Modeling thermogenic gas generation using carbon isotope ratios of natural gas hydrocarbons

Rooney, Melodye A.; Claypool, George E.; Chung, Hyen-Mi

doi:10.1016/0009-2541(95)00119-0

Cited by 347 publications

(167 citation statements)

References 15 publications

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“…Irrespective of post-genetic alterations, C 1 /C 2 and C 2 /C 3 ratios, δ 13 C values of methane, ethane and carbon dioxide as well as gas dryness indices suggest that thermogenic constituents of the vent gas are sourced from immature to early mature kerogen of the sapropelic, marine type II (Rice and Claypool, 1981;Rooney et al, 1995;Whiticar, 1989;Whiticar, 1999) and/or from secondary production due to oil cracking (Milkov and Dzou, 2007;Prinzhofer and Huc, 1995;Seewald, 2003). δ 13 C-CO 2 values likewise suggest that carbon dioxide in the vent gas primarily originates from the degradation of kerogen and soluble organic matter (Dai et al, 1996;Irwin et al, 1977;Wycherley et al, 1999).…”

Section: Station Nomentioning

confidence: 99%

Molecular and isotopic partitioning of low-molecular-weight hydrocarbons during migration and gas hydrate precipitation in deposits of a high-flux seepage site

et al. 2010

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Detailed knowledge of the extent of post-genetic modifications affecting shallow submarine hydrocarbons fueled from the deep subsurface is fundamental for evaluating source and reservoir properties. We investigated gases from a submarine high-flux seepage site in the anoxic Eastern Black Sea in order to elucidate molecular and isotopic alterations of low-molecular-weight hydrocarbons (LMWHC) associated with upward migration through the sediment and precipitation of shallow gas hydrates. For this, nearsurface sediment pressure cores and free gas venting from the seafloor were collected using autoclave technology at the Batumi seep area at 845 m water depth within the gas hydrate stability zone. Vent gas, gas from pressure core degassing, and from hydrate dissociation were strongly dominated by methane (> 99.85 mol.% of ∑[C 1 -C 4 , CO 2 ]). Molecular ratios of LMWHC (C 1 /[C 2 + C 3 ] > 1000) and stable isotopic compositions of methane (δ 13 C = − 53.5‰ V-PDB; D/H around − 175‰ SMOW) indicated predominant microbial methane formation. C 1 /C 2+ ratios and stable isotopic compositions of LMWHC distinguished three gas types prevailing in the seepage area. Vent gas discharged into bottom waters was depleted in methane by > 0.03 mol.% (∑[C 1 -C 4 , CO 2 ]) relative to the other gas types and the virtual lack of 14 C-CH 4 indicated a negligible input of methane from degradation of fresh organic matter. Of all gas types analyzed, vent gas was least affected by molecular fractionation, thus, its origin from the deep subsurface rather than from decomposing hydrates in near-surface sediments is likely. As a result of the anaerobic oxidation of methane, LMWHC in pressure cores in top sediments included smaller methane fractions [0.03 mol.% ∑(C 1 -C 4 , CO 2 )] than gas released from pressure cores of more deeply buried sediments, where the fraction of methane was maximal due to its preferential incorporation in hydrate lattices. No indications for stable carbon isotopic fractionations of methane during hydrate crystallization from vent gas were found. Enrichments of 14 C-CH 4 (1.4 pMC) in short cores relative to lower abundances (max. 0.6 pMC) in gas from long cores and gas hydrates substantiates recent methanogenesis utilizing modern organic matter deposited in top sediments of this high-flux hydrocarbon seep area.

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Section: Station Nomentioning

confidence: 99%

Molecular and isotopic partitioning of low-molecular-weight hydrocarbons during migration and gas hydrate precipitation in deposits of a high-flux seepage site

et al. 2010

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“…The extent of enrichment increases with chain length. Values of δ 13 C for ethane and propane typically range between -18 and -34‰, with propane 5 to 8‰ heavier than ethane (BERNER et al, 1995;DES MARAIS et al, 1988;LORANT et al, 1998;ROONEY et al, 1995). The isotopic ratios of thermogenic methane, ethane and propane typically increase with increasing maturity of the source material (BERNER et al, 1995;LORANT et al, 1998, and references therein;ROONEY et al, 1995;SHERWOOD LOLLAR et al, 2002).…”

Section: Sources Of Carbon Compoundsmentioning

confidence: 99%

Geochemistry of low-molecular weight hydrocarbons in hydrothermal fluids from Middle Valley, northern Juan de Fuca Ridge

Cruse

Seewald

2006

Geochimica et Cosmochimica Acta

108

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Hydrothermal vent fluids from Middle Valley, a sediment-covered mid-ocean ridge on the northern Juan de Fuca Ridge, were sampled in July, 2000. Eight different vents with exit temperatures of 186 to 281°C were sampled from two areas of venting: the Dead Dog and ODP Mound fields. Fluids from the Dead Dog field are characterized by higher concentrations of ΣNH 3 and organic compounds (C 1 -C 4 alkanes, ethene, propene, benzene and toluene) compared with fluids from the ODP Mound field. The ODP Mound fluids, however, are characterized by higher C 1 /(C 2 +C 3 ) and benzene:toluene ratios than those from the Dead Dog field. The aqueous organic compounds in these fluids have been derived from both bacterial processes (methanogenesis in low-temperature regions during recharge) as well as from thermogenic processes in higher-temperature portions of the subsurface reaction zone. As the sediments undergo hydrothermal alteration, carbon dioxide and hydrocarbons are released to solution as organic matter degrades via a stepwise oxidation process. Compositional and isotopic differences in the aqueous hydrocarbons indicate that maximum subsurface temperatures at the ODP Mound are greater than those at the Dead Dog field. Maximum subsurface temperatures were calculated assuming that thermodynamic equilibrium is attained between alkenes and alkanes, benzene and toluene, and carbon dioxide and methane. The calculated temperatures for alkene-alkane equilibrium are consistent with differences in the dissolved Cl concentrations in fluids from the two fields, and indicate that subsurface temperatures at the ODP Mound are hotter than those at the Dead Dog field. Temperatures calculated assuming benzene-toluene equilibrium and carbon dioxide-methane equilibrium are similar to observed exit temperatures, and do not record the hottest subsurface conditions. The difference in subsurface temperatures estimated using organic geochemical thermometers reflects subsurface cooling processes via mixing of a hot, low-salinity vapor with a cooler, seawater salinity fluid. Because of the disparate temperature dependence of alkene-alkane and benzene-toluene equilibria, the mixed fluid records both the high and low temperature equilibrium conditions. These calculations indicate that vapor-rich fluids are presently being formed in the crust beneath the ODP Mound, yet do not reach the surface due to mixing with the lower-temperature fluids.2

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“…Possible influence of hydrocarbon expulsion on the δ 13 C values of gaseous hydrocarbons Stable carbon isotopic compositions of gaseous hydrocarbons have been widely used to identify the origin of natural gases, and to assess their thermal maturity (Stahl, 1974;Schoell, 1983;James, 1990;Rooney et al, 1995). Our experimental results indicate that in addition to source and thermal maturity, hydrocarbon expulsion is also a possible factor influencing the carbon isotopic compositions of gaseous hydrocarbons.…”

Section: Origin Of Gaseous Hydrocarbonsmentioning

confidence: 71%

“…1). The cracking of oil, or expelled bitumen, is considered the major source of conventional gas generated from hydrogen-rich marine source rocks (Welte et al, 1988;Rooney et al, 1995). Conventional and unconventional gas fields commonly occur together within a hydrocarbon basin (Zou et al, 2015), where the cracking of expelled oil produces conventional gas, and the cracking of retained oil (i.e., bitumen) and kerogen generates unconventional gas.…”

Section: Introductionmentioning

confidence: 99%

The origin and evolution of thermogenic gases in organic-rich marine shales

Xiong

Zhang

Chen

et al. 2016

Journal of Petroleum Science and Engineering

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Modeling thermogenic gas generation using carbon isotope ratios of natural gas hydrocarbons

Cited by 347 publications

References 15 publications

Molecular and isotopic partitioning of low-molecular-weight hydrocarbons during migration and gas hydrate precipitation in deposits of a high-flux seepage site

Molecular and isotopic partitioning of low-molecular-weight hydrocarbons during migration and gas hydrate precipitation in deposits of a high-flux seepage site

Geochemistry of low-molecular weight hydrocarbons in hydrothermal fluids from Middle Valley, northern Juan de Fuca Ridge

The origin and evolution of thermogenic gases in organic-rich marine shales

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