Cause of shale gas geochemical anomalies and mechanisms for gas enrichment and depletion in high-maturity shales

•Geochemical anomalies of shale gases were characterized in detail.•A model was proposed to explain the observed geochemical anomalies of shale gases.•High TOC contents, high thermal maturity and late-stage closed system account for gas enrichment in shales.•Loss of free gases during post-generation evolution may result in gas depletion or even undersaturation in high-maturity organic-rich shales.

This article reviews the abnormal characteristics of shale gases (natural gases produced from organic-rich shales) and discusses the cause of the anomalies and mechanisms for gas enrichment and depletion in high-maturity organic-rich shales. The reported shale gas geochemical anomalies include rollover of iso-alkane/normal alkane ratios, rollover of ethane and propane isotopic compositions, abnormally light ethane and propane δ13C values as well as isotope reversals among methane, ethane and propane. These anomalies reflect the complex histories of gas generation and associated isotopic fractionation as well as in-situ “mixing and accumulation” of gases generated from different precursors at different thermal maturities. A model was proposed to explain the observed geochemical anomalies. Gas generation from kerogen cracking at relatively low thermal maturity accounted for the increase of iso-alkane/normal alkane ratios and ethane and propane δ13C values (normal trend). Simultaneous cracking of kerogen, retained oil and wet gas and associated isotopic fractionation at higher maturity caused decreasing iso-alkane/normal alkane ratios, lighter ethane and propane δ13C and corresponding conversion of carbon isotopic distribution patterns from normal through partial reversal to complete reversal. Relatively low oil expulsion efficiency at peak oil generation, low expulsion efficiency at peak gas generation and little gas loss during post-generation evolution are necessary for organic-rich shales to display the observed geochemical anomalies. High organic matter richness, high thermal maturity (high degrees of kerogen-gas and oil-gas conversions) and late-stage (the stage of peak gas generation and post-generation evolution) closed system accounted for gas enrichment in shales. Loss of free gases during post-generation evolution may result in gas depletion or even undersaturation (total gas content lower than the gas sorption capacity) in high-maturity organic-rich shales.