Shale sedimentary environments and their controlling factors in Lianggaoshan Formation of Fuxing area, Sichuan Basin

Taking the shale in the second member of the Jurassic Lianggaoshan Formation (Liang 2) in the Fuxing area of the Sichuan Basin as the research object, this study employed geochemical and sedimentary analysis to systematically investigate the paleoenvironmental characteristics of the shale in the study area during the sedimentary period and their controlling effects on shale quality. Sedimentary models for the shale were established. The results showed that the shale in the Lianggaoshan Formation was deposited in a freshwater to brackish water environment with a paleo-water depth of 4.9 to 39.4 m, averaging 17.2 m. The paleo-productivity level was high with an average bio-barium content of 567.24 μg/g. The overall environment was anaerobic and reducing, and the climate was warm and humid in general. The total organic carbon (TOC) content of the shale in the Lianggaoshan Formation was controlled by paleo-water depth, paleosalinity, paleoproductivity, and paleo-water redox environment. Higher TOC content was positively correlated with water depth, salinity, productivity, and warm and humid climate. Under redox conditions, minor differences in reducing environments had little impact on TOC content. During the sedimentary period of the lower sub-member of Liang 2, the upper gas layers had a deeper water body, higher paleoproductivity, and a warmer, more humid paleoclimate compared to the lower gas layers, resulting in richer organic matter accumulation. Based on these findings, two types of shale sedimentary models were established for shale in the lower sub-member of the Liang 2 in the study area: (1) As the lower gas layers deposited, under freshwater to brackish water, semi-humid to semi-arid climate, moderate paleoproductivity, and anaerobic-reducing conditions, fair to medium-quality shale was mainly deposited. (2) As the upper gas layers deposited, under freshwater to brackish water, warm and humid climate, moderate to high paleoproductivity, and anaerobic-reducing conditions, medium to high-quality shale was primarily deposited.