

this kind of fault on hydrocarbon ¯ow. An analogcharacterization e�ort and a subsurface case study atthe same locality will be presented later in this paper.The maximum hydrocarbon column height that thistype of faults can support is site speci®c and generallydepends on the clay content of rock (Knipe et al.,1997).

It is also possible to form a fault rock with a similarhydraulic response as the earlier case by a completelydi�erent faulting process: Shearing along joints (Cruik-shank, Zhao & Johnson, 1991; Martel, Pollard & Segall,1988) or joint zones (Myers & Aydin, 2000). Obviouslyfor smaller o�sets, fault rock generation is insigni®cantand sheared joint zones are favorable ¯ow pathways asmentioned earlier with reference to slightly sheared jointzones. However, increasing slip generates a fault rockwith smaller grain size and porosity (Fig. 8(b)) similar tothat of the fault rock formed by the deformation bandmechanism. The reduction in permeability normal to thefault rock produced by the two mechanisms is compar-able. However, faults formed by shearing along joints orjoint zones are always surrounded by a damage zonemade up of joints and slightly sheared joints (Fig. 10).Such a zone and a localized slip surface at the edge ofthe fault rock enhance the fault-normal permeability(kd) (as well as fault-parallel permeability) as shown inan idealized plot in Fig. 8(b). Detailed formulation todetermine e�ective block permeability for this case canbe found in Jourde, Aydin and Durlofsky (in prep-aration).

There is a large volume of work on the quanti®-cation of damage zone geometry and the interpretation

of its variation in terms of the process zone associatedwith the propagation, growth, and widening of thefault width with increasing slip (Walsh & Watterson,1988; Cowie & Scholz, 1992). In general, the width ofthe damage zone and the density of joints therein arerelated to the magnitude of slip across the fault.

Not all dilatant fractures around brittle faults areassociated with fault process zones. In some cases thefractures are proved to be older than the nearby faultsor are localized in a broad fold associated with abroader deformation ®eld. For example, the hydro-carbon-®lled joints (Fig. 11) in the hanging wall of athrust fault (Fig. 11(b) and (c)) are observed in out-crops directly above the Orcutt oil ®eld in central Cali-fornia (Lore et al., 2000b). Here the fault represents acomponent of a high-permeability hydrocarbon path-way that increases the drainage volume by simply con-necting the fracture system in the hanging wall.

The conceptual models for faults produced by defor-mation banding (Fig. 8(a)) and slip along joint zones(Fig. 8(b)) may also represent fault rocks composed ofshale or salt (Weber et al., 1978; Smith, 1980; Mooreand Vrolijk, 1992; Gibson, 1994; Lehner & Pilaar,1997). Thus, fault rock associated with faulting inmulti-layered brittle/ductile systems also produces alower permeability fault core similar to the per-meability structure in Fig. 8(a) and (b), and plays animportant role in hydrocarbon migration and entrap-ment. The distribution of shaly fault rock is closely re-lated to the process of shale smearing and the faultarchitecture resulting from such process (Koledoye,Aydin & May, 2000). It turns out that an increasing

Fig. 9. A tar sand domain bounded by a fault in sandstone and conglomerate of the Arroyo Grande oil ®eld, central California. Steam injection

breakthrough times show a signi®cant anisotropy which is consistent with the fault orientation and permeability. From Antonellini et al. (1999).

A. Aydin /Marine and Petroleum Geology 17 (2000) 797±814 805

Page 10: Fractures Faults and Hydrocarbon Entrapment

Fig. 10. Map showing01 m-wide fault rock at the fault core surrounded on either side by about 2 m damage zone made up of joints and sheared

joints in a sandstone formation in southeastern Nevada. This fault has formed by left-lateral shearing of 14 m across a joint zone whose right

stepping con®guration is still discernable. The fault rock is made up of ®ne grained material and has lower permeability than the undeformed

rock, whereas the damage zones on both sides have higher permeability due to a network of fractures. From Myers and Aydin (2000).

Fig. 11. Hydrocarbon ®lled fractures and faults in siliceous mudstone at the Orcutt oil ®eld, Santa Maria Basin (see Fig. 13 for location), central

California, USA. (a) Photograph showing hydrocarbon (dark) ®lled joints, (b) cross section showing the joint system, and (c) structural location

of (a) and (b) in the hanging wall of a reverse fault also ®lled by hydrocarbon. In this case, the joint system occurs in a broad fault-core anticline.

The section in (c) is from Munger, 1986; Dunham Bromley and Rosata (1991). Modi®ed from Lore, Eichhubl and Aydin (2000b).

A. Aydin /Marine and Petroleum Geology 17 (2000) 797±814806

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deformation may decrease the sealing capacity of sucha system. It is also possible that fault zones withsmeared shale may be accompanied by a relativelyconductive zone as depicted in Fig. 8(b).

2.3.3. Dilation associated with pull-aparts, brecciazones, and slip surfaces

Brittle faulting of rock results in volumetric changewithin the rock body. Dilational volume change takesplace either through the entire deformed rock or loca-lizes along discrete structures such as pull-apart open-ings, slip surfaces, breccia zones, and the associatedjoint system already described. These structures aregenerally conduits for geothermal ¯uids (Aydin & Nur,1982; Aydin, Schultz & Campagna, 1990; Martel &Peterson, 1991; Forster & Evans, 1991; Bruhn, Parry,Yonkee & Thompson, 1994; Sibson, 1968, 1986, 1994;Ohlmacher & Aydin, 1995), and for hydrocarbon(Reilly, Macdonald, Biegert & Brooks, 1996; Dholakia,Aydin, Pollard & Zoback, 1998; Moretti, 1998).

Hydraulic properties of pull-apart openings and slip

surfaces associated with faults are similar to those ofvoids and joints. Breccia zones with hydrocarbon(Fig. 12) have been described by Dholakia et al. (1998)and have been proposed to be e�ective hydrocarbonpathways. Unlike the conceptual models in Fig. 8(a)and (b), the model in Fig. 8(c) is di�cult to quantifybecause of the lack of actual ®eld measurement. Fur-thermore, the hydrodynamics of this type of fault isdi�erent from those of a simple parallel plate model(Fig. 2) in the sense that a typical zone includes bothbreccia of various sizes and shapes and hydrocarbon.However, the permeability (kf ) of such a zone may beapproximated as that of parallel plates with cylindricalor spherical asperities as given by Kumar, Zimmermanand Bodvarsson (1991, Eq. 8):

Kf � kb

�1ÿ tanh

ÿft=2 kp

b

�=ÿft=2 kp

b

���2�

where kb is a parameter representing permeability ofthe breccia without boundaries, f is the porosity, andt is the thickness. Using the Kozeny±Carman modeland assuming spherical particles of the same radius `a'within the fault zone would yield:

kb � f3a2=45�1ÿ f�2: �3�Substituting Eq. (3) into Eq. (2) gives

Kf � f3a2=45�1ÿ f�2�1ÿ tanh�CR�=�CR��: �4�Here, C is a function of porosity:

C � �3:4�ÿ1= fp ÿ f

p �,

R is the ratio of the thickness to the radius of thespherical particles:

R � t=a:

Thus, the permeability of such a system depends onthe porosity within the fault zone and the ratio of thefault thickness to particle size (or a measure of size dis-tribution for a more general case).

2.3.4. Other parameters crucial for fractures/faults andhydrocarbon ¯ow

2.3.4.1. Slip. The impact of slip on fault permeabilityis dependent upon speci®c faulting processes. Largeslip results in thicker and tighter fault zones in theconceptual model shown in Fig. 8(a). Small slipincreases fault-normal and fault-parallel permeabilityin the model shown in Fig. 8(b). Large slip across thefaults that are generated by the same conceptualmodel decreases fault-normal permeability andincreases fault-parallel permeability. For shale smear,large slip increases the distribution of the shaly faultrock along the dip and strike directions of the fault

Fig. 12. Photograph showing hydrocarbon-®lled breccia zone within

a fault zone in the Monterey Formation at Government Point,

coastal central California. Hydrocarbon focusing into faults of var-

ious sizes is ubiquitous in both coastal California and the San Joa-

quin Valley to the east of the San Andreas Fault, which include

major oil producing reservoirs in the region. From Dholakia et al.,

1998.

A. Aydin /Marine and Petroleum Geology 17 (2000) 797±814 807

Page 12: Fractures Faults and Hydrocarbon Entrapment

and provides a potential for a higher hydrocarbon col-umn. However, at some critical slip value, the continu-ity of the shaly fault rock may break down, therebyjeopardizing sealing integrity. The impact of slip mag-nitude on the permeability provided by faulting relateddilational structure is expected to be signi®cant as thesize of the opening is proportional to the slip (Aydin& Nur, 1982).

2.3.4.2. Cementation. Open space is prone to become®lled, if not with hydrocarbon, then with cements andmineral deposits. Once ®lled by cements, these struc-tures are less e�ective conduits. However, as noted ear-lier for dilatant fractures, a precipitant-®lled fault zonemore often has a higher pore volume (Eichhubl, 1997;Eichhubl & Beal, 1998) and perhaps a higher per-meability.

2.3.4.3. Present stress state. High fracture/fault-normalstress reduces aperture, thus inhibiting ¯uid ¯ow, andhigh fracture-parallel compressive stress increases theability of fractures/faults to stay open and transporthydrocarbon (Teuful & Lorenz, 1996). High ¯uidpressure is capable of opening fractures/faults at anydepth and thus facilitating vertical and lateral ¯ow(Pollard & Aydin, 1988; Engelder & Lacazette, 1990).Shearing, which is promoted by high di�erential stress,high ¯uid pressure, or low e�ective fault-normal stress,focuses ¯uid ¯ow (Aydin et al., 1990; Barton et al.,1995; Zoback & Moos, 1995; Finkbeiner, Barton &

Zoback, 1997; Dholakia et al., 1998). In fact, shearingis the most e�ective way to produce dilation at anydepth in the Earth's crust.

2.3.4.4. Time. Reservoirs are dynamic systems thatevolve through their geological history as well as theirproduction history (Long et al., 1996). Fractures/faultsare natural components of this ever-changing system.Earthquake-related ¯uid discharge is a good evidencefor the cyclic nature of fault zone hydraulics (Sibson,1968). The parameters such as time and ¯uid ¯ow dis-tance can also be evaluated for fault-related ancient¯uid ¯ow (Eichhubl, 1997).

3. Methodology for modeling hydrocarbon migrationand ¯ow: a case study

Perhaps one of the best case studies for fault-relatedhydrocarbon, entrapment, migration, and ¯ow is thatof the Monterey-type basins in California (Finkbeineret al., 1997; Eichhubl, 1997; Dholakia et al., 1998).Therefore, the Monterey basins in California are hereused to demonstrate construction of a conceptualmodel for hydrocarbon entrapment, migration, and¯ow in a particular environment.

Several sedimentary basins of Tertiary age existalong the San Andreas transform boundary through-out California (Pisciotto & Garison, 1981), the largestof which are the Los Angeles, Ventura, Santa Maria,San Joaquin, and Santa Cruz basins (Fig. 13). The so-called Monterey-type basins also occur along the Cir-cum Paci®c margins (Ingle, 1981) and contain huge hy-drocarbon and natural gas reserves. These basinsrepresent a unique depositional and diagenetic environ-ment (Graham & Williams, 1985) and have similarrocks composed of diatomaceous and phosphatic shaleand minor amounts of chert and dolostone (Fig. 14).The basins are usually ®lled with young detrital con-glomerates and sandstones deposits. The basinal rockscan be classi®ed into two major petrophysical groupsin terms of their permeability: The Monterey For-mation and related rocks have a low matrix per-meability (usually under one md, see for example,Johnston & Wachi, 1994), and the turbidite sand pock-ets and younger detrital units generally have a highmatrix permeability (Antonellini et al., 1999).

A general model of hydrocarbon maturation, mi-gration, and storage (Fig. 15(a)) depicts a system witha kitchen, conduit bed and a traditional reservoir inthe form of an anticlinal structural high (Dickinson,1976). We here propose a speci®c model for hydro-carbon migration and ¯ow within the Monterey-typebasins and other basins with similar petrophysicalunits and deformation process history. This model, il-lustrated schematically in Fig. 15(b), is primarily based

Fig. 13. Location map for major Tertiary basins of the Monterey

type along the San Andreas transform boundary, CA, USA.

A. Aydin /Marine and Petroleum Geology 17 (2000) 797±814808

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on sound structural principles established by both out-crop analogs and subsurface data from various reser-voirs in California. In this model, the organic richcarbonaceous shale is, of course, the source rock (Figs.13±15). The access pressure associated with transform-ation of the organic matter into oil and cracking andtranspressional tectonic deformation of extremely lowpermeable Oligocene±upper Miocene rocks are envi-sioned to produce a series of structures ranging fromhydrofractures, or pressurized and brecciated faultzones in orientations parallel and at high angle to bed-ding (Dholakia et al., 1998). The faulting process wasenvisioned to be cyclic (Eichhubl, 1997) similar toearthquake-related periodicity proposed for crustalscale faults (Sibson, 1968).

Brecciated fault zones and dilatant fractures aremajor pathways for hydrocarbon migration from thesource rock through the low permeability petrophysicalunits in the stratigraphic column. Interconnected faultsand fracture networks de®ne the drainage area andthus producability from a reservoir. The dilatant frac-tures are either hydrofractures or pressurized fracturezones with a small amount of slippage. As noted ear-lier, a small magnitude of shear is capable of increas-ing dimensions of fracture-fault systems and e�ciencyof ¯ow pathways. Brecciated fault zones both at highangle and parallel to bedding are by far the most e�ec-tive hydrocarbon conduits. Larger slip would normally

provide wider breccia zones. Two lines of evidencesuggest that the brecciated fault zones in the Montereybasins are propped open by high pressure: (1) severalcontractional (reverse) faults include tar and brecciazones in which fragments of siliceous rocks are totallydisconnected in a hydrocarbon matrix, and (2) bed-parallel breccia zones with hydrocarbon are preservedwithin the zone. The second case indicates that ¯uidpressure within the faults was high enough to lift upthe overburden.

There are three factors that play important roles inthis scenario: low permeability of the rock matrix; pet-roleum maturation, and diagenesis of the siliceousrocks. The ®rst is a typical characteristic of the silic-eous rocks of the Monterey Formation and is requiredto retain the pressure and hydrocarbon within faultsand fracture zones. The second may generate a volumeincrease within the source, usually organic rich shale,and nearby units in an incremental fashion. Thevolume change associated with petroleum maturationand cracking was brie¯y discussed earlier. Here itshould be noted that the permeability of a fracture (orfault) increases with increasing ¯uid pressure (Walsh,1981; Engelder & Scholz, 1981). The third is relevantto the formation of layer-parallel anisotropy, and, con-sequently, to the formation of layer-parallel brecciazones from highly brittle porcelanite (Dholakia et al.,1998), which is the end product of the diagenesis of

Fig. 14. Geologic columns (from right to left, Santa Maria basin, Salinas basin (Labarere well, Section 21, 23-10), and San Joaquin basins Har-

vester well, Section 25, 23-21) showing lithostratigraphic and petrophysical units within the Monterey type basins. Lower units are composed of

mostly diatomaceous, siliceous, and phosphatic shales with low permeability, whereas the younger deposits of sand and conglomerate have high

permeability. These two petrophysical units demonstrate contrasting styles of faulting and the permeability structure resulting from it. Lithologic

data from Dunham et al. (1991) and Graham, Seedorf, Walter and Bloch (1991).

A. Aydin /Marine and Petroleum Geology 17 (2000) 797±814 809

Page 14: Fractures Faults and Hydrocarbon Entrapment

the siliceous shale (Graham & Williams, 1985; Eich-hubl & Beal, 1998).

It is interesting to note that the hydraulic propertiesof faults within the same basin are controlled by thedi�erent rheology of the rocks in which the faultsoccur. The low-porosity and low-permeability rocksare deformed by brittle processes that produce per-meable fault and fracture zones. In contrast to thesedilatant and permeable faults, the faults in high-poros-ity and high-permeability rocks are contraction faultswith lower permeability than that of the matrix rock,and they form barriers to hydrocarbon ¯ow as indi-cated by outcrop study and core examples from theArroyo Grande sandstone reservoir in central Califor-nia (Antonellini et al., 1999). Steam injection break-through time from this ®eld demonstrates a nine-to-one lateral permeability anisotropy, the largest per-meability component being parallel to the strike of thefaults and the smallest being perpendicular to thefaults. These faults compartmentalize reservoirs, whichresult in a lack of communication between fault blockswith di�erent pressures.

4. Conclusions and discussion

Three major types of structural discontinuities andtheir geometric and hydraulic characteristics are con-sidered: dilatant, compaction/contraction, and shear.Important di�erences exist among the geometry anddistribution of these three types. Flow properties alsodi�er from one fundamental type to another and, for agiven structure type, from one lithologic and geome-chanical environment to another.

Joint networks are limited within brittle units butcan be e�ective ¯ow pathways for hydrocarbon pro-duction from these units. Horizontal drilling andspecial production designs such as gravity drainageand various injection methods may help to exploit thefractured reservoirs with joint networks.

The most e�ective elements in a joint network arejoint zones, which are prone to a small magnitude ofshearing. This, in turn, increases the aperture, verticaland lateral connectivity, and produces longer and tallerconduits for hydrocarbon ¯ow.

Hydrofractures and faults can be conduits that fa-

Fig. 15. (a) A general conceptual model of a hydrocarbon system showing source, kitchen, migration conduit, and reservoir with top seal (From

Dickinson, 1976, courtesy of Stephan A. Graham). (b) A structurally based conceptual model for the Monterey type basins showing permeable

faults and vertical hydrofractures as conduits in a low permeability petrophysical unit. Breccia zones with hydrocarbon also occur parallel to bed-

ding, which re¯ects the role of high ¯uid pressure in the process of brecciation and hydrocarbon invasion of the breccia zone. In top portion of

the basin, the petrophysical unit is of high permeability, but the faults are of low permeability, thereby behaving as barriers for hydrocarbon

¯ow. Faults of this type provide lateral seal for hydrocarbons and compartmentalize reservoirs.

A. Aydin /Marine and Petroleum Geology 17 (2000) 797±814810

Page 15: Fractures Faults and Hydrocarbon Entrapment

cilitate primary hydrocarbon migration. High ¯uidpressure, which is a prominent characteristic of hydro-carbon source rocks, and shearing are capable of pro-ducing open fractures and dilatant faults at all depthsof interest to hydrocarbon exploration and production.

Faults have complex architectures that may enhanceand/or impede hydrocarbon migration and ¯owdepending on the speci®c process of faulting and itsimpact on the characteristics of the three fundamentalelements of the fault zone architecture: juxtaposition,localized dilation, and fault rock. Other parameters,such as slip magnitudes, cementation, stress state, andtime, are also crucial for evaluating the e�ciency offault ¯ow and fault seal systems. A cursory surveysuggests that, at least, four categories of fault beha-viors exist. These are:

1. transmitting faults,2. sealing faults,3. vertically transmitting and laterally sealing faults,

and4. faults sealing or transmitting intermittently.

These are rather end-members. A systematic studyof the permeability structure of faults and fractures invarious geomechanical and depositional environmentsis required for constructing conceptual models, whichin turn can be tested in the ®eld as well as in simu-lation models. The methodology for doing this is nowestablished. It is now possible to conceptualize the sep-arate pieces of information about fractures and faultsand their hydraulic properties and to construct amodel for hydrocarbon migration, ¯ow and entrap-ment within a particular environment. In this respect,the example from the Monterey type basins in Califor-nia is encouraging and is in striking contrast with sedi-mentologic models for hydrocarbon migration andengineering models of fracture/fault networks com-monly used in reservoir simulation (see for example,Warren & Root, 1963). Hydrocarbon migration withina basin is largely a physical phenomenon and fracturesand faults are dominant components of it. Basinmodels without this physical perspective and its domi-nant elements are bound to be incomplete.

Acknowledgements

Research leading to this presentation has bene®ttedfrom many of my colleagues and former and presentgraduate students, a partial list of which includesMarco Antonellini, Charlie Brankman, DavidCampagna, James DeGra�, Sneha Dholakia, YijunDu, Peter Eichhubl, Radu Girbacea, Daniel Helgeson,Judson Jacobs, Simon Kattenhorn, Bashir Koledoye,Jason Lore, Stephan Matthai, Pauline Mollema,Rodrick Myers, Gregory Ohlmacher, David Pollard,

Thomas Roznovsky, Richard Schultz, Lans Taylor,Amgad Younes, and Manuel Willemse. I thankStephan Graham, Fikri Kuchuk, Herve Jourde, andRobert Zimmerman for their help in various stages ofthis manuscript. Financial support from the RockFracture Project (Agip, Anadarko, Aramco, Arco,BPAmoco, Chevron, Conoco, Elf, JNOC, Marathon,Mobil, Norsk Hydro, Phillips, Repsol/YPF/Maxus,Shell, Texaco, Total/Fina, and Western Atlas/BakerHughes) and Shale Smear Project (Chevron, Conoco,Elf, JNOC, and Marathon) at Stanford University,and from the US Department of Energy, Basic EnergySciences, Grant No. DE-FG03-94ER14462 is alsoacknowledged. I thank Victoria Doyle-Jones for hereditorial help.

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