Effect of hydrocarbon expulsion efficiency on shale gas formation and enrichment

Abstract

Shale gas enrichment is not only controlled by sufficient gas source but also by the intensity of later structural transformation.Gas volume is related to source rock quality and hydrocarbon expulsion efficiency, and the amount of shale gas generation depends on remaining hydrocarbon.Based on the identification and statistics of solid bitumen and the analysis of helium and carbon isotopes, the hydrocarbon expulsion efficiency, the in situ gas production of the Upper Ordovician Wufeng and Lower Silurian Longmaxi shales in Jiaoshiba and Pengshui exploration areas and its influence on shale gas enrichment were investigated. The hydrocarbon expulsion efficiency, residual oil volume and in situ gas volume for Jiaoshiba were 23%, 27.67 kg/t and 21.23 m3/t, while those values for Pengshui were 65%, 11.0 kg/t, and 18.99 m3/t, respectively, indicating differential hydrocarbon generation and expulsion related to various influence of the Indosinian Movement.According to the results of 4He isotope dating, Fuling shale gas began to be stored and accumulated at the initial stage (231 Ma) of the oil generation peak period with sufficient gas sources, while the closed system for Pengshui shale gas was formed at the later stage (183 Ma) than the gas generation peak with insufficient gas source. The δ13C2 and δ13C1-δ13C2 for Fuling shale gas were -35.8‰ and 4.8‰, while those values for Pengshui shale gas were -33.0‰ and 3.3‰, respectively.The different δ13C fractionation effects might be attributed to the distinct state of the hydrocarbon generation system. Therefore, the matching of the hydrocarbon generation peak period with the key structural transformation period in the process of hydrocarbon generation and evolution, the hydrocarbon retention amount in the maximum burial period and the structural transformation intensity in the process of uplift and denudation jointly controlled the generation, enrichment and preservation of shale gas.

Keywords

• sealing property
• noble gases
• isotope
• hydrocarbon expulsion efficiency
• shale gas
• wufeng-longmaxi formations
• sichuan basin