川西南地区下寒武统筇竹寺组页岩热演化史及页岩气成藏意义

页岩热演化史与页岩气的成烃、成藏过程关系极为密切.一方面，页岩热演化过程决定了页岩生烃过程、页岩气类型和生气量；另一方面，页岩有机孔隙的形成与页岩热演化过程密切相关.在前期热史恢复基础上，以锆石（U-Th）/He和沥青反射率R_b等古温标进行标定，模拟了川西南地区代表性钻孔下寒武统筇竹寺组页岩热演化史，探讨了页岩热演化过程与页岩气成烃、成藏的关系.研究结果表明，川西南地区下寒武统筇竹寺组页岩热演化过程和生烃史差异性明显，可以识别出两种模式:加里东期坳陷区，筇竹寺组页岩在加里东期成熟，中?晚二叠世期间快速演化定型，两个生烃高峰期分别出现在志留纪（生油高峰?湿气阶段）、中?晚二叠世（干气阶段），此后再无明显的生烃活动；加里东期古隆起区，筇竹寺组页岩在加里东期未熟?低熟，晚海西期?燕山期持续增熟，亦存在两期生烃高峰，分别是中?晚二叠世（生油高峰阶段）、晚侏罗世?早白垩世（湿气?干气阶段），筇竹寺组页岩生烃过程持续到晚白垩世末期.分析表明中?晚二叠世期间筇竹寺组页岩的埋深差异造成了其受峨眉山地幔柱热效应的影响不同，进而决定了加里东期坳陷区与隆起区筇竹寺组页岩热演化过程和生烃史差异，并最终导致了威远?犍为地区筇竹寺组页岩含气性优于盆地外围.综合川西南地区筇竹寺页岩生烃史和孔隙度演化模型，将川西南成藏条件相对优越的威远?犍为地区筇竹寺组页岩气成藏过程分为4个阶段:早古生代时期源?储?盖形成和生物气成藏阶段、中?晚二叠世期间初始成藏阶段、晚侏罗世?早白垩世主成藏阶段和晚白垩世以后调整改造阶段.该成果以页岩热演化过程为切入点解释了川西南威远?犍为地区与盆地外围下寒武统筇竹寺组页岩气成藏差异的原因. 

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.