hydrocarbon charge of a bacterial gas field by prolonged methanogenesis- an example from the east...

~ Pergamon Org. Geochem. Vol. 29, No. 1-3, pp. 301 314. 1998

© 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain

P l h S0146-6380(98)00064-3 0146-6380/98/$- see front matter

Hydrocarbon charge of a bacterial gas field by prolonged methanogenesis: an example from the East Java Sea, Indonesia

R. A. NOBLE* and F. H. H E N K J R . t

Atlantic Richfield Indonesia Inc., Landmark Tower B, Jalan Sudirman, Kav. 70, Jakarta, 12910, Indonesia; accepted 2 March 1998

Abstract The Terang-Sirasun Field in the East Java Sea of Indonesia contains 1.0 trillion cubic feet (TCF) of dry gas reserves, which are made up of over 99.5% methane with 613C of -65%0 and 6D of -185%o. The methane was formed exclusively by methanogenic bacteria via the CO2 reduction pathway. The primary source sediments for the methane were identified based on bulk geochemical and absolute biomarker concentrations. Specifically, the C25 acyclic isoprenoid 2,6,10,15,19-pentamethyleicosane (laME), and related isoprenyl glyceryl ethers, which are well known markers for methanogenic archaebacteria, were used as indicators for sediment layers thought to have had the most abundant methanogen activity. Burial history analysis and precise biostratigraphic age control provided the framework for assessing the timing of hydrocarbon fill. Our findings show that methanogenic activity was highest in marine shelfal claystones (mid-outer neritic) ranging in age from 8 Ma (Late Miocene) to present. The gas is reservoired in limestones and sands of the Paciran Member, which are dated from 6.5 to 1.3 Ma (Late Miocene Pliocene). The top seal for the present accumulation was deposited less than 0.5 Ma ago (Quaternary), although there is strong geological evidence that older seals existed. These older seals were catastrophically removed by submarine slumping resulting in the loss of earlier accumulated gas. Methanogenesis from older source beds continued, and the trap was recharged after deposition of the current top seal. The information obtained from this field study provides evidence that large accumulations of bacterial methane do not necessarily require early entrapment of methane from freshly deposited marine sediments. Traps may be filled with bacterial gas long after the deposition of source beds, provided that the required conditions for active methanogenesis are maintained throughout this period. © 1998 Elsevier Science Ltd. All rights reserved

Key words~bacterial gas, methanogenesis, biomarkers, acyclic isoprenoids, pentamethyleicosane, bisnorhopane, methane isotopes, East Java Sea, Indonesia, Paciran formation

INTRODUCTION

Explorat ion programs which specifically target natural gas systems are becoming increasingly important ~in the search for new hydrocarbon resources. Successful exploration in turn, requires a good understanding of all elements of the petroleum system, of which petroleum charge is an integral part. Methanogenic bacteria provide an important source of hydrocarbons in sedimentary en- vironments (Rice and Claypool, 1981). Very large quantities of methane are generated in near surface settings by bacterial degradation processes. However, only a small fraction of the bacterial methane accumulates in deposits of adequate size to be of commercial interest to the petroleum industry (Clayton, 1992). This is due to the very specific conditions required for trapping and accumulation of bacterial natural gas (reviewed by Rice, 1992; Brown, 1997).

*To whom correspondence should be addressed. E-mail: [email protected].

"~Present address. ARCO Exploration and Production Technology, 2300 West Plano Parkway, Plano, TX 75075, U.S.A.

Biochemical and geochemical processes involved in methanogenesis have been extensively researched and are well understood (reviewed by Schoell, 1988; Faber et al., 1992; Whiticar, 1994). Criteria have been established for recognizing the occurrence of bacterial gas and the metabolic pathways by which it is formed. The rates and duration of methanogenesis appear to be less well defined, and it has proven to be quite difficult to make generalizations about these variables for use in exploration well planning. In some studies, it has been shown that methanogenesis proceeds at extremely rapid rates in the first few metres of the methanogenic zone (Reeburgh, 1983; Martens and Klump, 1984; Kuivila et al,, 1990). At rates indicated by these studies, all metabolizable organic matter would be rapidly con- sumed, and methanogenic activity would cease in a relatively short time after sediment deposition. In this scenario, very early or contemporaneous development of reservoirs, traps and seals would be required for a significant gas accumulation. On the other hand, ocean drilling project (ODP) studies have shown that methanogens can survive in the sediment column for several million years at depths in excess of 500 m (Cragg et al., 1992). Under these

301

302 R.A. Noble and F. H. Henk Jr

conditions, additional time would be available to form and consolidate the geological elements required for an economic gas accumulation. Unless specific information is available about the stratigraphic location, age and burial history of potential bacterial gas source beds, meaningful predictions about the timing of hydrocarbon fill relative to reservoir and trap development may be very difficult to formulate.

In this study, we report on details of a bacterial gas field from the East Java Sea, Indonesia. The gas field, named Terang-Sirasun, is of commercial size, containing approximately 1 TCF of methane reserves. Using an extensive sample suite of conventional and sidewall cores from nine exploration and delineation wells, the probable identity and stratigraphic location of the source beds for the gas was determined. Specifically, biomarkers for methanogenic bacteria were identified, and their absolute concentrations in the sediments were used as indicators for the level of methanogenic activity. This information was integrated with the burial history and geological evolution of the region to recon- struct the filling history of the gas field.

GEOLOGICAL SETTING

Location and stratigraphy

The case study is taken from ARCO's Kangean block, which is located in the East Java Sea, approximately 140 km north of the island of Bali (Fig. 1). The study area falls within the Madura Basin province of the East Java Sea, which is part of a back-arc basinal setting north of the active Java Trench and Java-Lombok volcanic arc complex. The tectonic evolution of this region was dominated by extension during the Eocene- Oligocene period, resulting in large scale normal faulting and rift-induced sediment fill. Continuing subduction of the Australian-Indian plate beneath the Java arc has generated compressional stresses since the Miocene, resulting in reactivation of older normal faults and widespread inversion and wrench-related structures. Compressional forces have continued up to the present day, resulting in uplifted island segments and subsurface inversion features. A complete description of the tectono-stratigraphic evolution of the region is presented by Matthews and Bransden (1995).

The sedimentary section of interest is limited to the uppermost part of the stratigraphic column (Late Miocene to Recent). The older Eocene to Early Miocene section, although well characterized in this region by seismic and drilling results (Matthews and Bransden, 1995), will not be described in this work. Figure 2 shows a generalized stratigraphic column and typical well logs for the section of interest. Late Miocene to Pliocene sediments make up the Mundu Formation, which is

divided into three Paciran Members: the Lower Paciran Sandstone (L. PacSST), the Upper Paciran Sandstone (U. PacSST) and Paciran Limestone (PacLST). These deposits are typically deep water to shelfal deposits of claystones, siliclastic sandstones and carbonates. The Lower PacSST was deposited in an outer shelf fan/delta complex. The section coarsens upwards from shales and siltstones to fine and medium grained sands. The Upper PacSST is comprised of subangular quartz grains and a small percentage of lithic fragments. Occasional limestone stringers are also observed. This coarsening upwards interval was deposited in middle to outer shelf conditions. The overlying PacLST is a deposit of deep marine pelagic carbonates comprised of an illite smectite clay matrix and globeriginid foraminifera tests. The rock varies from a clay-rich marl to a globeriginid-rich grainstone, with the upper lithofacies being predominately marl and the lower lithofacies mainly grainstone/wackestone/packestone (hereafter simply called limestone).

The youngest sedimentary package deposited from Pleistocene to Recent comprises claystones, thin-bedded deep marine limestones, and thin shelfal glauconitic sand units. The lower part of this section is characterized by numerous submarine slump and shelf edge failure features. These are in- dicative of catastrophic sediment instability due to very high fluid content. Consequently, much of the Lower Pleistocene section (1.3 to 0.5 Ma), and in some areas part of the Upper Pliocene interval, is missing from wells drilled on basinal highs as a result of submarine slumping and other erosional processes. The upper part of the Lidah Formation, which mainly consists of claystones and muds, was deposited very rapidly after the slumping events during the last 0.44 Ma. The age and environmental constraints referred to above are based on extensive biostratigraphic and sedimentological studies per- formed by and on behalf of ARCO Bali North Inc. (unpublished internal reports).

Terang- Sirasun field

The Terang-Sirasun gas field was discovered by ARCO in 1982 with the Terang-I exploration well. With the emergence of a local gas market in East Java during the early 1990's, the field was further delineated for commercial development. Figure 3 shows a depth structure map for the top Paciran horizon. A total of 9 wells have been drilled to date as part of the exploration and delineation program: Terang 1 3, Sirasun 1-3, Bromo-1, Batur-1 and Kubu-1.

The Terang-Sirasun structure is an east-west trending elongate anticline with approximate dimen- sions of 10 km width and 40 km length. The structural ridge was formed as a result of compressional folding and diapirism of Early Miocene and older

Hydrocarbon charge of a bacterial gas field 303

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Fig. 1. Location maps of Kangean Block and Terang-Sirasun gas field in East Java Sea, Indonesia. A detailed structure map of Terang-Sirasun is shown in Fig. 3. Dark shaded areas are Indonesian island segments, and lightly shaded areas are hydrocarbon fields (Terang-Sirasun, W. Kangean, JS53 and

Pagerungan). Only Terang-Sirasun is discussed in this study.

shales. Structural uplift in the region is continuing up to the present day. A series of N E trending en

echelon faults dissect the area into smaller fault blocks. Individual gas accumulations at Terang,

Sirasun, Batur and Kubu are collectively referred to as the Terang-Sirasun Field.

Gas is reservoired in the Paciran Limestone and Paciran Sandstone Members. High quality sands


EPOCH

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Fig. 2. Generalized stratigraphic column for Late Miocene-Quaternary section in the Terang-Sirasun area. A typical set of well log traces taken from the Terang-3 well are shown (GR is gamma ray; RHOB is the density; DT is sonic and NPHI is neutron porosity). The shaded area of crossover

between the neutron and density curve is the interval of high gas saturation.

and porous limestones provide the best reservoir facies. A marly section at the top of the PacLST does not seal the underlying limestones and sands, and is considered part of the reservoir interval. The reservoir is effectively sealed by Lidah Formation claystones. Determination of the source for the gas at Terang-Sirasun is the main point addressed in this study.

EXPERIMENTAL

Samples

Samples from all nine wells drilled in the Terang- Sirasun area have been screened for geochemical properties (unpublished ARCO internal reports).

The rock units represented in these analyses are the Lidah Fm. (claystones), PacLST (marls and limestones), and PacSST (claystones, siltstones and fine- grained sands). Detailed geochemical analyses were conducted on 24 samples taken from five of the wells (Table 1). All of these samples were either from conventional core or sidewall core.

Geochemical screening

Total organic carbon (TOC) was determined by Leco analysis, and hydrogen index (HI) by Rock- Eval. Selected samples were analyzed by pyrolysis- gas chromatography (py GC) and visual kerogen description. Synthesis of these results provided information on source rock richness and kerogen type.


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Table 1. Geochemical and quantitative biomarker information for sample suite

EOM Hopanes 28,30- C25-PME Sample Depth TOC HI (ppm (ppb BNH (ppb (ppb No. Well (m) Formation Type Lithology (%) (mg/g) Rock) Rock) Rock) Rock)

1 Batur-I 959.3 Pac Lst core marl 0.80 206 81 55 7 0.4 2 Batur- 1 962.1 Pac Lst core marl 0.88 218 72 49 3 0.3 3 Batur-I 968.6 Pac Lst core marl 0.53 170 55 66 34 0.2 4 Batur-1 993.9 Pac Lst core limestone 0.78 165 59 35 1 0.2 5 Bromo-1 1105.2 Lidah swc claystone 0.69 41 39 118 0 0.5 6 Bromo-I 1140.4 Pac Lst core limestone 0.47 164 66 74 165 0.2 7 Bromo-1 1158.7 Pac Sst core limestone 0.41 90 39 55 111 0.2 8 Bromo-1 1179.6 Pac Sst core sandstone 0.52 44 11 14 13 0.1 9 Bromo-I 1333.8 Pac Sst swc siltstone 0.77 81 208 420 0 2.5 10 Kubu-1 789.0 Lidah swc claystone 0.94 44 151 187 0 1.0 II Kubu-I 797.0 Pac Lst swc marl 1.02 130 243 241 6 1.6 12 Kubu-I 802.4 Pac Lst swc claystone 0.67 196 172 209 4 6.9 13 Kubu-I 803.8 Pac Lst core sandstone 0.65 132 47 32 5 0.3 14 Sirasun-3 1030.8 Pae Lst core limestone 0.35 103 43 43 5 0.2 15 Sirasun-3 1033.7 Pac Lst core limestone 0.77 204 147 116 12 1.4 16 Terang-3 682.9 Lidah core claystone 0.76 47 13t 214 0 0.4 17 Terang-3 689.1 Lidab core claystone 0.74 57 183 307 0 0.6 18 Terang-3 697.9 Pac Lst core marl 0.60 107 115 121 0 0.6 19 Terang-3 707.6 Pac Lst core limestone 0.48 260 438 99 16 0.5 20 Terang-3 712.0 Pac Lst core limestone 0.53 175 85 135 12 0.8 21 Terang-3 829.6 Pac Sst swc claystone 0.73 44 176 290 0 3.3 22 Terang-3 917.1 Pac Sst swc claystone 0.70 90 476 328 0 2.7 23 Terang-3 939.3 Pac Sst swc claystone 0.69 72 290 402 0 7.8 24 Terang-3 942.1 Pac Sst swc claystone 0.86 77 737 450 0 9.1

Legend. TOC = total organic carbon; HI = hydrogen index; EOM = extractable organic matter; hopanes = sum of C27-C35 hopanes with ~/L/~[:~, [;~ configurations BNH = 28, 30, bisnorhopane; C25 PME = 2, 6, 10, 15, 19, pentamethyleicosane; swc = sidewall core.

Quantitative biomarker analysis

The twenty-four samples listed in Table 1 were extracted using the Soxhlet method (24 h, dichloro- methane). The solvent was removed by evaporation in a stream of nitrogen at 40°C. The weight of extractable organic matter (EOM) was recorded in ppm relative to the weight of rock (Table 1). The EOM was diluted with cyclohexane to a standard concentration of 100 g/L and an aliquot was taken for analysis by gas chromatography-mass spec- trometry (GC-MS). An internal standard (17fl-cholane) was added at a concentration of 100ppm relative to the weight of EOM. Integrated peak areas for components of interest were determined by GC-MS and their absolute concentration relative to the EOM were determined by comparison with the cholane standard (m/z 217). Response fac- tor corrections were not applied for any of the components. Absolute concentrations relative to the weight of rock were calculated based on EOM yields (Table l). Structures for three acyclic isoprenoid compounds referred to in the text (l, II, lII) are shown in the Appendix.

All GC MS data were acquired using an AutoSpecQ hybrid mass spectrometer with EBEQ geometry (Micromass, VG Analytical, Manchester). An HP5890 Series II gas chromatograph with an HP7673 autosampler was coupled to the instru- ment. Samples were injected directly on-column (0.2 pL). The gas chromatograph was fitted with a 60 m × 0.25 mm i.d., DB-1 fused silica capillary column (J and W Scientific). Helium was used as the carrier gas at constant inlet pressure (207 kPa). The oven was heated using a linear temperature pro-

gram from 150 to 300°C at 2°C/min. The mass spectrometer conditions were as follows: 1000 resolution, 70 eV ionization energy, 700 #A trap current, 250°C source temperature.

RESULTS AND DISCUSSION

Gas characterization

The gas composition across the field was found to be very uniform. Methane is the predominant component, making up over 99.5mol % of the total gas. Only trace levels of ethane are present, together with minor amounts of nitrogen and carbon dioxide (< 0.5%). fiJ3C and 6D isotopic ratios were determined for the methane (-65%o and -185%0, respectively). Figure 4 shows a crossplot of the isotope values for two representative gas samples from the Terang-2 and Sirasun-I wells. Using the established interpretative criteria for these parameters (Schoell, 1980; Jenden and Kaplan, 1986; Whiticar et al., 1986), together with the overwhelming predominance of methane, a bacterial origin for the gas is clearly indicated. Moreover, the methane was mostly likely formed by CO2 reduction as opposed to acetate fermentation. This result is in keeping with previous observations that most large accumulations of ancient bacterial gas are typically formed by methanogenic processes involving CO2 reduction (Schoell, 1988; Rice, 1992 and references therein).

Identification of bacterial source rocks

Current methodology trends in hydrocarbon exploration require that the source rocks for pet-


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Fig. 4. Gas characterization plot of C~13C VS 6D isotope ratios. The areas outlined in dashed lines are zones interpreted for each of the major gas generative processes (after Schoell, 1980; Whiticar et al., 1986). The Terang- Sirasun gas is interpreted to have formed by methanogen-

esis involving the CO2 reduction pathway.

roleum (oil or gas) be accurately identified and characterized. A firm knowledge of the origin of petroleum in any given area has typically led to improved exploration results in follow-up drilling. Such is the case for conventional thermogenic source rocks for oil and gas. However, when deal- ing with bacterial gas source rocks, the same methods for source rock recognition and characterization do not apply. New criteria for mapping the distribution and yield of methanogenic systems through geologic time must be established. The reasons for doing so are as compelling as those developed for thermogenic systems. This provides new challenges for locating the sedimentary layers which have effectively given rise to the bacterial gas and reconstructing their generative histories relative to reservoir and trap development.

Observations made in recent marine environ- ments have indicated that host sediments for active methanogenesis are not necessarily organic rich. TOC levels in the range of 0.5 to 1% may be adequate to support significant methanogenic activity (Rice and Claypool, 1981; Clayton, 1992). Metabolizable organic matter is typically in the form of land plant detritus, although other forms of organic matter may also be utilized by the microbial communities. The carbon components required for methanogenesis, i.e. acetate or carbon dioxide, are formed by other bacterial processes which degrade complex organic biopolymers into simple molecules (see reviews by Winfrey, 1984; Marry, 1992). Another potential source of CO2 is from diagenetic reactions of organic matter. These reactions are as- sociated with the early stages of kerogen evolution,

and are thermally controlled, taking place at depths that extend well below the known limits of microbial activity. The relative contribution of CO2 from these deeper, nonmicrobial reactions, to the overall production of bacterial methane in shallow sediments is not readily quantified. However, if diagenetic CO2 is a major feedstock for methanogenesis, then the type and quantity of organic matter deposited in the host sediments may be of little importance other than to initially support an active bacterial community.

With these observations in mind, data from wells in the Terang-Sirasun area were examined with the intention of locating specific source beds for the bacterial gas.

Organic matter-bulk properties. A variety of lithofacies from Late Miocene to Recent strata were analyzed for their bulk geochemical properties. This included the PacSST (claystones, siltstones and sandstones), the PacLST (marls and limestones) and the Lidah Formation (claystones). Geochemical data were acquired as part of routine well analysis procedures (unpublished ARCO internal reports). A summary of the observations drawn from these data is presented as follows:

TOC values were found to range from 0.2 to 1.6%. The vast majority of samples had values between 0.5 and 1.0%. The fine-grained elaystone lithofacies of the Paciran interval had consistently higher values than the coarser grained sandstone and limestone lithofacies. The average claystone TOC was about 1.0%, whereas the sandstones and limestones had average values around 0.6%. The Lidah claystones also had an average TOC value of 0.6%, which is considerably lower than the Paciran claystones. Rock-Eval HI values for all samples were in range of 40-260 mg HC/g C. A good correlation was found between the HI values and p y - G C characteristics. Samples with lower HI values (below 150 rag/g) typically displayed p y - G C signa- tures completely dominated by aromatic/phenolic components. Those with higher HI values (150- 260 rag/g) yielded a somewhat higher concentration of aliphatic compounds, although these were still subordinate relative to aromatics and phenols. A predorninately woody kerogen type was inferred in all cases, with a somewhat enhanced contribution of hydrogen-rich biomass in samples with higher HI values. Visual kerogen analysis confirmed the predominately woody nature of the kerogen in all samples.

in general, the fine grained lithofacies appear to be moderately enriched in organic matter compared with the coarser grained facies. Hence, the claystones might be expected to have supported a somewhat higher level of methanogenic activity compared with the sandstones and limestones. All samples contained predominately detrital organic matter derived from vascular land plants. Minor


enrichment with aquatic organic matter was noted in some samples. The claystones, therefore, provide the best candidates for bacterial gas source beds, although additional information was required to positively identify specific intervals that may have contributed differentially to the gas accumulation at Terang-Sirasun.

Molecular parameters. The biomarker composition of a set of 24 samples from representative lithofacies was examined in an effort to locate intervals showing enhanced evidence of methanogenic activity. There are a number of different biomarker classes with known affinity to microbial precursors. The most common of these are the hopanes, which are derived from cell-wall constituents of procaryotic micro-organisms (Ourisson et al., 1979). How- ever, methanogens are not procaryotes and do not typically biosynthesize hopanoids (reviewed by Ratledge and Wilkinson, 1989). Methanogens are classified as part of the archaebacterial form of life, and these micro-organisms have a very distinct biochemistry compared with procaryotic microbial groups (Ourisson et al., 1979; Tornabene et al., 1979). Extensive research on archaebacterial lipids has shown that methanogens synthesize acyclic isoprenoids as structural components of their biomem- branes (reviewed by Volkman and Maxwell, 1986). In particular, the C25 isoprenoid 2,6,10,15,19-pentamethyleicosane (PME, I) has been utilized as a specific biomarker for methanogenic archaebacteria

in marine sediments (Brassell et al., 1981; Schouten et al., 1997). Cooccurrence with the C30 isoprenoid squalane (II) further supports an archaebacterial origin (Tornabene et al., 1979; Brassell et al., 1981). Isoprenyl glyceryl ethers (III) are also important markers for this class of methanogens (reviewed by Langworthy et al., 1982).

Triterpane and acyclic isoprenoid distributions have been examined for various types of biomarkers. Figure 5 shows a partial m/z 191 fragmentogram for a typical sediment extract. Hopanes are abundant, indicating contributions from procaryotic micro-organisms which undoubtedly inhabited the sedimentary environment. The predominance of fl/3- hopanes relative to stereoisomers with the e/3 and /~ configurations illustrates the thermal immaturity of the strata at these depths (less than 1000m below sea level). The significant presence of bicadi- nanes shows the contribution of organic matter from land plant sources, which is consistent with the interpretation of bulk geochemical properties. 28,30-Bisnorhopane (BNH) was found in some samples only. The concentration of this compound was highly variable. In some cases, it was present in trace amounts, whereas in others, it was by far the most abundant compound in the m/z 191 trace. A discussion on the origin of this compound is presented in the next section.

A portion of the m/z 183 fragmentogram between nC21 and nC2s is shown in Fig. 6. The peaks labeled

m/z 191

"T" C30"R"

C30

C28

t r c27

C30 C30 C31

C29

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' I C33

Peak Legend ~ - Hopane ~ - Hopane ~ - H o p a n e r~_~Jtrans - Bicadinane f~i~!:i~:!i~:~ 28,30-Bisnorhopane

L ~ ~

Retention Time >

Fig. 5. Partial m/z 191 fragmentogram showing triterpanes in a sediment extract from Sirasun-3. The peak assignments were based on retention times and characteristic fragmentation ions.


nC25

nC23

nC21 ipC30

ipc2s | / /

Retention Time

Fig. 6. Partial m/z 183 fragmentogram showing n-alkanes and acyclic isoprenoids in a sediment extract from Terang-3. Peaks labeled nCX correspond to n-alkanes (X is number of carbon atoms). Peak ipC25 corresponds to 2,6,10,19,25-pentamethyleicosane (PME, I). Peak ipC30 corresponds to squalane

(liD. See text for discussion.

ipC25 and ipC30 represent acyclic isoprenoid alkanes with 25 and 30 carbon atoms, respectively. The mass spectrum and GC retention position for ipC25 compared favorably with published information for PME (I, Risatti et al., 1984). Direct comparison of these properties with those of an authentic standard confirmed its identity (reference compound kindly supplied by Professor S. Rowland from extract of Methanosarcina barkeri). The peak labeled ipC30 was identified as squalane (II, Holzer et al., 1979) based on its mass spectrum and GC retention position. Identification of these biomarkers in the extractable organic fraction was taken as positive evidence for methanogenic activity in the host sediments. Additional evidence for methanogenesis was the presence of isoprenyl glyceryl ethers (liD, which. were identified in the polar lipid fraction of the organic extracts (proprietary study conducted by University of Plymouth on behalf of ARCO).

Source horizons for bacterial gas. The identification of microbial lipids, specifically those that are biomarkers for methanogenic bacteria, provides a unique means to locate probable source horizons for the bacterial gas. Table 1 shows details of the sediments analyzed, and the absolute concentrations of bacterial biomarkers relative to rock weight (reported in ppb). An assumption is made that increased biomarker concentrations correspond to greater amounts of the precursor organism in the host sediments. Figure 7 shows a plot of biomarker

concentrations for each of the major sedimentary units. Data for total hopanes, BNH, and PME are shown. As pointed out above, the hopanes are derived from procaryotes, and their abundance is not a reflection of methanogen occurrence. Hopanes were present in all samples in concentrations ranging from ca. 10 to 450 ppb. BNH occurs most fre- quently in carbonates of the PacLST, in concentrations ranging from less than 5 to 165 ppb. It is also present in high abundance in a limestone stringer from the PacSST. Otherwise it is largely absent from this unit as well as from the Lidah claystones.

PME, which we are using as a specific marker for methanogens, was identified in variable quantities in all samples. Concentrations are in the range of 0.1 to 10 ppb. At the lower concentration levels, positive identification was less certain due to low peak intensity. Nevertheless, a component occurring at the exact retention position in the m/z 183 fragmentogram was consistently assigned as PME. As seen in Fig. 7, there are some specific intervals which contain elevated amounts of PME. Those with the highest abundances occur in claystones and siltstones of the PacSST and PacLST. Smaller amounts of this biomarker were recorded in all other lithofacies, including the coarser-grained sandstone and limestone reservoir facies. From these results we conclude that the highest levels of methanogenic activity, as recorded by the absolute


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Fig. 7. Stratigraphic column showing absolute concentrations of bacterial biomarkers, in parts per bil- lion (ppb) relative to weight of sediment. Samples were taken from several different wells, and their lo- cations within the stratigraphic column are depicted schematically in this figure. The lithologies are generalized and do not duplicate the lithologic section encountered in any single well. The sample #

refers to Table 1, and specific information on sample lithology is given therein.

concentration of PME, occurred within finer- grained facies, particularly claystones of the PacSST interval. These horizons are interbedded with the coarser grained facies which serve as the gas reservoir at Terang-Sirasun. Evidence for some methanogenic activity, as indicated by PME concentrations, was noted in the overlying Lidah claystones, as well as within the coarse-grained reservoir facies. However, on the basis of PME concentrations, we consider the Paciran claystones to be the primary source beds for bacterial methane.

Some observations concerning the origin of BNH can be made from this data set. BNH occurs in very high abundance in a few samples, and is totally absent from several others. It is consistently present in carbonates of the Paciran Limestone unit, which were deposited in a low energy, anoxic, deep marine, pelagic setting. There is no direct correlation between the abundance of BNH and PME, indicating no direct affinity of BNH and archaebacteria. Previous studies by Schoell et al. (1992) using compound specific isotope analysis of BNH and other biomarkers, similarly showed no obvious re- lationship of BNH and known archaebacterial lipids. Schoell and coworkers suggested that BNH was derived from chemoautotrophic bacteria which utilized 13C-depleted substrates. Although no isoto-

pic data were available for the BNH in our Paciran samples, an autochthonous source that thrived in deep water carbonate settings, seems probable for BNH.

Charge history o f Terang-Sirasun fieM

The information obtained from biomarker geochemistry on probable source bed horizons was integrated with geological observations to recon- struct the events which led to the charging of the Terang-Sirasun gas field. Variables that were of interest to our overall understanding of this bac-

.terial gas system included sedimentation rates and temperature history of the source beds, relative timing of source/reservoir/seal deposition, the duration of methanogenesis after source bed deposition, and the influence of erosional processes in disrupting the accumulation of gas.

A burial history diagram for the region was con- structed using average formation thicknesses and lithologies encountered in the nine wells drilled on the Terang-Sirasun structural ridge (Fig. 8). Age constraints were provided by detailed biostratigraphic analyses. An average geothermal gradient of 45°C/km in the Late Miocene and younger section was determined from drillstem test temperature measurements in several wells. This corresponded to

Hydrocarbon charge of a bacterial gas field

Geolog ica l T ime(Ma) 4 2

311

500 E , , . _ , .

..E

(D r~

1000

Fig. 8. Burial history diagram for the Terang-Sirasun structural ridge. Average thicknesses, lithologies, ages and temperatures from nine wells were used to construct the diagram (see text). Isotherms (10°C

intervals) are shown as dashed lines. Sedimentation rates are shown in m/Ma for the major units.

an average regional heat flow of 1.35 H F U (57 roW/m2).

The burial history model was used to obtain information about the likely duration of methanogenesis and the timing of hydrocarbon fill. The inferred claystone and marly source beds of the PacSST and PacLST units were deposited at sedimentation rates of approximately 100 to 200 m/Ma. Deposition of these source beds took place at various times between 8 and 1.3 Ma. Reservoir facies were deposited within the same time period, the sandstones being laid down at rates similar to those of the source beds (100-200 m/Ma), whereas the pelagic limestone facies were typically laid down at slower rates (10-20 m/Ma). Source bed temperatures along the structural ridge remained below 60°C throughout the burial history, and hence the survival of methanogens would not be limited by thermal constraints (Rice and Claypool, 1981; Whiticar, 1994 and references therein). A more important limi- tation to methanogenic activity could potentially be the supply of adequate nutrients. However, this does not appear to be the case, since present day TOC levels of around 1% in the claystones indicate that sufficient quantities of metabolizable plant-derived organic matter probably still remain in the source beds. Additional CO2 for methanogenesis may also have been supplied from diagenetic reactions taking place deeper in the section from the underlying Early Miocene-Eocene interval. Hence, it is probable that methanogenesis in the Paciran section would not have been limited by nutrient

supply or excessively high temperatures. We consider it likely that methanogenesis has continued over the entire period of source/reservoir deposition. In support of this scenario of prolonged bacterial activity, ODP studies have shown that methanogens can continue to survive in sediments that are more than 4 Ma old at burial depths in excess of 500 m (Cragg et al. , 1992).

The most critical issues to the accumulation of significant quantities of gas at Terang Sirasun are the timing and integrity of the top seal. Several fine-grained facies with adequate sealing properties are interbedded within the coarser grained reservoirs. As discussed above, some of these fine- grained rocks are the primary source beds for the gas. However, their geometry and lateral continuity indicate that they do not provide the ultimate top seal for the gas field. The overall top seal for the accumulation was laid down after deposition of the PacLST unit, which ended at ca. 1.3 Ma. Seismic data, supported by core information, has shown that the period from 1.3 to 0.5 Ma was one of extreme sediment instability. Sediments that were deposited along the structural ridge during this time were catastrophically removed by submarine slumping and other erosional processes. These fine- grained deposits may have provided a temporary seal for the gas accumulation, but any gas trapped prior to 0.5 Ma would probably have been lost to the surface during the slumping event. The effective top seal which exists today was deposited over a very short time period from about 0.44 Ma to pre-

312 R .A . Noble and F. H. Henk Jr

FiME1 =0:6 Ma I 8ROMO-1 SlRASUN-2

s I I FUTURE N

FIME;

SLIDE MASSES MOVING BASINWARD

• g ; , : ; : ,11,

]TIME 3 = present] BROMO-, SIRASUN-2

SCAR ~ ~ INFILL [ J ~ , , . - - - ' - - " ' ~

t ;,=~. , '

Fig. 9. Filling history model at three time slices across a N-S section of the gas accumulation in the Sirasun area. TIME 1 = 0.6 Ma. Just prior to an enormous submarine slide event. An older top seal (#1) exists over the entire closure. Gas fills down to northern spill point, sourced from interbedded claystones of the Paciran Sandstone interval. TIME 2 = 0.5 Ma. Submarine slide breaches seal and gas escapes to surface. TIME 3 = present day. A new seal (#2) has been deposited from 0.44 Ma to present over the Paciran reservoir. Gas refills the trap from surrounding claystone source beds, which have con-

tinued to yield gas over a prolonged period.


sent at an extremely high sedimentation rate of 1600 m/Ma.

The geological events leading to the accumulation of gas in the Sirasun area are shown schematically in Fig. 9. A similar set of events, but on a reduced scale, occurred in the Terang area, which lies to the west of Sirasun (Fig. 3). Prior to about 0.6 Ma, source beds within the Paciran unit charged the reservoir sands and limestones, which were sealed at that time by late Pleistocene claystones. Massive submarine slides at around 0.6-0.45 Ma removed the poorly consolidated seal facies, exposing the reservoir to the sea floor. A new seal was then laid down at 0.44 Ma over the entire section, and the reservoir was recharged with gas. Based on the geometry of source/reservoir/seal units over the structure, the original Paciran source beds must have been largely responsible for once again filling the reservoir with bacterial gas. Where the bed geome- tries are favorable, some gas may also have been derived from the overlying sealing claystones them- selves, although these claystones have only small amounts of methanogen biomarkers and have low TOC values. The results of this case study indicate that large accumulations of bacterial methane are not necessarily restricted to early entrapment of methane from freshly deposited marine sediments. Traps may be filled with bacterial gas long after initial deposition of source beds, provided that the required conditions for active methanogenesis are maintained throughout this period. In the case of Terang-Sirasun, a prolonged period of methanogenesis continued for several million years after the initial deposition of source beds. This continuous gas charge was sufficient to emplace 1 TCF of gas in a trap which only attained its final configuration less than half a million years ago.

CONCLUSIONS

The gas reservoired in the Terang-Sirasun Field, which contains over 99.5% methane, was generated exclusively by bacterial processes. The primary source beds for the methane are interpreted to be claystones and other fine-grained lithofacies that are interbedded within the reservoir section. TOC levels in these claystones are about 1%, the organic matter being derived primarily from land plants. Specific source intervals were identified based on their elevated concentrations of PME and other acyclic isoprenoids that are biomarkers for methanogenic archaebacteria. BNH was also found in some samples, although it appears that the origin of this compound is not related to methanogens, but rather to some other autochthonous source that thrived in deep water carbonate settings. Burial history analysis shows that the gas source beds were laid down at sedimentation rates of 100-200 m/Ma. An earlier seal that trapped the bacterial gas was

lost during a catastrophic submarine slumping event. A second seal was laid down at an extremely high sedimentation rate of 1600m/Ma, and the reservoir was recharged with bacterial gas. The geometry of source/reservoir/seal over the Terang- Sirasun structural ridge indicate that the older source beds must have continued to generate gas, possibly supplemented in places by gas from younger strata. The case study provides evidence that methanogenesis can continue for extended periods of time (in the range of 5-6 Ma) and over considerable depths (up to 1000m) provided that conditions for methanogenesis are maintained.

Acknowledgements--We would like to express our appreci- ation to the ARCO Bali North exploration team, who were responsible for the discovery and delineation of the Terang-Sirasun gas field. The exploration team includes Bo Henk, Dharmawan Samsu, Wayne Basden, Maria Rahardja, Sudarta, and Chris Ponto. We are also grateful to Professor Steve Rowland of the University of Plymouth for reference compounds and for proprietary analytical work. Technical support was provided by Rick Tharp and David Stansbury (Baseline Resolution Inc.) and Ann Fincannon and Susan Singletary (ARCO Exploration and Production Technology). Technical and editorial reviews which greatly improved the manuscript were provided by Jim Howes (ARCO Indonesia) and two EAOG reviewers. Finally, we thank the managements of ARCO Indonesia, ARCO International and PERTAMINA for their support and encouragement, and for their permission to publish this paper.

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APPENDIX

Structures

II

III

hydrocarbon charge of a bacterial gas field by prolonged methanogenesis- an example from the east...

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