Abstract
The compositional carbon isotopic seriesδ13C-CH4<δ13C-C2H6<δ13C-C3H8<δ13C-C4H10is common in thermogenic gases. With the exploration of deeper strata, however, isotopic reversals (δ13C-CH4>δ13C-C2H6>δ13C-C3H8) in overmature unconventional shale gases and conventional (coal-derived) gases have been identified. Paleozoic gases in the southern Ordos Basin, China, with partial or complete isotopic reversals, were studied as examples of isotopic fractionation in overmature coal-derived gases. Isotopic compositions of gases of different maturities from the Ordos Basin and shale gases from around the world were compared. Results indicate that carbon isotopic series are related to maturity. Complete isotopic reversal occurs mostly in regions with vitrinite reflectanceRo>2.4%. Where2.4%>Ro>2.0%, almost all gases display partial isotopic reversal, withδ13C-CH4>δ13C-C2H6orδ13C-C2H6>δ13C-C3H8. Carbon isotopic reversal in coal-derived gases is not caused by abiotic origin, the mixing of gases from different types of source rock, abiotic polymerization, wet gas cracking, and other mechanisms that contribute to reversal in shale gases. Based on the unique structure of coaly source rock and the geology of the Ordos Basin, closed-system aromatization-polycondensation reactions are considered the most likely cause of carbon isotopic reversal. During the reactions, isotopically light gases are generated by recombination of previously formed hydrocarbons and residual kerogen-coal. Hydrogen isotopic reversal in the southern Ordos Basin might also be caused by aromatization-polycondensation reactions.
Highlights
The carbon isotopic composition of natural gas can be used to determine its origin, source, and maturity [1, 2]
12C–12C bonds usually break before 12C–13C bonds, resulting in kinetic isotopic fractionation [2] and causing natural gas to follow two evolutionary trends, with carbon isotopes becoming heavier as the component carbon number increases, forming a positive isotopic series with δ13C-CH4
There are problems with this explanation as follows: (1) shale gas is an oil-type gas generated from type I and type II kerogen, whereas Ordos gas is mainly coal derived and generated from type III kerogen, with different gas-generation reactions occurring in these sources; (2) complete carbon isotopic reversal is universal in overmature shale gases [13,14,15], but not all overmature coal-derived gas has complete carbon isotopic reversals, as in gas from the southern Ordos Basin [16]; and
Summary
The carbon isotopic composition of natural gas can be used to determine its origin, source, and maturity [1, 2]. As exploration of Paleozoic gas in the Ordos Basin has expanded into the southern part of the basin, more gases have been found to have carbon and hydrogen isotopic reversals. There are problems with this explanation as follows: (1) shale gas is an oil-type gas generated from type I and type II kerogen, whereas Ordos gas is mainly coal derived and generated from type III kerogen, with different gas-generation reactions occurring in these sources; (2) complete carbon isotopic reversal is universal in overmature shale gases [13,14,15], but not all overmature coal-derived gas has complete carbon isotopic reversals, as in gas from the southern Ordos Basin [16]; and (3) gases in the southern Ordos Basin display unique characteristics that differ from those of overmature shale gas, such as the evolutionary trend of hydrogen isotopes. The cause of hydrogen isotopic reversal in the southern Ordos Basin is discussed
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