gas geochemistry – a key to understanding formation

7/30/2019 GAS GEOCHEMISTRY – A KEY TO UNDERSTANDING FORMATION

http://slidepdf.com/reader/full/gas-geochemistry-a-key-to-understanding-formation 1/14

Vol. 1 - 789

IPA01-G-032

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

GAS GEOCHEMISTRY – A KEY TO UNDERSTANDING FORMATIONAND ALTERATION PROCESSES

Barry J. Katz*

ABSTRACT

Globally there has been growing interest in gasexploration. A better understanding of the processesassociated with the formation and alteration of gascan 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 modeof formation is reflected in a gas’ bulk and isotopegeochemistry. Examples are used to show howgeochemical attributes can establish the mode of formation and alteration history. For example, withsome limitations the significance of biogenic gascontributions can be estimated using the isotopicdiscordance of methane and the wet gas components.When a biogenic contribution is present methaneappears isotopically lighter than would be anticipated.

The presence of a biogenic component is significantin that it addresses the timing of trap development.The importance of primary vs. secondary crackingcan also be established using the differences in theisotopic composition and the relative abundance of ethane and propane. This information and theestimated thermal maturity of the gas based on itsisotope composition is key to establishing the sourceof the gas.

Just like oil, gas may undergo significant alterationthrough both water washing and biodegradation.

Water washing is established through increases in gaswetness and in the iC4/nC4 ratio. Biodegradationdecreases gas wetness and results in isotopicallyheavier C3, nC4, and nC5. These processes not onlyalter 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-hydrocarboncontent varies among basins and fields, and withinindividual fields. These components may also havemultiple origins and their presence decreases thevalue of an individual accumulation. Their origin isestablished 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 usagelevels (Energy Information Administration, 2001).Within the industrialized world nearly half of the

projected increase in total energy use is associatedwith natural gas. These same projections expect the

largest growth in natural gas consumption in LatinAmerica and in Asia. This increase in consumptionhas resulted for a number of reasons includingenvironmental, the need for fuel diversification and/or energy self-sufficiency issues, as well as marketfactors.

With this growing interest in gas exploration, many of the questions typically associated with oil explorationare now being asked, including those associated withhydrocarbon charge. The number of availablegeochemical tools is, however, limited relative to oil

systems because of the limited number of compounds present and their simplicity. Nevertheless, thenumber of tools available is growing along with theassociated need. The focus of this paper will be onthe tools available to examine a gas accumulation’shistory, including origin, thermal maturity, andalteration of the hydrocarbon components and theorigin of non-hydrocarbon components. Examples,largely from Indonesia, Southeast Asia, andAustralasia, are used as a means of illustrating the

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

Contents

Search



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 fifteenlargest 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 lowtemperatures though a series of biochemical reactionsacting on both sedimentary organic matter (Rice andClaypool, 1981) and accumulated hydrocarbons(Pallasser, 2000), at more elevated temperaturesthrough 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 gaswithin the main phase of hydrocarbon generation.Burnham and Sweeney (1991) note that type I andtype II kerogens will yield, through primary cracking,about 65% of the methane that type III kerogen yieldsand can ultimately yield larger volumes of gas if oilcracking is considered.

Biogenic gas accumulations are both locally andglobally significant; at least 20% of known gasreserves may be biogenic in origin (Rice, 1992).

Biogenic gas typically contains less than 0.2% ethane(Schoell, 1983). Primary biogenic methane isisotopically light, the ä13C values are typically lessthan –60‰ (i.e., more negative). Exceptions have,however, been noted where isotopically heavy

biogenic methane is present. These isotopicallyheavier biogenic gas accumulations have been termed“secondary” having formed through the

biodegradation of preexisting hydrocarbonaccumulations (Pallasser, 2000). There are two

primary mechanisms of methanogenesis, CO2 reduction and fermentation. These two mechanisms

can be differentiated by the methane’s deuteriumcontent (Whiticar et al., 1986; and Whiticar, 1999).Isotopically lighter (δD < -200‰) methane formsthrough fermentation. Isotopically heavier (δD > -200‰) methane forms through CO2 reduction.Methane production through fermentation typicallyoccurs earlier in the diagenetic sequence thanmethanogenesis via CO2 reduction. It has also beensuggested that fermentation dominates in freshwater systems, with CO2 reduction dominating in marine

systems (Whiticar et al., 1986). Gas accumulationswithin the Powder River Basin (Wyoming) have beencited as examples of the rare commercial occurrencesof biogenic gas formed through fermentation (Law etal., 1991). Available data suggest that severalaccumulations 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 mayalso exist in the region (Baylis et al., 1997; Lowry etal., 1998; and Dobson et al., 1998).

Thermogenic gas may be either wet (C2+ > 5%) or drydepending on the level of thermal maturity. Drygases are associated with higher levels of thermalstress. Thermogenic methane will typically display astable carbon isotope composition greater than –55‰(Schoell, 1980), with values increasing with

increasing thermal maturity. As noted above,thermogenic gas may be derived directly fromkerogen (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 isotopiccomposition of ethane and propane can be used todifferentiate between primary and secondarycracking. Wiese and Kvenvolden (1993) suggest thatthe thermal cracking of higher molecular weighthydrocarbons to methane begins at temperatures of ~150oC. A review of available data reveals that both

primary and secondary cracking are active in theregion (Figure 1).

As a result of the distribution of organic matter inmany basins and the low threshold requirements for

potential gas source rocks, many individual gasaccumulations may have multiple sources (i.e., amixed gas). Gas source rocks may only need tocontain between 0.1 and 0.5% organic carbon to beeffective (Rice and Claypool, 1981 and Clayton,1992). One method proposed to identify the presenceof multiple sources for a gas is the use of Chung et

al.’s (1988) “natural gas plot” where the carbonisotope ratio is plotted against the inverse carbonnumber (Figure 2). If unaltered, a gas accumulationin which all of the gas components are co-generatedresults in a linear plot. If there is a biogenic gascontribution, the methane will appear isotopicallylight 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 valueestablishes the isotopic composition of the “pure”thermogenic methane formed in association with thewet gas components. This estimated isotope value isthen used to calculate the biogenic gas contributionassuming a “pure” end-member biogenic gas value of

approximately -70‰. If the methane appearsisotopically heavier than would be suggested by theC2+ components it implies a second thermally moremature source. However, unlike the biogenic gas noestimate of the relative contribution of the secondsource is possible because an isotope end-member isnot 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 theresidual gas and depletion in the diffused product.Prinzhofer and Pernaton (1997) have suggested thatthe effects of diffusive fractionation are sufficient tocause some thermogenic gas accumulations to bemisidentified as biogenic.

THERMAL MATURITY

The isotopic composition of individual gascomponents is a function of thermal maturity and thenature of the original starting material. Individual gas

components become isotopically heavier withincreasing thermal maturity. The magnitude of theincrease decreases with increasing carbon number.

Stahl (1977) proposed a pair of relationships betweenthe methane carbon isotope composition and vitrinitereflectance, one for coal-derived methane and asecond for gas derived from a sapropelic source.Berner and Faber (1988 and 1997) also providedempirical relationships between the carbon isotopiccomposition of methane, ethane, and propane andvitrinite 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 thermalmaturity of a co-generated wet gas using thedifference in isotopic composition of the C1 throughC5 components. Clayton (1991) proposed a similar means of estimating thermal maturity using thedifference in isotope composition among the differentgas components. Boreham et al. (2001) suggest,

however, that although there is an increase in isotopiccomposition for the individual gases with increasingmaturity the proposed correlations with thermalmaturity 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 gasaccumulation may have multiple sources and the gasmay not be co-generated. Under such circumstances,the thermal maturity estimates for the differentcomponents vary. If the maturity estimate for methane is less than that of either ethane or propane itmay indicate that there has been a biogenic gascontribution. When the methane thermal maturityvalue estimate is greater than that of the wet gascomponents, a more mature source rock system is

inferred. In either case, as a result of the mixing, theestimated methane maturity values are notrepresentative 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 thetemperature of gas generation. This is accomplishedusing the difference in isotopic composition of ethaneand methane (ä13C2- ä

13C1) and propane and methane(ä13C3- ä13C1) for those samples in which the gasesappear to have been derived solely by thermogenic

processes. Using this approach Rooney estimated gasgeneration temperatures for Gorgon gas (CarnarvonBasin, Australia) between 185 and 190oC (Figure 4).In some instances the estimated temperatures basedon the ä13C2- ä13C1 values are slightly lower thanthose calculated based on the ä13C3- ä13C1 value.These differences may be the result of either differenttemperatures 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 derivedfrom deltaic or coaly sequences than from marine

sequences.

In addition to changes in isotopic composition thereare also changes in gas composition associated withincreases in thermal maturity. For example, theiC4/nC4 ratio decreases with increasing thermalmaturity as a result of the generation of nC4, andremains constant at ~0.75 once thermal maturitylevels equivalent to the “oil-window” are obtained(Connan and Cassou, 1980).



Vol. 1 - 792

ALTERATION

The methods outlined above make an assumption thatthe gases have not been altered. Just as with a crudeoil accumulation, a gas accumulation may undergoalteration 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 whichcan be used to identify the process and its level of intensity.

Biodegradation

Bacterial alteration of wet gases typically appears asthe preferential removal of the C3+ components, withthe normal alkanes being more susceptible to

microbial attack than the corresponding iso-alkanes.Consequently, there is a well-defined progression inchanges in gas character and composition. With theonset of microbial alteration 12C is preferentiallyremoved from propane, n- butane, and n-pentaneresulting in isotopically enriched wet gas components(Figure 5). James and Burns (1984) suggest that thisisotopic enrichment may exceed 20 ‰.

Microbial alteration of methane can also occur. It is,however, much more difficult to recognize comparedto that of the wet gas components (James and Burns,

1984). The major indicator of such alteration mayreside in the deuterium content of the gas. Methane-oxidizing bacteria fractionate both the carbon andhydrogen in methane. The residual methane isisotopically enriched in both 13C and 2D compared tothe initial methane (Coleman et al., 1981). Thechange in the deuterium content is significantlygreater than that of 13C. This enrichment can result invery heavy deuterium values (δ2D > -100 ‰). It isinteresting to note that Coleman et al. (1981)suggested that as a result of this enrichment the modeof formation and/or thermal maturity of the source

may be misinterpreted (Coleman et al., 1981), i.e., themethane would appear more mature. Schoell (1983)suggested, however, that oxidation of commercialquantities of methane is unlikely.

As biodegradation proceeds, the normal alkanes areslowly eliminated. This results in an increase in theiC4/nC4 ratio. This may result in a gas compositionthat appears to be thermally immature. In extremecases, the preferential removal of the wet gas

components can also result in a significant decrease ingas wetness.

In those cases where both oil and gas co-exist,microbial alteration or degradation of a wet gas neednot be associated with biodegradation of an associated

crude oil since different bacterial populations may beinvolved (James and Burns, 1984).

Water Washing

Gas wetness may increase through water washing as aresult of the preferential removal of methane.Methane solubility is about 3, 13, and 50 timesgreater than ethane, propane, and n-butane,respectively (McAuliffe, 1979). Water washing in theBonaparte Basin (Australia) may aid in explainingsome 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 northernBonaparte Basin. He suggested hydrocarbon volumereductions in gas/condensate fields may be as muchas 90%.

NON-HYDROCARBON COMPONENTS

An understanding of non-hydrocarbon gases is of economic importance because of both the associatedreduction in BTU content and the acidic nature of

many of these gases, which results in a significantincrease in production costs, through the increasedcosts of tubular goods and the need for pre-processing

prior to going market.

Carbon Dioxide

Within the Australasian region CO2 may beconsidered the major contaminant, and may in factrepresent the dominant gas species in anaccumulation. For example, in the Natuna D Alphagas 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 differentmeans. These include derivation from an organicsource, the decomposition of carbonate mineralsthrough catalysis with clays, hydrolysis, hightemperature processes, as well as through mantledegassing. The source and means of formation areconsidered important because it has been suggestedthat volumetrically important carbon dioxide is



Vol. 1 - 793

typically derived from outside of the petroleumsystem.

Although imperfect, the stable carbon isotopiccomposition of CO2 is the primary means todifferentiate among the different modes of formation.

Thrasher and Fleet (1995) have presented a generalinterpretation scheme. They suggest that isotopicallylight CO2 (δ13CCarbon Dioxide < -10‰) is derived fromorganic matter through maturation. δ

13C values between -4 and -7‰ are typically associated withmantle degassing. Isotope ratios associated withcarbonate decomposition partially overlap those of mantle degassing (–10 and 2‰) and reflect that of thecarbonate minerals.

Multiple formation processes may be active in a basin. For example, in the Carnarvon Basin the

carbon isotopic composition reveals multiple originsfor CO2 (Figure 6). Lower concentrations of the gasare associated with isotopically lighter valuessuggesting an organic origin. Isotopically heavier values are associated with the higher CO2 concentrations and are thought to have an inorganicorigin. A further examination of this dataset suggeststhat with increasing depth (and temperature) the CO2 content generally increases suggesting that an organic

background signal is being masked by an increasinginorganic contribution (Figure 7). Such a relationshipwould be consistent with either in situ CO2 generation

or increasing proximity to a basal source for the gas.

Although the Carnarvon Basin dataset suggests thatCO2 content behaves in a regular fashion and is

predictable, other data from the region reveals thecomplexity of the problem. For example, in theAndaman Sea Miocene Play, CO2 content rangedfrom ~10% in Yetagun to ~40+% in Myeil to ~90% inYemanhnuang (Imbus et al., 1998). Another exampleof this complexity is revealed in the distribution of carbon dioxide content offshore Sarawak, whichshows no regular pattern with depth, but appears to be

in large part associated with reservoir lithology. Idris(1992) noted that elevated CO2 contents wereassociated with limestone reservoirs and overlyingsandstone reservoirs. The lack of a clear depth trendin this dataset suggests that low temperature processesare responsible.

Nitrogen

Another common contaminant is nitrogen. It does not

typically display a correlation with carbon dioxidesuggesting separate origins and/or different timing of generation (Boreham et al., 2001). Nitrogen may bederived through the thermal evolution of kerogen, therelease 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 meteoricwaters or introduced during sampling. When elevatedconcentrations of non-contaminant nitrogen are

present they are often associated with coals andelevated temperatures (Krooss et al., 1993).

Within the region, elevated nitrogen contents wereobserved in several of the Australian basins (Borehamet al., 2001) as well in the Andaman Sea (Lepage,1998) and offshore Sarawak (Idris, 1992). Nitrogenaccumulations within these fields may represent more

than 50% of the accumulated gas.

SUMMARY AND CONCLUSIONS

Exploration for natural gas is growing in importanceas a result of increased demand. This increase inexploration requires a better understanding of howgas accumulations form and change through time.The relative simplicity of gas compared to that of oillimits that amount of information that can be derivedfrom 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 levelof thermal maturity of the generative sequence. Suchinformation can have a major impact on determiningthe relative timing of gas generation and trapdevelopment. It can also be used to indicate whether the gas has been altered through biodegradationand/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 significantconcentration. The available data reveal multiple

origins acting within individual basins and individualfields 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 andsuggestions 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 fromonshore Papua New Guinea. Marine Geology, v. 137,

p. 109-120.

Berner, U., and Faber, E., 1988. Maturity relatedmixing model for methane, ethane and propane, basedon carbon isotopes. Organic Geochemistry, v. 13, p.67-72.

Berner, U., and Faber, E., 1997. Carbonisotope/maturity relationships for gases from algalkerogens 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. Modelingthe maturation and migration of petroleum. Foster,

N.H. and Beaumont, E.A. (eds) Source and MigrationProcesses and Evaluation Techniques. AmericanAssociation of Petroleum Geologists (Tulsa), p. 55-63.

Chung, H.M., Gormly, J.R., and Squires, R.M., 1988.Origin of gaseous hydrocarbons in subsurfaceenvironments: Theoretical considerations of carbonisotope distribution. Chemical Geology, v. 71, p. 97-103.

Clayton, C., 1991. Carbon isotope fractionationduring natural gas generation from kerogen. Marineand Petroleum Geology, 8:232-240.

Clayton, C., 1992. Source volumetrics of biogenicgas 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 etCosmochimica Acta, 45:1033-1037.

Connan, J., and Cassou, A.M., 1980. Properties of gases and petroleum liquids derived from terrestrialkerogen 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, WestSumatra, Indonesia. Proceedings of the IndonesianPetroleum Association, v. 26/1, p. 343.

Dunn, P.A., Kozar, M.G., and Budiyono, 1996.Application of geoscience technology in a geologicstudy of the Natuna gas field, Natuna Sea, offshore

Indonesia. Proceedings of the Indonesian PetroleumAssociation, v. 25/1, p. 117-128.

Howes, J.V.C., 1997. Petroleum resources and petroleum systems of SE Asia, Australia, Papua NewGuinea, and New Zealand. Proceedings of theIndonesian Petroleum Association Conference onPetroleum 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 AndamanSea, offshore Myanmar. Proceedings IndonesianPetroleum Association Conference on Gas Habitats of SE Asia and Australasia, p. 99-111.

James, A.T., 1983. Correlation of natural gas by useof carbon isotopic distribution between hydrocarboncomponents. American Association of PetroleumGeologists Bulletin, v. 67, p. 1176-1191.

James, A.T., and Burns, B.J., 1984. Microbialalteration of subsurface natural gas accumulations.American Association of Petroleum GeologistsBulletin, 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 FortUnion 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 onthe formation and the expulsion of thermogenichydrocarbon 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 ZealandPetroleum Conference Proceedings, p. 207-221.

McAuliffe, C.D., 1979. Oil and gas migration:Chemical and physical constraints. Roberts, W.H., IIIand Cordell, R.J. (eds) Problems of PetroleumMigration. American Association of PetroleumGeologists (Tulsa), Studies in Geology, v. 10, p. 89-107.

Newell, N.A., 1999. Water washing in the northernBonaparte Basin. Australian Petroleum Production &Exploration Association Journal, v. 39/1, p. 227-247.

Pallasser, R.J., 2000. Recognising biodegradation ingas/oil accumulations through the ä13C compositionsof gas components. Organic Geochemistry, v. 31;p.1363-1373.

Prinzhofer, A. and Pernaton, É., 1997. Isotopicallylight 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 GeologistsBulletin, v. 65, p. 5-25.

Rooney, M.A., Claypool, G.E., and Chung, H.M.,1995. Modeling thermogenic gas generation usingcarbon isotope ratios. Chemical Geology, v. 126, p.219-232.

Schoell, M., 1980. The hydrogen and carbon isotopiccomposition of methane from natural gases of variousorigins. Geochimica et Cosmochimica Acta, v. 44, p.649-661.

Schoell, M., 1983. Genetic characterization of natural

gases. American Association of PetroleumGeologists Bulletin, v. 67, p. 2225-2238.

Stahl, W.J., 1977. Carbon and nitrogen isotopes inhydrocarbon research and exploration. ChemicalGeology, v. 20, p. 121-149.

Thrasher, J., and Fleet, A.J., 1995. Distribution,origin and prediction of carbon dioxide in petroleumreservoirs. American Association of PetroleumGeologists Bulletin, v. 79, p. 1252 (abstract).

Whiticar, M.J., 1999. Carbon and hydrogen isotopesystematics 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 CosmochimicaActa, 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 biogenicand thermogenic gas (square; Central Sumatra Basin), and a mixed gas with a portion of themethane having been derived from a source with a more advanced level of thermal maturitythan 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 effectivesource 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 theBonaparte 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 backgroundsignal 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 theCarnavon Basin, Australia. The observed relationship suggests that CO2 is either a functionof temperature within the basin or that there is a basal source.

gas geochemistry – a key to understanding formation

Documents