Abstract

We have assessed the genetic types of hydrocarbon gas in the Fangzheng Basin by analyzing the effects of geologic settings on gas generation, kerogen types in source rocks, gas compositions, stable carbon isotopes of individual alkanes, and biomarkers in gas-associated oil. The primary compounds of source rocks in the Eocene Xinancun Formation and Paleocene Wuyun Formation are found as type II and III kerogens, respectively. The hydrocarbon gas in the Fangzheng Basin can be classified into three families. Family I is affected by biodegradation, and it is dry gas generated from low-maturity lacustrine mudstones (i.e., oil-prone source rocks) of the Xinancun Formation. Family II is coal-derived wet gas accompanied by oil, and it is typically generated by type III kerogen of mudstones in coal measures of the Wuyun Formation. Family III is mixed-type wet gas whose primary compound is oil-associated gas, and it is mainly generated by type II kerogen in the Xinancun Formation and partly from type III kerogen in the Wuyun Formation in the Daluomi (DLM) Uplift. The family I and II hydrocarbon gases are located in the Zhuozhugang (ZSG) Sag. Family III hydrocarbon gas was formed in the mixing process of different genesis gas through the active faults because the late Miocene transpressional strength of uplift in the DLM Uplift was more intense than that in the ZSG Sag after the development of increased accommodation space coeval with intrabasinal rifting before Oligocene.

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