PA95 - 1.3 - 220

PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Twenty Fourth Annual Convention, October 1995

ABSTRACT

BIOGENIC GAS - ITS FORMATION AND ECONOMIC SIGNIFICANCE

Bany J. Kab"

Recently there has been growing interest in gas exploration. Much of this focus has been directed toward thermogenic gas derived from cracking kerogen andlor previously generated liquid hydrocarbons. Such gases form late in a basin's history at elevated temperatures. A significant proportion of the global gas reserve-base is, however, not thermogenic but of bacterial origin. Biogenic gases form early in a basin's history at low temperatures. Because of their different origins, different exploration strategies are required for the two types of gas. The focus of this study is to examine those factors which ultimately result in the accumulation of commercial quantities of biogenic gas, to review the characteristics of several major biogenic gas accumulations, and to briefly comment on the potential economic significance of biogenic gas in Indonesia.

Much of our current understanding of the formation of biogenic gas accumulations has been derived from the study of known occurrences. In order to gain insight into the constraints on formation of commercial biogenic gas deposits in the Po Valley (Italy), Cook Inlet (Alaska), Powder River basin (Wyoming) and West Siberian basin (Russia) are examined. Information from this review and an examination of biological and physical processes can then be used to establish the characteristics of gas source rocks, the influence of sedimentation rate, the role of thermal history, and the importance of relative timing.

Currently available data suggests that significant potential for biogenic gas exists within the Indonesian archipelago.

SIGNIFICANCE OF BIOGENIC GAS

Although significant quantities of biogenic gas are known to be generated during early diagenesis, it has commonly been assumed that much of this gas is generated prior to the development of viable traps and is, therefore, lost. It has, however, been estimated that at least 20% of the known (Rice, 1992) and potentially up to 30% (Grunau, 1984; Figure 1) of the global gas reserves are of a biogenic origin and that accumulations may be locally quite significant (Table 1).

The regions noted in Table 1 provide important information on the geochemicallgeologic conditions associated with the formation of commercial quantities of biogenic gas. Presented below is a series of brief summaries of some of these biogenic gas provinces. These summaries provide a general reference-frame for the development of a biogenic gas exploration model.

Cook Inlet, Alaska

The Cook Inlet basin is 'a polycyclic basin located along the southern Alaskan margin (Figure 1). During the Jurassic, the basin formed in a back-arc setting. As a result of a change in the polarity of subduction in the North Pacific, the basin evolved into a fore-arc basin during the Tertiary (Johnsson et al., 1993).

The sedimentary section ranges in age from Jurassic through Quaternary (Kelly, 1963). Included in this sedimentary sequence is as much as 7 km of Tertiary age nonmarine sediments. It is within this nonmarine sequence that both sandstone and conglomerate reservoirs as well as coal and carbonaceous shale source rocks developed.

* Texaco. Inc. Gas accumulations are of both biogenic and

© IPA, 2006 - 24th Annual Convention Proceedings, 1995

462

thermogenic origin. This duality is clearly established through an examination of their carbon isotopic compositions (Claypool et al., 1980) and is a result of the wide stratigraphic distribution and the resulting wide-range of thermal maturities of the Tertiary coals and carbonaceous shales(Johnsson et al., 1993).

The biogenic gas accumulations are principally found within the Pliocene Sterling Formation, although they also occur within the Miocene Beluga Formation and the Miocene portions of the Tyonek Formation (Claypool et al., 1980). Typical reservoir temperatures for these accumulations is -43"C, at depths commonly less than 1.5 km. Such thermal conditions imply that active biogenic gas production is presently continuing.

Structural traps have been forming since Sterling time (Magoon and Egbert, 1986). Most of the structures in the region are simple, slightly asymmetric anticlines displaying little faulting or compartmentalization.

Among the most significant of the individual biogenic gas accumulations in the Cook Inlet is the Kenai gas field with reserves of > 2 Tcf of dry gas (Claypool et al., 1980). Biogenic gas reserves in the field are contained within five shallow (-1220 m)pools (Sterling Fm.), with an average pay thickness across the field of -160 m (Alaska Geological Society, 1975). A deeper (-2680 m) pool (Tyonek Fm.) contains thermogenic gas.

Po Valley and offshore extension

The Po Valley in northern Italy (Figure 1) has had a complex geologic history. The region was originally part of an extensional setting with Triassic rifting evolving into a passive margin during the Jurassic. During Cretaceous and Tertiary times the area became one of active compression as a result of the closing of the Tethys (Dewey et al., 1973).

The sedimentary sequence ranges in thickness from -4 km along the northern margin to greater than 12 km near the Northern Apennines along the valley's southern margin (Mattavelli et al., 1983). Of significance to this study are the Oligo-Miocene through Pleistocene turbiditic deposits which achieve a thickness of -8 km and may have locally achieved accumulation rates in excess of 1 kmlmy.

The closing of Tethys was also responsible for the

initial formation of a series of folds which continued to grow throughout the Pleistocene. These folds provided for early or syndepositional trap development.

The average organic carbon content of the Upper Miocene through Pleistocene sequence is 0.7 wt.% (Mattavelli et al., 1993), with occasional values exceeding 1.0 wt.% (Mattavelli et al., 1983). Much of the organic matter is found within the turbiditic clays, where the level of organic enrichment is 2 to 5 times that of the hemipelagites. The organic matter within the Miocene and Pliocene interval is rather uniform in character, composed principally of terrestrial material (40% woody material and 45% herbaceous material) with only a minor marine component (15%; Mattavelli et al., 1983).

The isotopic composition of the methane indicates that the most (-85%) of the region's gas is biogenic (Mattavelli et al., 1983). The deuterium content of the biogenic gases (6D values are between -209 and -179%0; Mattavelli et al., 1983) suggests that CO, reduction was the primary mechanism of formation (Figure 2).

Within the region the biogenic gas window appears to be quite deep and wide as a result of the low geothermal gradients (10 to 20 "Ckm; Mattavelli and Novelli, 1988). These low geothermal gradients are, in part, a result of the rapid Plio-Pleistocene sedimentation rates.

Included within the region are two giant fields from which data are available, Agostino-Porto Garibaldi and Barbara. The Agostino-Porto Garibaldi field contains B3.5 Tcf gas of bacterial origin. This gas is contained in about 60 sands over a gross interval of 1600 m (Mattavelli et al., 1993). One of the major sands, PL2-L, had -159 Bcf original gas in-place and is capped by a thin clay layer with a thickness of slightly more than 0.6 m (Mattavelli and Novelli, 1988). The field is a gentle anticline with about 100 m of closure, covering an area of about 50 km2 (Mattavelli et al., 1993). Hydrocarbon generation began -3 MYBP (Mattavelli and Novelli, 1988). Although continued in situ replenishment of gas has been suggested for the field, current temperatures (48 to 63°C) are no longer considered optimum for bacterial activity.

463

The Barbara field contains -1.4 Tcf (Ianniello et al., 1992) of biogenic gas in 13 turbiditic sands intercalated within Pleistocene clays. Structurally the field is a gentle drape anticline, where the Plio- Pleistocene clastic sequence onlaps a Paleogene- Cretaceous partially eroded limestone sequence. The field covers an area of -80 km2 and has a vertical closure of -50 m (Mattavelli et al., 1993).

The Po Valley contains the deepest known biogenic gas accumulation, Antonella at -4.5 km. Temperature data (75°C at 4500 m; Mattavelli et al., 1983), suggest that this gas accumulation represents a "fossil" deposit (i.e., gas entrapment and production occurred at a shallow depth and has been buried).

Powder River basin gas accumulations are the only ancient bacterial gas deposits known to have been produced by fermentation (Law et al., 1991). An accumulation of gas derived through fermentation is thought to infer early trapping (i.e., nearly syndepositional; Law et al., 1991).

Many of the structural features associated with the coal source beds are considered penecontemporaneous compaction structures which formed in response to facies changes. These features may show as much as 76 m of relief and formed shortly after peat formation. In addition to folds, compaction faults also formed in response to high stress.

West Siberia, Russia Powder River hasin

The Powder River basin of Wyoming and Montana (Figure 1) formed as a result of Laramide (Late Cretaceous-Early Tertiary) tectonics. The sedimentary sequence includes as much as 2,400 m of Tertiary rocks ranging in age from Paleocene to Miocene age (Law et al., 1991). Included within this sequence are the coals of the Paleocenc Fort Union and the Eocene Wasatch Formations.

The Fort Union Fm. has been subdivided into three members, the Tullock, Lebo Shale and Tongue River. The oldest member, the Tullock, was deposited within a fluvial setting. Both the overlying Lebo Shale and the younger Tongue River members were deposited within fluvial/lacustrine settings. Net coal thickness, although variable, can exceed 60 m (Law et al., 1991), in the Tongue River member alone. Coal resources have been estimated to be 1.18 x 10" metric tons.

The Fort Union coals are dominated by vitrinite precursors (88%), with minor amounts of liptinite (5%) and inertinite (7%). Coal rank within the Fort Union range from lignite to subbituminous-B (R, = 0.28 to 0.45%).

The gas within the coals and associated sandstones is dominated by methane (>99% of the total hydrocarbons). The methane 6°C values range from - 60.9 to -54.0%0, with 6D values ranging from -354 to 307 %O (Scott, 1993). Such isotopic compositions are consistent with generation via acetate fermentation (Figure 2). Such an origin is considered unusual. The

The West Siberian basin is a large (3.5 x lo6 km') intracratonic basin (Figure 1). In the northern portion of the basin, the sedimentary section reaches a maximum thickness of -11,800 m (Grace and Hart, 1986), with the Mesozoic-Cenozoic accounting for -3 to 4 km of section away from the basin margins (Peterson and Clarke, 1991).

Of primary interest to this study is the Aptian- Cenomanian Pokur Suite. The Pokur Suite obtains a thickness of up to 790 m and is composed of sandstones, interbedded shales and well-developed, laterally extensive coal seams. These coals and associated shales, which contain on the average 1.11 wt.% TOC (Peterson and Clarke, 1991), are considered the primary sources for the biogenic gas reservoired within the Cenomanian sandstones. The Pokur Suite contains > 21 x lo1* metric tons of organic matter, with the coals accounting for only 3 to 5 % of the total organic carbon (Peterson and Clarke, 1991).

Within the northeastern portion of the West Siberia basin there are at least 34 fields which are thought to contain biogenic gas. These gas sands are located at depths less than 1300 m. Individual pools are sealed by the shale interbeds as well as by the overlying Turonian shales. There are at least nineteen productive sands within the Pokur Suite. Total sand thickness within the suite may be as much as 500 to 800 m (Grace and Hart, 1986). The most significant accumulation is the Urengoy Field which, within the PK, sand, contains 218 Tcf of biogenic gas (Grace and Hart, 1986).

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The biogenic origin of the Cenomanian gas is suggested by the 6I3C values, which range from -68.7 to -57.5%0 (Rice and Claypool, 1981; and Grace and Hart, 1986), and by the low rank (lignite) of the Pokur coals. The maximum temperatures achieved by the Cenomanian complex were between 40 and 50°C. In addition to the biogenic gas accumulations there is, in both Cenomanian and Upper Jurassic-Neocomian reservoirs, thermogenic gas.

The Cenomanian biogenic accumulations of West Siberia are among the oldest biogenic gas accumulations known. In part, the presence of these accumulations is due to the growth of positive structural features during the Early Cretaceous with growth continuing during gas generation throughout the Cenozoic. Many of the broad gentle structures formed as a result of differential compaction (Peterson and Clarke, 1991). These structures have experienced only mild tectonic disturbance since their formation.

BIOGENIC GAS GENERATION

An analysis of the various biogenic gas provinces summarized above suggests that there are a number of basic conditions which appear to be associated with the formation of commercial biogenic gas deposits. These include high rates of sedimentation, low geothermal gradients, early trap development, simple trap styles with uncomplicated histories, and closely associated source and reservoir systems. Coals also appear to be common to these systems.

Utilizing the above criteria and an understanding of the biologic, chemical and physical processes associated with methanogenesis an effective gas generation model can be constructed. Such a model must examine both possible sources and mechanisms of formation. Gas may be derived directly from sedimentary organic matter or as a secondary product through the thermal alteration of petroleum. The transformation of these larger macromolecules to gas may occur through either bacterially mediated (biogenic) reactions or through thermal cracking (thermogenic). Only aspects of biogenic gas generation are discussed here. Readers are referred to Welte et al. (1988) for a discussion of thermal gas generation.

Souxe Rock Criteria

The commonly invoked source rock thresholds, based on the level of organic enrichment (1.0 wt.%; Bissada, 1982) and total generation potential (2.5 mg HC/g rock; Bissada, 1982), for oil-prone source rocks are not valid for gas-prone systems. Different thresholds exist because of different "expulsion" mechanismsand the elevated expulsion efficiencies of gas-prone systems.

Primary migration or expulsion of liquid hydrocarbons is generally believed to occur as a separate phase (Leythaeuser et al., 1987). Expulsion is largely driven by pressure and capillary forces near the edge of the effective source rock (Leythaeuser et al., 1987). Expulsion as a separate phase requires that the source rock's pore system becomes saturated with petroleum prior to expulsion. It is this requirement that establishes the commonly invoked threshold values. A minimum generation potential of -2.5 mg HC/g rock is generally required in order to saturate the source rock pore system near the top of the "oil-window" assuming typical compaction profiles.

In contrast, diffusion is considered the means of primary migration of light hydrocarbons (less than C,; Leythaeuser et al., 1982). Diffusion migration is controlled by concentration gradients and does not require that the pores be filled with free gas for expulsion to occur. Clayton (1992) did, however, suggest that a threshold for free gas within a source rock system does exist. He noted that -0.12 wt.% TOC is typically required for source rock pore fluids to become saturated with gas and is thus the minimum for the development of free gas in the source rock. Higher levels of organic carbon would be necessary to achieve saturation throughout the pore network with even higher levels of organic enrichment being necessary in hydrodynamic systems where there is substantial groundwater movement (Clayton, 1992). Rice and Claypool (1981) suggested that for commercial biogenic gas accumulations a minimum of -0.5 wt.% TOC is necessary, assuming that the organic matter is not inert. This more elevated level of organic enrichment takes into consideration both saturation and gas losses which may occur as a result of such factors as transfer efficiency.

The effectiveness of methane expulsion is also significantly greater than that of heavy hydrocarbons.

465

Zhang (1994) noted that less than 0.001% of generated methane is retained within the source rock which contrasts with potential oil retention by the source of up to 90+%.

Gas Generation

Biogenic gas is produced as part of the sequential decomposition of sedimentary organic matter. Methanogens are strict anaerobes and are active only during the final stages of decomposition (Zeikus, 1977). Their position in the anaerobic decomposition or remineralization sequence is largely the result of their relative efficiency in deriving energy from the available substrate (Figure 3). Marty (1992) noted that denitrifiers, which are more efficient in extracting energy, dominate over sulfate reducers, which dominate over methanogens. Consequently, methanogenesis does not effectively begin in marine systems until pore-water sulfate has been significantly reduced and sulfate reducers are no longer active. The depth at which this transition occurs is controlled by the availability of sulfate, the burial rate and the nature of the organic matter. The depth to the onset of methanogenesis is greatest when the organic matter is more refractory and the rates of sedimentation are slow (Berner, 1978). Sulfate reduction may persist to depths of several hundred meters. The general absence of sulfate in nonmarine systems permits methanogenesis to begin at much shallower depths.

Methanogens do not directly decompose available organic matter but metabolize decomposition products of earlier bacterial mediated reactions (Figure 4). Two primary pathways (Figure 5 ) have been identified for methanogenesis: acetate fermentation (CH,COOH -+ CH, + CO, ) and CO, reduction CO, + 8(H) -+ CH, + 2H,O). Law et al. (1991) suggested that CO, reduction represents the later phases of biogenic gas production. Methane formed by these two different pathways can be isotopically discriminated between (Figure 2; Whiticar et al., 1986).

Methanogenic activity is strongly influenced by temperature. Although methanogens can survive over a wide temperature range (0 to 97°C; Konig, 1992; Figure 6), the optimum temperature for methanogenic bacterial activity of interest to the energy industry is between 30 and 40°C (Mah et al., 1977; and Konig, 1992). The group of methanogens which display a lower temperature for optimum metabolic activity

would commonly be in competition with sulfate reducers and once established would, under most circumstances, produce gas prior to seal development. The thermophilic forms would produce most of their hydrocarbons coincident with thermogenic gas generation with their products being indistinguishable from those of thermal cracking.

An understanding of the rate of bacterial methanogenesis as a function of temperature permits the development of a qualitative cumulative methane generation curve. The curve presented in Figure 7 assumes both a constant geothermal gradient and sedimentation rate. This cumulative curve suggests that as much as 113 of the potential biogenic methane yield occurs at temperatures below 30" C, -1/3 takes place in the 30-40" C window, and that the remaining 1/3 of the biogenic gas is produced between 40' and 70°C. Variations in sedimentation rate and/or geothermal gradient through time would alter the shape of the cumulative generation curve.

Attempts to quantitatively estimate biogenic hydrocarbon yields have been limited. Clayton (1992) suggested that under normal geologic conditions -1 0% of the available organic matter can be converted to methane through biogenic processes within those geologic settings conducive to biogenic gas formation. This corresponds to a CH, yield of 4.9 m3 CH,/% TOC/m3 rock. Through the compiling of this "ultimate" yield value with the cumulative generation curve (Figure 7) and a burial history a semiquantitative estimate of biogenic gas production can be made.

POTENTIAL FOR BIOGENIC GAS IN INDONESIA

A preliminary evaluation of several of the sedimentary provinces of Indonesia reveals that several basins display favorable characteristics for the formation of significant biogenic gas deposits.

Many of the Indonesian basins formed during the Tertiary and have rather thick sedimentary accumulations (>3 km; Figure 8). Individual stratigraphic units may display very high sedimentation rates. For example, sedimentation rates in excess of 300 m/my existed during deposition of the Baong Fm. (North Sumatra basin), Walanae Fm. (Bone basin), and Steenkool Fm. (Bintuni basin). Such rapid sedimentation rates reduces time within the

466

sulfate reduction zone and increases the percentage of organic matter which may act as a substrate for methanogenesis.

In addition, many of these sedimentary basins are cool, displaying below-average to average geothermal gradients (Figure 9). These low gradients would maximize the duration of conditions suitable for biogenic gas production.

Limited organic geochemical data also reveals that many of these depositional settings contain sufficient quantities of organic matter for commercial. quantities of biogenic gas to be produced. Organic carbon contents in the Mahakam Delta (Kutei basin) are generally high (3-10%) in the shaley facies, with lignitic accumulations containing 20-46% TOC (Combaz and De Matharel, 1978).

Sufficient quantities of organic matter even appear to be present within the "normal" marine stratigraphic sequence. For example, the Early and Middle Miocene turbiditic sequence of the Celebes Sea contains up to 4.9 wt.% TOC (Rangin et al., 1990).

In addition to the presence of suitable conditions for biogenic gas generation, a limited amount of published data confirm the presence of biogenic gas in the greater Indonesian area (Rice, 1992; Figure 1). It is, however, important to note that many of the offshore regions which display the potential for commercial biogenic gas accumulations are in deepwater (1 1000 meters).

CONCLUSIONS

Optimum conditions for biogenic gas accumulations are typically associated with series of interbedded sources (claystones, shales and coals) and reservoirs, deposited rapidly. These conditions are commonly associated with turbidites and marginal marine settings where repeated transgressions and regressions dominate sedimentation.

A wide window of biogenic gas production is favored when geothermal gradients are low. These low gradients may be the result of rapid sedimentation andkectonic setting.

Potential trap formation occurs early relative to the deposition of the source rock sequence. It does not

appear, however, that the initial bacterially derived gases are trapped. An exception may exist in the Powder River basin where traps appear to have formed penecontemporaneously with source rock deposition and the trapped gases were derived from acetate fermentation.

Conditions appear favorable in several of the Indonesian basins for the production and accumulation of biogenic gas. One of the major restrictions on the exploration for these deposits may be the associated water depths.

ACKNOWLEDGMENTS

The author thanks Texaco Inc. for permission to publish this work. An early version of this manuscript was critically reviewed by Dr. C. R. Robison.

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467

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Biogenic Methane Formation in Marine and Zhang Yi Gang, 1994, Factors Affecting the Dynamic Freshwater Environments : CO, Reduction vs. Acetate Equilibrium of Gas Accumulations, J. Pet. Geol., v. Fermentation - Isotope Evidence, Geochim. 17, 339-350. Cosmochim. Acta, v. 50, 693-709.

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TABLE 1 SUMMARY OF SOME SIGNIFICANT BIOGENIC GAS RESERVES

Geographic Region

____

Cook Inlet, Alaska Greater Green River bash Guajira Region, Colombia Gulf of Mexico Japan Powder River basin Po Valley & offshore extension West Siberia

Biogenk Gas Reserves

(Tcf)

>7.5

29

3.3

> 12

4.7 40

> 5

> 460

470

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FIGURE 1 - Location index map of significant biogenic gas deposits (after Rice, 1992). Regions highlighted are discussed in the text.

." -400 -350 -300 -250 -200 -150 -100

sDMethane

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