Genetic and source differences of gases in the Middle Permian Qixia and Maokou formations in the Sichuan Basin, SW China

The Middle Permian Qixia and Maokou formations (Fms), are important targets for current gas exploration in the Sichuan Basin. In recent years, high-yield industrial gas flow has been obtained in different structural zones of the basin. Based on the molecular and isotopic compositions of natural gas and reservoir bitumen, biomarker composition, and geological conditions, the genetic types, maturity, and sources of natural gas reservoired in the Qixia and Maokou Fms in different structural zones in the basin were analyzed. The following conclusions can be drawn: (1) the molecular and isotopic compositions of natural gas reservoired in the Middle Permian Qixia and Maokou Fms from different areas in the Sichuan Basin differ considerably; (2) the Middle Permian natural gas is oil-type gas, which is mainly generated by oil cracking; (3) most of the natural gas reservoired in the Qixia and Maokou Fms in the Sichuan Basin has reached the over-mature stage. The maturity of the natural gas from the northwestern Sichuan Basin is the highest, followed by gas from eastern and southern Sichuan, and part of natural gas in Southern Sichuan is high-maturity; (4) except for the slight thermochemical sulfate reduction (TSR) secondary alteration in the central Sichuan Basin, an effect of TSR was not observed in other areas; and (5) natural gas is controlled by the regional hydrocarbon kitchen. Gas in the eastern and southern Sichuan areas mainly originates from the Lower Silurian Longmaxi Fm shale, with a small contribution from marl in the Maokou Fm. The gas of the first member of the Maokou Fm has self-generation and self-storage characteristics, with a small contribution of the Longmaxi Fm shale. The Middle Permian gas in northwestern Sichuan is a mixture of gas generated from source rocks of the Lower Cambrian Qiongzhusi and Maokou Fms, whereas that in central Sichuan mainly originates from source rocks of the Qiongzhusi Fm. The Middle Permian gas in southwestern Sichuan mainly originates from its own source rock, and the natural gas reservoired in Permian volcanic rocks in the Chengdu-Jianyang area was generated by the mudstone of the Qiongzhusi Fm.