PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Twenty Third Annual Convention, October 1994

HIGH QUALITY VOLCANICLASTIC SANDSTONE RESERVOlRS IN EAST JAVA, INDONESIA

Peter Willumsen* David M. Schiller* *

ABSTRACT

Volcaniclastics are generally regarded as very poor reservoirs by most explorationists, however, recent studies suggest that the reservoir potential of these rocks within Indonesia and elsewhere may be underrated.

The paper describes the reservoir quality of selected Plio-Pleistocene Indonesian volcaniclastic sandstones and compares these with examples of volcaniclastic reservoirs and oillgas fields productive from volcaniclastic horizons worldwide.

The studied rocks are from the Porong-1 and WD-8 wells in East Java as well as various East Java outcrops and were deposited in a wide variety of depositional settings from non-marine (lacustrine, fluvial) to transitional marine (deltaic) and deep marine (turbidite). The deposits consist of mainly sand and gravel sized volcanic sediments that have been extensively reworked by marine or fluviatile sedimentary processes. The sandstones are composed of predominantly plagioclase feldspar and andesitic rock fragments with subordinate heavy minerals and bioclasts. Quartz is almost completely absent and clays are dominated by sodium smectite.

The volcaniclastics have good to excellent reservoir properties, with total porosity greater than 30% at shallow depths, decreasing to approximately 20% at a depth of 7,500 feet. Permeability ranges between 3-550md in the Porong-1 well, averaging lOOmd at shallow depths and decreasing to 20md at 5,000 feet. Outcrop samples have generally higher permeability, in some cases up to 20,000md.

The main macroporosity types are primary intergranular porosity and secondary porosity created by leaching of feldspars, volcanic rock fragments and heavy minerals. Porosity shows a distinct change with depth and age. At depths shallower than 4,500 feet within Pleistocene horizons, intergranular porosity is greater than secondary. Deeper than 4,500 feet within Late Pliocene horizons, secondary porosity is greater than primary intergranular porosity. Brine production of up to 20,000 barrels of water per day from volcaniclastics in the WD-8 area illustrates the potential of these horizons as high quality reservoirs.

Volcaniclastics of Tertiary and Quaternary age are very common throughout Indonesia, however, very few have yielded commercial hydrocarbon discoveries and volcaniclastics are generally regarded as very poor reservoirs by most explorationists. Recent studies of Pliocene and Pleistocene volcaniclastics from the Porong-1 and WD-8 wells and various outcrops within onshore East Java, show that volcaniclastic sandstones can provide excellent quality reservoirs. The results suggest that the negative aspects of volcaniclastic reservoirs may have been overemphasized in the past, and that Indonesia and other similar regions may contain considerable overlooked potential within these horizons. This conclusion is supported by worldwide examples of volcaniclastic reservoirs and oillgas fields productive from volcaniclastic intervals.

WHAT ARE VOLCANICLASTICS ?: A BRIEF REVIEW

Definition

* Iluffco Brantas, Inc. * * Crawford Consultants, Inc.

Volcaniclastics can be defined as "any clastic material composed in part or entirely of volcanic fragments,"

IPA, 2006 - 23rd Annual Convention Proceedings, 1994

without regard to its rock forming mechanism, mode of transport, environment of deposition or admixture of non-volcanic fragments (Fisher, 196 1). This non-genetic definition is very broad and encompasses a wide variety of rocks, including the instantaneous deposits of volcanic eruptions (agglomerates, ashfalls, pyroclastic flows etc.) that have poor reservoir potential, as well as porous sandstones generated through long term weathering and transport.

Volcaniclastic Sediments and Facies

Three major classes of volcaniclastic sediments are recognized, based on the process through which they are created: pyroclastic (formed from rapidly expanding magma); hydroclastic (formed by magma-water interactions); and epiclastic (lithic clasts and crystals derived from "pre-existing rock" by weathering and erosion) (from Fisher and Smith, 1991). Epiclastics usually offer the best reservoir potential.

Volcaniclastics can be divided into five general facies based on the relative distance from the volcanic source ( Figure 1). Of these generally only the distal non-marine facies and distal deltaic/marine facies have acceptable reservoir quality. The sorting and reservoir quality of volcaniclastics generally increases as the particles are transported farther away from the source, thus distal volcaniclastic facies have far greater reservoir potential than the more poorly sorted proximal and medial facies. For the oil-finder, the volcaniclastics of greatest interest should be epiclastics that have been extensively reworked and deposited in fluvial to marine distal settings. A notable exception are pyroclastic ashfalls or tuffs which in some cases can form well sorted and highly porous deposits (Mathisen and McPherson, 1991 ).

DATABASE AND METHODS

The present study is based on data from the Porong-1 exploration we11 and the WD-8 brine production well, both drilled in 1993, in addition to data from age equivalent outcrops in the ailticlinorium known as the Kendeng Zone (van Bemmelen, 1949) and at Bangil in East Java, Indonesia (Figure 2).

Well And Outcrop Data

The Porong-1 well was drilled to evaluate an Early

Miocene reef objective at 8,481 feet, with a secondary objective of evaluating the reservoir potential of overlying Plio-Pleistocene volcaniclastic intervals. Analysis of more than 100 sidewall cores cut between the depths of 800 and 8,478 feet were incorporated into this Study together with standard petrophysical and sample data.

The WD-8 well was drilled to a total depth of 555 meters by P.T. Kimia Farma at Watu Dakon on the Mojokerto Anticline (Figure 2) for production of iodide-rich formation water. A typical water well evaluation program was performed, with the exception of 24 m of conventional core.

Outcrop data includes field samples and measured sections collected from a number of East Java localities including the Pondok Dam site near Ngawi, the Ploso Anticline and the Bangil Anticline (Figure

2).

Analyses

Selected well and outcrop samples were subjected to an extensive series of analyses, including the following:

Conventional core: core gamma ray, profile permeametry, sedimentological description and core photography.

Conventional core plugs and sidewall cores: micropaleontology (foraminifera and nannofossils), poroperm and density measurements, thin sectioning (with vacuum-impregnated blue-epoxy), petrography, infrared spectroscopy ("Mineralog"), X-ray diffraction, scanning electron microscopy and "a", "m" and "n" exponent determination.

Outcrop samples: micropaleontology (foraminifera and nannofossils), poroperm and density measurements, thin sectioning (with vacuum-impregnated blue-epoxy), petrography and X-ray diffraction.

STRATIGRAPHY

Age and Depositional Scquence

The studied horizons range from Late Pliocene to Pleistocene in age and were deposited in a wide variety of depositional settings. The sections display

a regressive sequence ranging from outer to middle neritic conditions during the Late Pliocene (up to 250m water depth) to non-marine sedimentation during the Late Pleistocene ( Figure 3).

The Pliocene volcanic sediments may have been sourced from erosion of more distal Oligo-Miocene volcanic centers (van Bemmelen's "Old Andesite Formation") as major Pliocene volcanism has yet to be conclusively documented within the region (van Bemmelen, 1949; Soeria-Atmadja et al, 1988).

During the Pleistocene the region was transformed from a deep marine basin to the present day setting of East Java. The high sedimentation rates, sometimes up to 1,000 mlmillion years, appear associated with the onset of massive, mostly andesitic, Pleistocene volcanism along the still active volcanic arc located less than 100 km south of the study area (Figure 2) (van Bemmelen, 1949; Widjonarko, 1990).

The volcaniclastic rocks in the Porong-1 and WD-8 wells and in outcrop are penecontemporaneous, occupying a time span from Late Pliocene (N19121 [el) to Recent (Figure 3), and are thus younger than approximately 3.0 Ma and largely equivalent to the last Ice Age.

Formations

The outer to middle neritic Late Pliocene volcaniclastic horizons are partly equivalent to the upper part of what is traditionally called the Upper Kalibeng Formation in Kendeng Zone outcrops. Siliciclastic sand-rich horizons of this age have been referred to as the "Sonde Member" (Marks, 1957; de Genevraye and Samuel, 1972) (Figure 3).

The middle to inner neritic to transitional marine beds appear correlative with the Pleistocene Pucangan Formation in outcrop, whereas the non-marine sediments (including vertebrate fossils of Homo erectus, "Java Man") belong to the Pleistocene Kabuh Formation (van Bemmelen, 1949; Marks, 1957; de Genevraye and Samuel, 1972). The Kabuh and Pucangan Formations have often been described as discrete time-stratigraphic units, however the present regional data suggests that these formations represent partly penecontemporaneous and time transgressive facies belts that become progressively younger in an easterly direction.

DEPOSITIONAL FA CIES

The majority of porous volcaniclastics described in this study are moderately to well sorted sandstones and conglomerates composed of volcanic epiclastic sediments that have been extensively reworked by marine and fluviatile processes and deposited in a distal facies. No proximal or medial volcaniclastic facies are represented other than subordinate lahars, debris flows and pyroclastic ashfall tuffs.

The Late Pliocene volcaniclastic sandstones in the Porong-1 well are middle to outer neritic (Figure 3). Apart from some massive sands thicker than 100 feet below a depth of 6,500 feet, individual sandstone units range in thickness from 10-30 feet. The Pleistocene volcaniclastics are inner neritic to transitional marine with individual sandstone units ranging from 10-100 feet thick, with the majority averaging 10-20 feet.

The Late Pliocene volcaniclastic sandstone in the conventional core of the WD-8 well is interpreted as a series of stacked, high density, outer neritic turbidite deposits. The core displays many classic sandy turbidite features (partial and complete Bouma sequences, fluidization structures, scoured basal contacts and interbeds of planktonic foram-rich hemipelagic mud) and is composed of mostly B and C turbidite facies (Mutti and Ricci Lucchi, 1972). Individual turbidite flows range from 0.2-2.0 meters in thickness and some stacked turbidite sand units are up to 5 meters thick.

The outcrop locations used in this study are indicated on Figure 2, which also shows the position of the well known Solo River section described by van Gorsel and Troelstra (1981), and van Gorsel et. al. (1989). The sections represent depositional facies ranging from non-marine to outer neritic (Figure 3). Non- Marine fluviatile to fluvial/lacustrine environments, with largely cross-bedded sandstones up to 30 m thick, are seen at Bangil, the Pondok Dam Site and at Ploso. Transitional marine facies, including distributary channel, tidal channel, delta mouth bar and shallow marine bar sandstones, occur in the Pleistocene section of the Ploso outcrops. Reservoir sandstones up to 20 m thick are present in this facies. Marine facies, including shales, marls and argillaceous volcaniclastic sandstones, occur in the Pliocene part of the Ploso outcrop.

Photomicrographs of some of these reservoirs are shown in Figure 6.

LITHOLOGY

Most of the reservoir sandstones are variably fine to very coarse grained, moderately to well sorted, feldspathic litharenites and lithic arkoses. Some porous conglomeratic sandstones and conglomerates are also present. Framework grains are predominantly plagioclase feldspar (35-45% of framework grains; mostly andesine) and volcanic rock fragments (35- 45% of framework grains; mostly andesitic)' with fewer heavy minerals (pyroxene, amphibole, opaques and biotite) and occasional bioclasts and plant fragments. Quartz is conspicuous by its almost total absence (< I%), indicating a basic composition for the original volcanic source. Smectite occurs as detrital, pore-filling and authigenic pore-fillinglgrain-replacing clay ( 8-40%, by XRD).

Pomng-1 and WD-8 Lithology

The framework grain mineralogy of the Porong-1 and WD-8 volcaniclastic sandstones are almost identical, however, altered grains, bioclasts and plant fragments are more abundant in the WD-8 core ( up to 5% by point count). Heavy minerals are also more abundant in the Porong-1 sandstones (5-12% by point count) than the WD-8 sandstones (1-3% by point count). Silicified volcanic rock fragments are also much more common in the WD-8 samples. For comparison, a present day sand sample recovered from the banks of the Porong River consists of 55% plagioclase, 36% volcanic rock fragments, 7% pyroxene and 2% opaques.

Outcmp Lithology

The Pleistocene volcaniclastic sediments from the field localities are in most cases texturally and compositionally identical to those observed within the Porong-1 well. Bioclasts are relatively common in the Ploso Anticline Pucangan Formation sandstones, but are virtually absent in all Kabuh Formation samples and the Pondok Dam site area Pucangan Formation samples.

Pleistocene sandstones from the Bangil Anticline are friable, variably medium to very coarse grained, moderately well sorted and locally pebbly. The

framework grains are mostly volcanic rock fragments (50-75%) and plagioclase (10-25%) with minor hornblende, augite and opaque minerals.

RESERVOIR QUALITY

Based on structural restoration from seismic data, it has been determined that the Mojokerto Anticline was subjected to a Pleistocene structural uplift of approximately 3,000 feet. This figure has been verified by vitrinite reflectance measurements (Ro= *0.4%), which indicate that the WD-8 core horizon was once buried to a depth of approximately 4,500 feet. Under this assumption, the Late Pliocene section of both the WD-8 and Porong-1 wells were once buried to nearly identical maximum depths.

Pomng-1 Reservoir Quality

Porous volcaniclastic sandstone facies were encountered from surface down to depths of 7,500 feet in the Porong-1 well. Porosity development is generally fair to good in most samples with helium porosity values of 20-35% and thin section point count porosities of 5-25%. There is a distinct decrease in porosity with depth in the Porong-1 well (Figure 4). Helium porosity is around 35% at a depth of 800 feet and decreases to around 20% at 7,500 feet. Point count porosity (i.e. macroporosity) shows a similar trend, but is generally 5-10% lower than the helium porosity (i.e. macro- and microporosity). The main porosity types are primary intergranular porosity, and secondary dissolutionlintragranularporosity created by leaching of feldspars, volcanic rock fragments and to a lesser extent heavy minerals. The ratio of primary to secondary porosity has a very distinct distribution with depth and age. Primary intergranular porosity dominates between surface and a depth of approximately 4,045 feet throughout the Pleistocene section, whereas secondary/dissolution porosity dominates in the deeper Late Pliocene section (Figure

5).

Air permeabilities measured from sidewall cores recovered between 800 to 7,600 feet range from 3-550md, averaging lOOmd (Figure 4). Between 800 and 5,000 feet there is a trend of decreasing permeability with depth, averaging approximately lOOmd at the top and 20md at the bottom. Data is too sparse below 5,000 feet to establish a trend.

WD-8 Reservoir Quality dissolution porosity,

Porosity development in the WD-8 core is generally good in most samples with helium porosity values of 35-40% and thin section point count porosities of 10-25% (Figure 4). The core was recovered from a depth of 450m (1,500 feet). This horizon is Late Pliocene in age, and the porosity is dominantly secondary and created by dissolution of feldspars and volcanic rock fragments. Most primary intergranular porosity had been destroyed by a combination of compaction and cementation.

Air permeability measurements on core plugs from the WD-8 core indicate an obvious small scale facies dependency. The best reservoir facies consist of medium to coarse grained, moderately sorted turbidite sandstone beds with low detrital clay, porosity greater than 20% and air permeability between 80-200md. In fine to medium grained, moderately to poorly sorted turbidites containing significantly more detrital clay, porosities range from 12-25% and permeabilities from 0.2-1 0md (Figure 4).

An approximately 1OOm thick section within the interval from 418 to 544 m was screened off and produced a natural flow of 3,000 barrels of water per day. Two other recent iodide wells in the area, completed in the same reservoir zone, flowed 7,000 and 20,000 barrels of water per day, respectively. This production data is further proof of the excellent reservoir quality of these volcaniclastic reservoirs.

Outcrop Reselvoir Quality

The outcrop samples of reservoir quality volcaniclastic facies have porosity and permeability development nearly identical to those observed in the subsurface Pleistocene samples. The reservoir properties in some cases have been exaggerated due to surface weathering and sample disaggregation.

a. Ploso Anticline Tmsitional Marine Facies

Shallow marine to transitional marine facies of the Pleistocene Pucangan Formation have helium injection porosities that range from 32-43% (38% average) and thin section porosities from 12-35% (28% average). Permeabilities range from 100-1,00Omd, averaging 500 md. The main porosity types are primary intergranular porosity and to a lesser extent secondary

b. Pondok Dam Site Area and Ploso Anticline Non-Mluine Facies

Fluviatile non-marine facies of both the Kabuh and Pucangan Formations have slightly better porosity and permeability developnlent than the marine facies. Helium injection porosities range from 35-46% (41% average) and thin section porosities from 18-37% (3 1% average). Permeabilities range from 85-4,00Omd, averaging 2,500md.

c. Bangil Anticline Lacustrine/Fluviatile Facies

Pleistocene volcaniclastic sandstones analyzed from the Bangil Anticline have helium porosity from 34- 46% and thin section porosity from 27-40%. The best reservoirs have permeabilities of up to 20,000 md.

Log Evaluation

The Porong-l well was evaluated with a standard suite of oil field wireline logs. Porosity calculated from the sonic and neutronldensity logs tends to be similar to helium-injection porosity, and thus on average 5-10% larger than point count porosity ( i.e. macroporosity).

The resistivity of volcaniclastic sands is generally low even in the presence of hydrocarbons due to the effect of water bound within the smectite clay coating most grains, and thus renders reservoir identification and evaluation difficult ( Mathiesen, 1984; Itoh et al, 1982). The exponents a, m and n used in water saturation calculations were measured on WD-8 core material to be 1.0, 1.75 and 1.98, respectively. Formation factor calculations based on the measured exponents corresponds to the Humble formula used for quartz sands with porosity greater than 20%. For porosities less than 20%, the standard exponents will cause an erroneously high calculated water saturation. For tighter volcaniclastics it is thus important to apply the proper exponents in order not to overlook potential hydrocarbon-bearing zones.

The difference in density between volcaniclastics and quartz sandstones affects log evaluation. With the exception of a few denser siderite-cemented samples, the matrix density of the volcaniclastics in the studied material ranges between 2.45 and 2.70 g/cm3, with an

average from 2.60 to 2.62 g/cm3. This is slightly lower than the matrix density of a quartz sandstone (2.65-2.70 g/cm3). The reduced density is believed mainly due to the abundance of sodic plagioclase (2.61-2.65 g/cm3; 2.62 g/cm3 average) and smectite clay (2.00-3.00 g/cm3; 2.35 g/cm3 average) (Gearhart- Owen, 1978).

DIAGENESIS

Volcaniclastic rocks typically consist of chemically unstable minerals and framework grains that are very prone to diagenetic alteration, and these reactions often lead to the precipitation of large amounts of pore-filling cements such as calcite, clays and zeolites (Surdam and Boles, 1979; Davies et al, 1979; Galloway et al, 1979). The abundance of mechanically unstable framework grains such as volcanic rock fragments can also greatly increase the amount of porosity destruction from compaction. Some porosity reduction from cementation and compaction is evident in the volcaniclastics examined in this study, however, the cementation is in most cases relatively minor or localized, and reservoir quality has been enhanced through dissolution of unstable framework grains.

The generalized diagenetic sequence consists of localized early ferroan-calcite cementation followed in turn by the formation of smectite clay, zeolite cement and later stage grain dissolution.

Cementation

Fenoan-calcite was found to have only a localized distribution within the East Java volcaniclastic sand- stones. Field and well evidence show that calcite cementation is more common in the marine to tran- sitional marine facies than in the non-marine facies. Heavily -calcite cemented beds were generally encountered within or in proximity to bioclast-rich or calcareous beds acting as seed crystals, particularly in association with shell rich lags at the base of deltaic or tidal channel sands (Figure 6E).

The most common pore-filling cement is authigenic smectite which occurs as a mostly grain-coating and pore-lining "webby" clay (Figure 6F). The smectite appears to have formed early at shallow burial depths, prior to major compaction. XRD and SEM analyses indicate the clay variety is highly expandable sodium

smectite. Smectite comprises from 10-35% of most samples (by XRDIMineralog), but thin section and SEM analyses indicate that approximately 30 to 50% of this is either detrital smectite clay or smectite associated with partly clay-replaced volcanic rock fragments.

Clinoptilolite is a zeolite mineral that occurs in some samples as small euhedral crystals in pore-filling and volcanic rock fragment-replacing habit (Figure 6F). The amount of clinoptilolite is extremely variable (0- 30%) but most samples contain only small amounts of this cement.

Small amounts (generally < 4%) of plagioclase, pyrite, hematite, kaolinite, chlorite and siderite cements were also detected in a limited number of samples.

Compaction

As expected compaction increases with depth and is evident as squashing of grains, tighter grain-packing and an increase in straight to concave/convex grain contacts as opposed to point contacts. Compaction is largely responsible for loss of primary intergranular porosity with depth (Figure 5), particularly in the Pliocene section, while cementing plays a secondary role.

Secondary Porosity Development

Major secondary porosity has been developed in most of the volcaniclastics by dissolution of unstable framework grains such as plagioclase feldspar, volcanic rock fragments and occasional heavy minerals. Secondary porosity represents approximately a third of the macroporosity within the Pleistocene horizons and nearly all the porosity in the Late Pliocene horizons.

In the Porong-1 well, secondary porosity is found as shallow as 850 feet in PleistoceneIRecent sediments, indicating that this process can occur very early. Diagenetic textures show that most of the grain dissolution post-dates the precipitation of authigenic smectite and other cements. Secondary porosity generally increases with depth as primary porosity decreases from compaction, and virtually all porosity below 4,500 feet within the Late Pliocene sandstones is secondary (Figure 5). The deepest volcaniclastic samples at 7,500 feet still display porosities of 10%

and it is unknown how deep secondary porosity can be formed and preserved in these sequences, but the process could potentially continue as long as unstable minerals are present.

Secondary porosity is the dominant porosity type within the Late Pliocene sandstones of the WD-8 core. This is consistent with the estimate that the sequence was once buried to depths similar to those of the Porong-1 Late Pliocene sequence and exposed to similar diagenetic conditions.

Controls on Secondary Porosity

It is unclear at this time as to whether the porosity difference between the Pleistocene and Late Pliocene h o r i z o n s i s m a i n l y a f u n c t i o n o f depth/temperaturelcompaction relationships, or is alternately controlled by differences in the grain composition and diagenetic history of these horizons. The evidence thus far, however, suggests the following diagenetic relationships.

a. Eady Venus Late Secondary Porosity

The predominance of secondary porosity within the Porong-1 sequence appears to occur very abruptly at approximately 4,067 feet, close to the Plio- Pleistocene boundary (Figure 5). The abruptness could arguably be an artifact of the lower sampling density below this depth. Diagenetic fabrics seen in thin section, however, suggest that the style and timing of secondary porosity evolution may be different for the Pleistocene and Late Pliocene horizons. Secondary porosity evolution within the Pleistocene samples appears to be very early, prior to major compaction, while virtually all the secondary porosity within the Late Pliocene samples appears to be a late stage, post-major compaction phenomenon.

The style of grain-leaching also appears different. In thc Pleistocene samples, plagioclase dissolution is less complete with development of mainly elongate intragranular pores along cleavage planes (Figure 6A). Dissolution is more advanced within the Late Pliocene samples as the central portion of the plagioclase grains usually is completely leached out, leaving a thin rim of plagioclase surrounding a large grain-mouldic pore (Figurcs 6B and 6C) .

b. Grain-Dissolving Mechanism

The grainlpore-fluid interactions that created the secondary porosity are not fully understood. Plagioclase and volcanic rock fragments can be dissolved through a variety of reactions, including an increase in fornlation water pH resulting from early hydration reactions. The rate of plagioclase dissolution tends to increase with increasing salinity and pH, however, the dissolved ions must be removed in order to prevent their precipitation as cement (Surdam and Boles, 1979; Mathisen, 1984).

Plagioclase can also be dissolved through a decrease in pH caused by carbon dioxide and organic acids released from the thermal maturation of source rocks. Organic acids are only stable within a temperature window of approximately 80-120" C and typically reach their maximum concentrations within a reservoir at these temperatures (Surdanl et al, 1989). Downhole temperatures measured in the Porong-1 well show that the Late Pliocene secondary porosity zone is situated exactly within this temperature window (Figure 5). Seismic data suggest that the Porong-1 Late Pliocene section was never buried deeper than its current depth, and the WD-8 Late Pliocene sequence is presumed to have experienced a similar burial and temperature history.

No analysis of organic acids were performed on formation waters, but the high concentrations of iodide (100 ppm) and bromide (4,000 ppm) within water produced from Late Pliocene horizons in the WD-8 area are intriguing. A correlation between high organic acid and iodidelbromide content has been noted by some researchers (pers. comm. J.R. Boles and S.G. Franks, 311994). Significantly lower iodide (4 ppm) and bromide (1 ppm) concentrations were noted in water produced from Pleistocene horizons within a Porong-1 area water well.

c. Reasons For Low Cementation

The relatively low cementation within the Plio-Pleistocene horizons could possibly be related to the reservoirs being "open" rather than "closed" systems. High rates of porewater flow can inhibit cement precipitation by continually removing cement-forming ions (Hayes et al, 1976; Mathisen, 1984).

DISCUSSlON AND LITERATURE REVEW OF WORLDWIDE VOLCANICLASTIC RESERVOIRS AND FIELDS.

Negative Aspects

While the majority of the paper is concerned with volcaniclastics that have good reservoir quality, it should not be forgotten that poor quality volcaniclastic reservoirs do exist. A local example from East Java is the Late Miocene Kerek Formation which contains turbidite sandstones and conglomerates composed of a mixture of volcaniclastic and calcareous bioclastic sediments (de Gcnevraye and Samuel, 1972; van Gorse1 and Troelstra, 198 1). The porosity observed in outcrop is generally less than 5-10% due to early calcite cementation brought about largely by the abundance of calcareous bioclasts.

Calcite and other cements such as zeolite and clays are the major cause of poor reservoir quality in other examples such as Neogene volcaniclastics from Slovakia (Reed and Gipson, 1991) and arc-derived sands from Alaska (Galloway, 1979) and New Zealand (Surdam and Boles, 1979). In these examples the cements were formed as a result of early diagenetic alteration of the volcaniclastic sediments. Another example is Stewart (1976) who writes-off the hydrocarbon potential of the Aleutian chain based upon a single ODP well.

Prior to 1985, most authors seem to have generally focused on the negative aspects of volcaniclastics. This is no more apparent than in the 1979 SEPM special publication "Aspects of Diagenesis" in which no less than four very negative papers appeared (Surdam and Boles, 1979; Davies et al, 1979; Bums and Ethridge, 1979; Galloway, 1979) (Table 1). This publication may to a large extent be responsible for the negative attitude to volcaniclastics that still persists today.

Positive Aspects

Since 1985 the literature has presented a more balanced view of volcaniclastic reservoirs. Conolly (1985) and Hawlader (1990) both mention the oil and gas production within Mesozoic volcaniclastics in the Surat and Bowen Basins of Australia, and the untested potential of new reservoir fairways. Mathisen (1984) describes excellent reservoir quality in shallow

Plio-Pleistocene volcaniclastics from the Philippines that are very similar to the East Java examples of this study. The high porosity is attributed to a combination of non-marine deposition, shallow burial (400-900m), early grain dissolution and high porefluid flow rates.

Mathisen and McPherson (1991) present a good review of the various factors that control porosity preservation and destruction in volcaniclastic sandstones. They indicate that the most favorable conditions arc achieved in reworked, distal non-marine, epiclastic and pyroclastic fall deposits with shallow burial, high geothermal gradients and high-rate porewater flow (Table 1).

Hydrocahon Production from Volcaniclastics.

Even though volcaniclastics represent reservoirs far less common than carbonates and quartzose sandstones, oil and gas is commercially produced from or associated with them in examples from all over the world (Table 1). Productive volcaniclastics are most often of Tertiary age, but examples as old as Permian are known. Most of the productive fields are located along past or present subduction zone. / volcanic-arc tracts. These incIude the ancient vol- canic belts of the former Soviet Union (Georgia, Azerbaidzhan), Australia and China, as well as the present-day "ring of fire" which rims the Pacific Basin (Indonesia, Japan, New Zealand and Argentina).

Indonesian examples include the Jatibarang volcaniclastics which produce oil and gas from the Jatibarang Field (Table 1). The long abandoned Kuti and Metatu Fields were discovered near Surabaya in Northeast Java over 100 years ago (Figure 2). The Kuti Field produced 0.75 MMBO from Pleistocene tuffaceous sandstone while Metatu produced 0.3 MMBO from the same Pleistocene volcaniclastics and from underlying Pliocene calcarenites (Soetantri et al, 1973).

The Samgori Field in Georgia, the former Soviet Union, represents the largest volcaniclastic oil field described in the available literature. The field has produced over 165 MMBO since its discovery in 1974 from hydrothermally altered tuffs and tuffaceous sandstones. The porosity is reportedly mostly secondary and the result of zeolitization and fracturing (Grcnberg et al, 1991; Patton, 1993).

ACKNOWLEGMENTS reservoir fairway in back-arc basins, Eastern Australia, AAPG Bulletin 69, p.246 (abstract).

The authors are indebted to the Brantas PSC group (Huffco Brantas, Inc. (Operator), Inpex Brantas, Ltd, Norcen Brantas Ltd. and Oryx Indonesia Brantas Company) and to the Tuban JOB group (Santa Fe, Pertamina, Total, RS Resources and Ensearch) as well as PT Kimia Farma for making the present data available for publication.

REFERENCES

This publication is to a large extent based on unpublished analyses performed by many fine earth scientists at service companies in Jakarta, including PT Geoservices (paleo), PT Corelab (petrography, core and plug analysis, geochemistry), PT Rocktech Sejatera (paleo), Sedimentological Services (field work and petrology) and Simon Petroleum Services (paleo, petrography). Being unavailable to the general public, these reports have been excluded from the references.

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Baldwin, H.L., 1944. Tupungata Oil Field, Mendoza, Argentina, A A PG Bulletin 28, 1455-1484.

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FIGURE 6A: Poroag-1 ~ Sidewaa Core - ~1586 It, lqeistoceae Transitional marine facies volcaniclutic sandstone, Med, med well-sorted, with primary inte~ranular porosity (1), plngiodase (white grains), volcan- ic roCk fragments O r) and heavy mlncrals (H). N0t low compaction, loose gzein-packing and minor zeolite cementation (Z). (4b'X)

FIGURE 6B: Poreag-I WeD Sidewall Core - 6S28 It., Late Pliocene Secondary porosity from plngioclnsc dissolution is major porosity type i*~ middle-outer neritic Late Pliocene facies. Note solution style with large grain-monldic pore center (G), surrounded by remnant rim of ptagiocla ~c (arrows). Compactionhasdeatmyedmnatintergranularpomsity. (IlOX)

FIGURE 6C: ~ Web Cmventkmal Core- 464.05m, Late Plineme Secondary porosity is similar to that in Figure 6B. Note plngioclase dissolution style with grain-mouldic center ((3) and remnant rim (R), leached volcanic rock fragment w/plngioclase microlites (V), intergrane- lar porosity (1) and pore-lining zcoflte cement (arrows). (ItOX)

PLATE6D: Outcrep Near Poadok Dam Site, PleistoceeeKalmhFm. Well developed primary intcrgranular porosity (I) (30%) in Med-Cs, rood-well sorted, non-marine fluviatile volcaniclastic sandstone. Note loose grain packing; plagioclase OF), volcanic rock fragments (V), heavy ~ncrm (m and ~ched ~raim (L). (4sx)

I'LATIg 6E: ~ Anticl~e Oat~mp, Ple/stecxme ! ~ , ~ - - , ~ Fro. Localized heavy fetzoan--calclte cementation in bioclast-rich lag deposit at base of deltaic channd. All intetgramilaf porosity is ferroan calcite-filled

(C). Note pelecypod fragment OF), planktonic f0ram OF), volcanic rock fragment (3/) and plagioclase (while grains). (4b'X)

FIGURE 6F: WD-g Well Conventional Core - 448.13m, Late l'iioome SEM photomicrograph showing the dingenctie sequence of several frc- queer cements. The "webby" grain-coating authigenic smectite clay (C) was formed prior to the tabular-shaped crystals of zeolite (clinoptilolitc) cement. (370X)

FIGURE 6 - Thin Section and SEM Photomicrographs

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