Fracture vein characteristics and paleofluid activities in the Lower Cambrian Qiongzhusi shale in the central portion of the Mianyang-Changning intracratonic Sag, Sichuan Basin

Huge and thick black shale was deposited in the Mianyang-Changning intracratonic sag of the Sichuan Basin, which is a favorable target for shale gas exploration in the future. Taking the paleofluid preserved in the Qiongzhusi Formation shale as the research object, based on the petrological and optical characteristics of fracture veins observed in cores and thin sections, the source and origin of the paleofluid were analyzed by carbon and oxygen isotope testing, the formation pressure evolution of shale gas was simulated by using BasinMod software, and the preservation conditions of Qiongzhusi Formation shale gas in the central portion and surrounding areas of the Mianyang-Changning intracratonic sag were discussed. The results show that fracture vein rarely appears in the Qiongzhusi Formation shale of wells GS17 and MX9. While multiple groups of bed-parallel and high-angle fractures appear in Well W201, which are filled with mesocrystalline to coarse crystalline calcite, fibrous calcite, saddle dolomite, barite, and quartz. Carbon and oxygen isotopes indicate that fracture veins were generated by fluids of hydrothermal solution or organic matter decarboxylation. Hydrocarbon generation, particularly the crude oil cracking, induced the strong overpressure in the Qiongzhusi Formation of the intracratonic sag and the Gaoshiti-Moxi area, and the overpressure has been preserved thus far. Although overpressure was also developed in the Weiyuan structure, it dissipated in the late tectonic movement. Compared with the intracratonic sag and Gaoshiti-Moxi area, the late preservation conditions of shale gas in the Weiyuan area were relatively harsh, but some local overpressure areas need to be focused on.