Thermal Evolution History and Shale Gas Accumulation Significance of Lower Cambrian Qiongzhusi Formation in Southwest Sichuan Basin

The thermal evolution history is closely related to shale gas generation and accumulation process. On the one hand, the maturity evolution history determines the hydrocarbon generation process, type and amount of shale gas in the geological history. On the other hand, the formation of organic matter hosted pore is extremely relevant to the thermal evolution history of gas shale. In this paper, through analysis of burial and thermal history, it examined representative boreholes for the thermal evolution simulation of the Qiongzhusi Formation shale in the Southwest Sichuan basin, where the zircon (U-Th)/He and bitumen reflectance (R_b) were used for calibration. Then, the relationship between the thermal evolution history and the shale gas generation and accumulation was discussed. The results show that they differed from borehole to borehole on the thermal evolution and hydrocarbon generation history of Qiongzhusi Formation shale in the Southwest Sichuan basin and two patterns were summarized. The Qiongzhusi Formation shale in the Caledonian depression entered the mature stage during Caledonian period and typing stage during the Middle-Late Permian because of Emeishan mantle plume. Accordingly, the two hydrocarbon generation peaks occurred in the Silurian (oil and wet gas generation stage) and the Middle-Late Permian (dry gas generation stage), respectively, which indicates that the organic matter had been nearly exhausted during the Middle-Late Permian and there was no obvious hydrocarbon generating activities since then. The Qiongzhusi Formation shale in the Caledonian uplift area was very different from the former on the thermal evolution and hydrocarbon generation history. They had not yet or just exceeded the generation threshold during the Caledonian, and continually entered the mature and overmature stage during the late Hercynian to Yanshanian. There were also two rapid hydrocarbon generation stages of the Qiongzhusi Formation shale in the Caledonian uplift area, namely during the Middle-Late Permian (oil generation stage) and the Late Jurassic to Late Cretaceous (wet and dry gas generation stage). With the basin uplift and cooling, the hydrocarbon generation was effectively halted at the end of the Late Cretaceous. It shows that the different burial depths of Qiongzhusi Formation shale during the Middle-Late Permian caused dominantly the different influence by high heat flow associated with Emeishan mantle plume, and eventually led to the differences on the thermal evolution, hydrocarbon generation history and gas-bearing characteristics of Qiongzhusi Formation shale between the Caledonian depression and uplift area. The Qiongzhusi Formation shale gas accumulation process in Weiyuan-Qianwei area was divided into four phases on the basis of hydrocarbon generating and porosity evolution history analysis: the source-reservoir-cap deposition and biogenic gas accumulation stage during Early Paleozoic, the initial accumulation stage during the Middle-Late Permian, the main accumulation stage from the Late Jurassic to Early Cretaceous, and the adjustment stage since the Late Cretaceous. The thermal evolution analysis explains the Qiongzhusi Formation shale gas accumulation differences between the Weiyuan-Qianwei and periphery in the Southwest Sichuan basin.