--> --> Interpreting δ13C Reversed Gas Produced from the Silurian Qusaiba Shale in NW Saudi Arabia

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Interpreting δ13C Reversed Gas Produced from the Silurian Qusaiba Shale in NW Saudi Arabia

Abstract

Recent studies of unconventional shale plays from the U.S., China, and Canada have demonstrated that gas geochemistry and unconventional gas productivity are highly correlated. In particular, isotope reversal has been considered a sign for high productivity; however, the mechanism of isotope reversal is not yet well understood and remains controversial. Moreover, isotopically reversed gas has been noted from low production zones. Chemical composition and carbon isotope ratios of unconventional gas produced from the Silurian Qusaiba shale in NW Saudi Arabia are analyzed to study its origin and potential. The fractured tight shale gas shows partial reversal (δ 13C1> δ13C2 δ13C3) from the normal trend (δ13C1 < δ13C2 < δ13C3). One interpretation is that secondary cracking of hydrocarbon components, including wet gases along with mixing with primary gases, is responsible for the isotope reversal and therefore the reversal is linked to high gas potential. δ13C2 increases with decreasing gas wetness (e.g., wetness < 5%), implying ethane was a reactant during the cracking. The carbon dioxide (CO2) coexisting with light hydrocarbon gas in the shale shows light δ13CO2 values (-9‰ to −13‰), which decreases as wetness decreases; implying that isotopically light CO2 is linked to the decomposition of some wet gas components. Potential mechanisms for the isotope reversal are discussed.