Vol. 1 - 789

IPA01-G-032

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Twenty-Eighth Annual Convention & Exhibition, October 2001

GAS GEOCHEMISTRY – A KEY TO UNDERSTANDING FORMATION

AND ALTERATION PROCESSES

Barry J. Katz*

ABSTRACT Globally there has been growing interest in gas exploration. A better understanding of the processes associated with the formation and alteration of gas can result in more efficient exploration. This paper examines these processes and how geochemistry can be used to decipher a gas accumulation’s history. Gas has multiple formation mechanisms. The mode of formation is reflected in a gas’ bulk and isotope geochemistry. Examples are used to show how geochemical attributes can establish the mode of formation and alteration history. For example, with some limitations the significance of biogenic gas contributions can be estimated using the isotopic discordance of methane and the wet gas components. When a biogenic contribution is present methane appears isotopically lighter than would be anticipated. The presence of a biogenic component is significant in that it addresses the timing of trap development. The importance of primary vs. secondary cracking can also be established using the differences in the isotopic composition and the relative abundance of ethane and propane. This information and the estimated thermal maturity of the gas based on its isotope composition is key to establishing the source of the gas. Just like oil, gas may undergo significant alteration through both water washing and biodegradation. Water washing is established through increases in gas wetness and in the iC4/nC4 ratio. Biodegradation decreases gas wetness and results in isotopically heavier C3, nC4, and nC5. These processes not only alter gas composition but may reduce the volume of gas present. ___________________________________________________________ * Texaco Group Inc., USA

Non-hydrocarbon components, principally CO2 and N2, can be significant in the region. Non-hydrocarbon content varies among basins and fields, and within individual fields. These components may also have multiple origins and their presence decreases the value of an individual accumulation. Their origin is established through the integration of isotope and geologic data.

INTRODUCTION

There has been a growing interest in gas exploration. In large part, this interest has developed as a result of energy consumption projections. Gas demand is projected to nearly double by 2020 from 1999 usage levels (Energy Information Administration, 2001). Within the industrialized world nearly half of the projected increase in total energy use is associated with natural gas. These same projections expect the largest growth in natural gas consumption in Latin America and in Asia. This increase in consumption has resulted for a number of reasons including environmental, the need for fuel diversification and/or energy self-sufficiency issues, as well as market factors. With this growing interest in gas exploration, many of the questions typically associated with oil exploration are now being asked, including those associated with hydrocarbon charge. The number of available geochemical tools is, however, limited relative to oil systems because of the limited number of compounds present and their simplicity. Nevertheless, the number of tools available is growing along with the associated need. The focus of this paper will be on the tools available to examine a gas accumulation’s history, including origin, thermal maturity, and alteration of the hydrocarbon components and the origin of non-hydrocarbon components. Examples, largely from Indonesia, Southeast Asia, and Australasia, are used as a means of illustrating the

© IPA, 2006 - 28th Annual Convention Proceedings, 2002

Vol. 1 - 790

different tools available. A better understanding of the processes associated with generation, accumulation, and destruction of natural gas deposits is of particular importance in the area because nine of the fifteen largest petroleum systems are dominated by gas, based on a barrels of oil equivalent basis (Howes, 1997).

ORIGIN Unlike oil there are several modes of formation for commercial gas accumulations. Gas may form at low temperatures though a series of biochemical reactions acting on both sedimentary organic matter (Rice and Claypool, 1981) and accumulated hydrocarbons (Pallasser, 2000), at more elevated temperatures through the cracking of kerogen, and at even higher temperatures through the cracking of oil. Although the different kerogen types are often referred to as oil- and gas-prone, both kerogen types will yield gas within the main phase of hydrocarbon generation. Burnham and Sweeney (1991) note that type I and type II kerogens will yield, through primary cracking, about 65% of the methane that type III kerogen yields and can ultimately yield larger volumes of gas if oil cracking is considered. Biogenic gas accumulations are both locally and globally significant; at least 20% of known gas reserves may be biogenic in origin (Rice, 1992). Biogenic gas typically contains less than 0.2% ethane (Schoell, 1983). Primary biogenic methane is isotopically light, the ä13C values are typically less than –60‰ (i.e., more negative). Exceptions have, however, been noted where isotopically heavy biogenic methane is present. These isotopically heavier biogenic gas accumulations have been termed “secondary” having formed through the biodegradation of preexisting hydrocarbon accumulations (Pallasser, 2000). There are two primary mechanisms of methanogenesis, CO2 reduction and fermentation. These two mechanisms can be differentiated by the methane’s deuterium content (Whiticar et al., 1986; and Whiticar, 1999). Isotopically lighter (δD < -200‰) methane forms through fermentation. Isotopically heavier (δD > -200‰) methane forms through CO2 reduction. Methane production through fermentation typically occurs earlier in the diagenetic sequence than methanogenesis via CO2 reduction. It has also been suggested that fermentation dominates in freshwater systems, with CO2 reduction dominating in marine

systems (Whiticar et al., 1986). Gas accumulations within the Powder River Basin (Wyoming) have been cited as examples of the rare commercial occurrences of biogenic gas formed through fermentation (Law et al., 1991). Available data suggest that several accumulations in Indonesia and Australasia have a biogenic origin (e.g., Terang-Sirasun, Madura Basin; East Java and Niengo, Waipoga Basin, New Guinea) and that additional biogenic gas accumulations may also exist in the region (Baylis et al., 1997; Lowry et al., 1998; and Dobson et al., 1998). Thermogenic gas may be either wet (C2+ > 5%) or dry depending on the level of thermal maturity. Dry gases are associated with higher levels of thermal stress. Thermogenic methane will typically display a stable carbon isotope composition greater than –55‰ (Schoell, 1980), with values increasing with increasing thermal maturity. As noted above, thermogenic gas may be derived directly from kerogen (primary) or through the (secondary) cracking of crude oil. Work by Lorant et al. (1998) has suggested that the relationship between the C2/C3 ratio and the difference in the carbon isotopic composition of ethane and propane can be used to differentiate between primary and secondary cracking. Wiese and Kvenvolden (1993) suggest that the thermal cracking of higher molecular weight hydrocarbons to methane begins at temperatures of ~150oC. A review of available data reveals that both primary and secondary cracking are active in the region (Figure 1). As a result of the distribution of organic matter in many basins and the low threshold requirements for potential gas source rocks, many individual gas accumulations may have multiple sources (i.e., a mixed gas). Gas source rocks may only need to contain between 0.1 and 0.5% organic carbon to be effective (Rice and Claypool, 1981 and Clayton, 1992). One method proposed to identify the presence of multiple sources for a gas is the use of Chung et al.’s (1988) “natural gas plot” where the carbon isotope ratio is plotted against the inverse carbon number (Figure 2). If unaltered, a gas accumulation in which all of the gas components are co-generated results in a linear plot. If there is a biogenic gas contribution, the methane will appear isotopically light relative to the C2+ components. Chung et al. (1988) suggested that the deviation from the wet gas (C2 to nC5) trend could be used to estimate the actual biogenic methane contribution. This is accomplished

Vol. 1 - 791

by projecting the isotope trend. This projected value establishes the isotopic composition of the “pure” thermogenic methane formed in association with the wet gas components. This estimated isotope value is then used to calculate the biogenic gas contribution assuming a “pure” end-member biogenic gas value of approximately -70‰. If the methane appears isotopically heavier than would be suggested by the C2+ components it implies a second thermally more mature source. However, unlike the biogenic gas no estimate of the relative contribution of the second source is possible because an isotope end-member is not available. A nonlinear character for the C2+ components may also indicate multiple sources for the wet gas components. Clayton et al. (1997) has, however, suggested that deviations from this “normal” trend may also result from the diffusion of methane and to a lesser extent ethane and propane. Diffusion results in the isotopic enrichment of the residual gas and depletion in the diffused product. Prinzhofer and Pernaton (1997) have suggested that the effects of diffusive fractionation are sufficient to cause some thermogenic gas accumulations to be misidentified as biogenic. THERMAL MATURITY

The isotopic composition of individual gas components is a function of thermal maturity and the nature of the original starting material. Individual gas components become isotopically heavier with increasing thermal maturity. The magnitude of the increase decreases with increasing carbon number. Stahl (1977) proposed a pair of relationships between the methane carbon isotope composition and vitrinite reflectance, one for coal-derived methane and a second for gas derived from a sapropelic source. Berner and Faber (1988 and 1997) also provided empirical relationships between the carbon isotopic composition of methane, ethane, and propane and vitrinite maturity level for different organic matter types (Figure 3). These empirical relationships could, therefore, be used to estimate the thermal maturity of the effective source rock system for each component. James (1983) proposed a means of estimating thermal maturity of a co-generated wet gas using the difference in isotopic composition of the C1 through C5 components. Clayton (1991) proposed a similar means of estimating thermal maturity using the difference in isotope composition among the different gas components. Boreham et al. (2001) suggest,

however, that although there is an increase in isotopic composition for the individual gases with increasing maturity the proposed correlations with thermal maturity may not be universally valid. When the gas is unaltered (see discussion below) and co-generated, the estimated thermal maturity level for each component is approximately the same. However, as noted above an individual gas accumulation may have multiple sources and the gas may not be co-generated. Under such circumstances, the thermal maturity estimates for the different components vary. If the maturity estimate for methane is less than that of either ethane or propane it may indicate that there has been a biogenic gas contribution. When the methane thermal maturity value estimate is greater than that of the wet gas components, a more mature source rock system is inferred. In either case, as a result of the mixing, the estimated methane maturity values are not representative of a specific generative interval. In addition to thermal maturity, Rooney et al. (1995) suggested that the isotopic composition of the C1 through C3 components could be used to estimate the temperature of gas generation. This is accomplished using the difference in isotopic composition of ethane and methane (ä13C2- ä

13C1) and propane and methane (ä13C3- ä13C1) for those samples in which the gases appear to have been derived solely by thermogenic processes. Using this approach Rooney estimated gas generation temperatures for Gorgon gas (Carnarvon Basin, Australia) between 185 and 190oC (Figure 4). In some instances the estimated temperatures based on the ä13C2- ä13C1 values are slightly lower than those calculated based on the ä13C3- ä13C1 value. These differences may be the result of either different temperatures of generation for ethane and propane or they may be a result of the kerogen character. Rooney et al. (1995) noted that the differences between ä13C1 and ä13C2 are greater for gases derived from deltaic or coaly sequences than from marine sequences. In addition to changes in isotopic composition there are also changes in gas composition associated with increases in thermal maturity. For example, the iC4/nC4 ratio decreases with increasing thermal maturity as a result of the generation of nC4, and remains constant at ~0.75 once thermal maturity levels equivalent to the “oil-window” are obtained (Connan and Cassou, 1980).

Vol. 1 - 792

ALTERATION

The methods outlined above make an assumption that the gases have not been altered. Just as with a crude oil accumulation, a gas accumulation may undergo alteration through several different processes, including biodegradation and water washing. And, just as when examining an oil accumulation, both processes result in a regular series of changes which can be used to identify the process and its level of intensity.

Biodegradation Bacterial alteration of wet gases typically appears as the preferential removal of the C3+ components, with the normal alkanes being more susceptible to microbial attack than the corresponding iso-alkanes. Consequently, there is a well-defined progression in changes in gas character and composition. With the onset of microbial alteration 12C is preferentially removed from propane, n-butane, and n-pentane resulting in isotopically enriched wet gas components (Figure 5). James and Burns (1984) suggest that this isotopic enrichment may exceed 20 ‰. Microbial alteration of methane can also occur. It is, however, much more difficult to recognize compared to that of the wet gas components (James and Burns, 1984). The major indicator of such alteration may reside in the deuterium content of the gas. Methane-oxidizing bacteria fractionate both the carbon and hydrogen in methane. The residual methane is isotopically enriched in both 13C and 2D compared to the initial methane (Coleman et al., 1981). The change in the deuterium content is significantly greater than that of 13C. This enrichment can result in very heavy deuterium values (δ2D > -100 ‰). It is interesting to note that Coleman et al. (1981) suggested that as a result of this enrichment the mode of formation and/or thermal maturity of the source may be misinterpreted (Coleman et al., 1981), i.e., the methane would appear more mature. Schoell (1983) suggested, however, that oxidation of commercial quantities of methane is unlikely. As biodegradation proceeds, the normal alkanes are slowly eliminated. This results in an increase in the iC4/nC4 ratio. This may result in a gas composition that appears to be thermally immature. In extreme cases, the preferential removal of the wet gas

components can also result in a significant decrease in gas wetness. In those cases where both oil and gas co-exist, microbial alteration or degradation of a wet gas need not be associated with biodegradation of an associated crude oil since different bacterial populations may be involved (James and Burns, 1984).

Water Washing

Gas wetness may increase through water washing as a result of the preferential removal of methane. Methane solubility is about 3, 13, and 50 times greater than ethane, propane, and n-butane, respectively (McAuliffe, 1979). Water washing in the Bonaparte Basin (Australia) may aid in explaining some of the basin’s elevated (> 35%) gas wetness values. Newell (1999) further suggested that water washing was a major destructive process for hydrocarbon accumulations within the northern Bonaparte Basin. He suggested hydrocarbon volume reductions in gas/condensate fields may be as much as 90%.

NON-HYDROCARBON COMPONENTS

An understanding of non-hydrocarbon gases is of economic importance because of both the associated reduction in BTU content and the acidic nature of many of these gases, which results in a significant increase in production costs, through the increased costs of tubular goods and the need for pre-processing prior to going market.

Carbon Dioxide

Within the Australasian region CO2 may be considered the major contaminant, and may in fact represent the dominant gas species in an accumulation. For example, in the Natuna D Alpha gas field an estimated 157 TCF of CO2 is present, representing 71% of the gas in-place (Dunn et al., 1996). Carbon dioxide may be introduced into a petroleum system through a number of different means. These include derivation from an organic source, the decomposition of carbonate minerals through catalysis with clays, hydrolysis, high temperature processes, as well as through mantle degassing. The source and means of formation are considered important because it has been suggested that volumetrically important carbon dioxide is

Vol. 1 - 793

typically derived from outside of the petroleum system. Although imperfect, the stable carbon isotopic composition of CO2 is the primary means to differentiate among the different modes of formation. Thrasher and Fleet (1995) have presented a general interpretation scheme. They suggest that isotopically light CO2 (δ13CCarbon Dioxide < -10‰) is derived from organic matter through maturation. δ13C values between -4 and -7‰ are typically associated with mantle degassing. Isotope ratios associated with carbonate decomposition partially overlap those of mantle degassing (–10 and 2‰) and reflect that of the carbonate minerals. Multiple formation processes may be active in a basin. For example, in the Carnarvon Basin the carbon isotopic composition reveals multiple origins for CO2 (Figure 6). Lower concentrations of the gas are associated with isotopically lighter values suggesting an organic origin. Isotopically heavier values are associated with the higher CO2 concentrations and are thought to have an inorganic origin. A further examination of this dataset suggests that with increasing depth (and temperature) the CO2 content generally increases suggesting that an organic background signal is being masked by an increasing inorganic contribution (Figure 7). Such a relationship would be consistent with either in situ CO2 generation or increasing proximity to a basal source for the gas. Although the Carnarvon Basin dataset suggests that CO2 content behaves in a regular fashion and is predictable, other data from the region reveals the complexity of the problem. For example, in the Andaman Sea Miocene Play, CO2 content ranged from ~10% in Yetagun to ~40+% in Myeil to ~90% in Yemanhnuang (Imbus et al., 1998). Another example of this complexity is revealed in the distribution of carbon dioxide content offshore Sarawak, which shows no regular pattern with depth, but appears to be in large part associated with reservoir lithology. Idris (1992) noted that elevated CO2 contents were associated with limestone reservoirs and overlying sandstone reservoirs. The lack of a clear depth trend in this dataset suggests that low temperature processes are responsible. Nitrogen Another common contaminant is nitrogen. It does not

typically display a correlation with carbon dioxide suggesting separate origins and/or different timing of generation (Boreham et al., 2001). Nitrogen may be derived through the thermal evolution of kerogen, the release from igneous and metamorphic rocks, oxidation of ammonia, or it may represent contamination (i.e., the presence of trace quantities of air) associated with the introduction of meteoric waters or introduced during sampling. When elevated concentrations of non-contaminant nitrogen are present they are often associated with coals and elevated temperatures (Krooss et al., 1993). Within the region, elevated nitrogen contents were observed in several of the Australian basins (Boreham et al., 2001) as well in the Andaman Sea (Lepage, 1998) and offshore Sarawak (Idris, 1992). Nitrogen accumulations within these fields may represent more than 50% of the accumulated gas.

SUMMARY AND CONCLUSIONS

Exploration for natural gas is growing in importance as a result of increased demand. This increase in exploration requires a better understanding of how gas accumulations form and change through time. The relative simplicity of gas compared to that of oil limits that amount of information that can be derived from a given accumulation. However, the molecular and isotopic composition of a gas can be used to infer its mode of formation (biogenic versus thermogenic; and primary versus secondary cracking) and the level of thermal maturity of the generative sequence. Such information can have a major impact on determining the relative timing of gas generation and trap development. It can also be used to indicate whether the gas has been altered through biodegradation and/or water washing. Both processes have the potential to significantly reduce available resources. Non-hydrocarbon components, particularly CO2, are present within the region, often in significant concentration. The available data reveal multiple origins acting within individual basins and individual fields complicating their predictability.

ACKNOWLEDGEMENTS

The author would like to thank Texaco Group Inc. for permission to present this work. Coleman Robison, Bob Davis, and Mark Chamberlain read an earlier version of this manuscript. Their comments and suggestions are appreciated.

Vol. 1 - 794

REFERENCES CITED Baylis, S.A., Cawley, S.J., Clayton, C.J., and Savell, M.A., 1997. The origin of unusual gas seeps from onshore Papua New Guinea. Marine Geology, v. 137, p. 109-120. Berner, U., and Faber, E., 1988. Maturity related mixing model for methane, ethane and propane, based on carbon isotopes. Organic Geochemistry, v. 13, p. 67-72. Berner, U., and Faber, E., 1997. Carbon isotope/maturity relationships for gases from algal kerogens and terrigenous organic matter. Geologische Jahrbuch, v. 103D, p. 129-145. Boreham, C.J., Hope, J.M., and Hartung-Kagi, B., 2001. Understanding source, distribution and preservation of Australian natural gas: A geochemical perspective. Australian Petroleum Production & Exploration Association Journal, v. 41, p. 523-547. Burnham, A.K., and Sweeney, J.J., 1991. Modeling the maturation and migration of petroleum. Foster, N.H. and Beaumont, E.A. (eds) Source and Migration Processes and Evaluation Techniques. American Association of Petroleum Geologists (Tulsa), p. 55-63. Chung, H.M., Gormly, J.R., and Squires, R.M., 1988. Origin of gaseous hydrocarbons in subsurface environments: Theoretical considerations of carbon isotope distribution. Chemical Geology, v. 71, p. 97-103. Clayton, C., 1991. Carbon isotope fractionation during natural gas generation from kerogen. Marine and Petroleum Geology, 8:232-240. Clayton, C., 1992. Source volumetrics of biogenic gas generation. Vially, R. (ed) Bacterial Gas. Éditions Technip (Paris), p. 191-204. Clayton, C.J., Hay, S.J., Baylis, S.A., and Dipper, B., 1997. Alteration of natural gas during leakage from a North Sea salt diapir field. Marine Geology, v. 137, p. 69-80. Coleman, D. D., Risatti, J.B. and Schoell, M., 1981. Fractionation of carbon and hydrogen isotopes by

methane-oxidizing bacteria. Geochemica et Cosmochimica Acta, 45:1033-1037. Connan, J., and Cassou, A.M., 1980. Properties of gases and petroleum liquids derived from terrestrial kerogen at various maturation levels. Geochimica et Cosmochimica Acta, v. 44, p. 1-23. Dobson, P.B., Rahardjo, T., Atallah, C.A., Frasse, F.I., Specht, T.D., Djamil, A.S., Marhardi, Netherwood, R.E., and Montaggioni, P.J.M., 1998. Biogenic gas exploration in Miocene carbonate, West Sumatra, Indonesia. Proceedings of the Indonesian Petroleum Association, v. 26/1, p. 343. Dunn, P.A., Kozar, M.G., and Budiyono, 1996. Application of geoscience technology in a geologic study of the Natuna gas field, Natuna Sea, offshore Indonesia. Proceedings of the Indonesian Petroleum Association, v. 25/1, p. 117-128. Howes, J.V.C., 1997. Petroleum resources and petroleum systems of SE Asia, Australia, Papua New Guinea, and New Zealand. Proceedings of the Indonesian Petroleum Association Conference on Petroleum Systems of SE Asia and Australasia, p. 81-100. Idris, M.B., 1992. CO2 and N2 contamination in J32-1, SW Luconia, offshore Sarawak. Geological Society Malaysia Bulletin, v. 32, p. 239-246. Imbus, S.W., Wind, F.H., and Ephraim, D., 1998. Origin and occurrence of CO2 in the eastern Andaman Sea, offshore Myanmar. Proceedings Indonesian Petroleum Association Conference on Gas Habitats of SE Asia and Australasia, p. 99-111. James, A.T., 1983. Correlation of natural gas by use of carbon isotopic distribution between hydrocarbon components. American Association of Petroleum Geologists Bulletin, v. 67, p. 1176-1191. James, A.T., and Burns, B.J., 1984. Microbial alteration of subsurface natural gas accumulations. American Association of Petroleum Geologists Bulletin, v. 68, p. 957-960. Krooss, B.M., Lillack, J., and Leythaeuser, D., 1993. Nitrogen-rich natural gases. Erdoel Kohle -Erdgas-Petrochem, v. 46, p. 271-276.

Vol. 1 - 795

Law, B.E., Rice, D.D., and Flores, R.M., 1991, Coalbed gas accumulations in the Paleocene Fort Union Formation, Powder River Basin, Wyoming. Schwochow, S.D., Murray, D.K., and Fahy, M.F. (eds) Coalbed Methane of Western North America, Rocky Mountain Association of Geologists (Denver), p. 179-190. Lepage, A., 1998. Myanmar production meets first-gas targets. Oil and Gas Journal, v. 96/36, p. 88-90, 92-94. Lorant, F., Prinzhofer, A., Behar, F., and Huc, A.Y., 1998. Carbon isotopic and molecular constraints on the formation and the expulsion of thermogenic hydrocarbon gases. Chemical Geology, v. 147, p. 249-264. Lowry, D.C., Francis, D.A., and Bennett, D.J., 1998. Biogenic gas, a new play in the East Coast Basin of New Zealand. Proceedings, 1998 New Zealand Petroleum Conference Proceedings, p. 207-221. McAuliffe, C.D., 1979. Oil and gas migration: Chemical and physical constraints. Roberts, W.H., III and Cordell, R.J. (eds) Problems of Petroleum Migration. American Association of Petroleum Geologists (Tulsa), Studies in Geology, v. 10, p. 89-107. Newell, N.A., 1999. Water washing in the northern Bonaparte Basin. Australian Petroleum Production & Exploration Association Journal, v. 39/1, p. 227-247. Pallasser, R.J., 2000. Recognising biodegradation in gas/oil accumulations through the ä13C compositions of gas components. Organic Geochemistry, v. 31;p. 1363-1373. Prinzhofer, A. and Pernaton, É., 1997. Isotopically light methane in natural gas: bacterial imprint or diffusive fractionation? Chemical Geology, v. 142; p. 193-200. Rice, D.D., 1992. Controls, habitat, and resource potential of ancient bacterial gas. Vially, R. (ed) Bacterial Gas. Éditions Technip (Paris), p. 13-24.

Rice, D.D. and Claypool, G.E., 1981. Generation, accumulation, and resource potential of biogenic gas. American Association of Petroleum Geologists Bulletin, v. 65, p. 5-25. Rooney, M.A., Claypool, G.E., and Chung, H.M., 1995. Modeling thermogenic gas generation using carbon isotope ratios. Chemical Geology, v. 126, p. 219-232. Schoell, M., 1980. The hydrogen and carbon isotopic composition of methane from natural gases of various origins. Geochimica et Cosmochimica Acta, v. 44, p. 649-661. Schoell, M., 1983. Genetic characterization of natural gases. American Association of Petroleum Geologists Bulletin, v. 67, p. 2225-2238. Stahl, W.J., 1977. Carbon and nitrogen isotopes in hydrocarbon research and exploration. Chemical Geology, v. 20, p. 121-149. Thrasher, J., and Fleet, A.J., 1995. Distribution, origin and prediction of carbon dioxide in petroleum reservoirs. American Association of Petroleum Geologists Bulletin, v. 79, p. 1252 (abstract). Whiticar, M.J., 1999. Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane. Chemical Geology, v. 161, p. 291-314. Whiticar, M.J., Faber, E., and Schoell, M., 1986. Biogenic methane formation in marine and freshwater environments: CO2 reduction vs. acetate fermentation – isotope evidence. Geochimica et Cosmochimica Acta, v. 50, p. 693-709. Wiese, K., and Kvenvolden, K.A., 1993. Introduction to microbial and thermal methane. Howell, D.G., Wiese, K., Fanelli, M., Zink, L. and Cole, F. (eds) The Future of Energy Gases. U.S. Geological Survey (Washington, D.C.), Professional Paper 1570, p. 13-20.

Vol. 1 - 796

FIGURE 1 - Relationship between the C2/C3 ratio and the difference in stable carbon isotope of ethane and propane used to establish whether the gas was derived through primary or secondary cracking (after Lorant et al., 1998). Gas samples plotted are from the Central Sumatra (circle), Carnarvon (square), and Bonaparte (triangle) Basins.

Vol. 1 - 797

FIGURE 2 – “Natural gas plot” showing a co-generated gas (circle; Bonaparte Basin), a mixed biogenic and thermogenic gas (square; Central Sumatra Basin), and a mixed gas with a portion of the methane having been derived from a source with a more advanced level of thermal maturity than the methane co-generated with the wet gas components (triangle; Bonaparte Basin).

Vol. 1 - 798

FIGURE 3 – Empirical relationship between isotope composition of methane, ethane, and propane and effective source rock thermal maturity (assuming an algal kerogen; after Berner and Faber, 1997).

Vol. 1 - 799

FIGURE 4 – Estimated generation temperatures for gases from the Gorgon Field, Carnarvon Basin, Australia (after Rooney et al., 1995).

Vol. 1 - 800

FIGURE 5 – Stable carbon isotope composition patterns associated with two biodegraded gas accumulations (circle-Bonaparte Basin and the square- Carnarvon Basin) and an unaltered gas from the Bonaparte Basin (triangle). Note the “saw-tooth” pattern of the biodegraded gases.

Vol. 1 - 801

FIGURE 6 – Stable carbon isotope composition of CO2 as a function of abundance in the outboard portion of the Carnarvon Basin, Australia. These data suggest that an organic background signal is present and is masked by increasing amounts of inorganic -derived CO2.

Vol. 1 - 802

FIGURE 7 – The observed relationship between CO2 abundance and depth in the outboard portion of the Carnavon Basin, Australia. The observed relationship suggests that CO2 is either a function of temperature within the basin or that there is a basal source.