川南地区五峰组—龙马溪组页岩岩相特征及沉积环境演化OA北大核心

The black shale of the Wufeng and Longmaxi formations in the southern Sichuan region is a primary target for shale gas exploration.This study investigates the lithofacies characteristics,sedimentary environment evolution,and depositional processes of the Wufeng and Longmaxi formations shale using core and thin-section observations,2D XRF elemental scanning,scanning electron microscopy(SEM),micro-area QEMSCAN analysis,whole-rock XRD,and geochemical analysis.The results indicate the following:(1)Based on a multi-element lithofacies classification scheme that con-siders"TOC content-mineral content-genesis-sedimentary structure",the Wufeng and Longmaxi for-mations shale is categorized into eight lithofacies types:high-TOC massive biosiliceous shale,extremely high-TOC thin-laminated biosiliceous shale,high-TOC very thin-laminated bio-detrital siliceous shale,medium-TOC laminated detrital siliceous shale,high-TOC laminated chemo-bio-calcareous-siliceous mixed shale,high-TOC very thin-laminated bio-detrital siliceous-argillaceous mixed shale,medium-TOC laminated detrital siliceous-argillaceous mixed shale,and medium-TOC laminated detrital ternary mixed shale.The lithofacies characteristics,vertical distribution,and spatial distribution of these lithofacies were thoroughly characterized.(2)The sedimentary environment evolved through five stages from bottom to top.The paleoclimate transitioned from warm and humid at the base of the Wufeng Formation to dry and hot in the middle,then to dry and cold at the top,followed by warm and humid in the lower Longmaxi Formation,leading to warm and wet conditions in the middle-upper Longmaxi Formation.Two large-scale marine transgressions occurred in the lower Wufeng Formation and the lower Longmaxi Formation,resulting in sea-level rise.During periods of high sea levels,the water body exhibited weak confinement,strong reducibility,low salinity,and high paleoproductivity.A glacial event at the top of the Wufeng Formation caused a drop in sea-level,leading to strong water confinement,weak reducibility,high salinity,and low paleoproductivity.(3)High-frequency changes in the sedimentary environment signi-ficantly influenced shale lithofacies development and organic matter enrichment.The organic matter content of different lithofacies shows a strong positive correlation with paleoproductivity and water reducibility.Extremely high-TOC thin-laminated biosiliceous shale and high-TOC very thin-laminated bio-detrital siliceous shale were predominantly deposited during periods of high paleoproductivity and strong water reducibility.These findings not only enhance the understanding of organic matter enrichment and shale sedimentation,but also provide a theoretical basis for predicting favorable layers for exploration and development in the southern Sichuan Basin in the future.