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TitleThe stable carbon isotope geochemistry of natural gas after 20 years of shale gas development
 
AuthorCesar, JORCID logo; Mayer, B; Ardakani, O HORCID logo
SourceConference proceedings, 30th International Meeting on Organic Geochemistry (IMOG 2021); 2021 p. 1-2, https://doi.org/10.3997/2214-4609.202134075 Open Access logo Open Access
Image
Year2021
Alt SeriesNatural Resources Canada, Contribution Series 20210162
PublisherEuropean Association of Geoscientists and Engineers
MeetingInternational Meeting on Organic Geochemistry; September 12-17, 2021
Documentserial
Lang.English
Mediaon-line; digital
File formatpdf; html
Subjectsfossil fuels; geochemistry; Science and Technology; petroleum resources; hydrocarbons; gas; hydrocarbon generation; stable isotope studies; carbon isotopes; reservoir rocks; permeability; thermal maturation; organic geochemistry; ethane; source rocks
ProgramGeoscience for New Energy Supply (GNES) Shale Reservoir Characterization
Released2021 09 12
AbstractAfter completion of numerous studies of hydrocarbon generation processes in petroleum basins, the generation of natural gas is well understood, and represents a process of simple non-catalytic thermal cracking (e.g. Snowdon, 2001). From the perspective of conventional hydrocarbon accumulations, natural gas has been characterized as part of a dynamic system where generation, migration, and accumulation of hydrocarbons affect not only the molecular composition of the gas but also the isotopic composition of the individual molecules. Stable carbon isotopes are known to distribute according to kinetic isotope fractionation as a consequence of thermal cracking and migration of hydrocarbons from the source rock (e.g. Clayton, 1991).
During the rapid development of shale gas (and low-permeability reservoirs in general) in the last twenty years, the carbon isotope concepts that were conceived for conventional petroleum systems were also applied to these new types of hydrocarbon reservoirs. However, geochemical evidence has shown that shale gas exhibits different isotope fractionation phenomena. On the one hand, a well-documented distinct isotope feature of gases from low-permeability reservoirs is the occurrence of ethane carbon isotope reversal (delta-13Cethane< delta-13Cmethane), typically found in gases with thermal maturity above 2.0 %Ro. The isotope reversal has been attributed to partly reversible free radical reactions under kinetic isotope fractionation (Xia and Gao, 2018). On the other hand, at lower thermal maturity in shale gas plays, it is yet a question whether the processes governing the carbon isotope distribution are the same as those conceived for conventional hydrocarbon accumulations, especially in the wet-gas window, which defines condensate rich trends of major economic significance in producing low-permeability hydrocarbon reservoirs around the world.
Recently, we have found that stable carbon isotopes of natural gas from low-permeability reservoirs display a tendency towards equilibrium isotopic fractionation in the oil window and wet-gas window (Cesar et al., 2020). Carbon isotope quasi-equilibrium has also been proposed for gases from conventional accumulations using clumped-isotope geochemistry (Thiagarajan et al., 2020). The possibility of tendencies towards isotope equilibrium does not change the understanding of gas generation as a kinetic process via thermal cracking of kerogen and/or other hydrocarbons, but it implies the likelihood of other processes facilitating isotope equilibration of the generated gas that is retained in the source rock in unconventional hydrocarbon reservoirs over extended residence times.
In this study, we present new compound-specific carbon isotope evidence suggesting a tendency towards isotope equilibration in gases from low-permeability hydrocarbon reservoirs in North America. We also discuss how these isotope proxies are related to the molecular composition of the gas and the evolution of key molecular parameters such as the ethane to propane ratio (C2/C3) during the thermal maturation process.
Summary(Plain Language Summary, not published)
In this study, we present new carbon isotope evidence suggesting a tendency towards isotope equilibration in gases from low-permeability hydrocarbon reservoirs in North America. We also discuss how these isotope proxies are related to the molecular composition of the gas and the evolution of key molecular parameters during the thermal maturation process.
GEOSCAN ID328591

 
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