Abstract:

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 considers “TOC content-mineral content-genesis-sedimentary structure”,the Wufeng and Longmaxi formations 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 significantly 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.

Key words: lithofacies characteristics, palaeoenvironment, sedimentary model, Wufeng Formation, Longmaxi Formation, Sichuan Basin

摘要： 川南地区五峰组—龙马溪组黑色页岩是页岩气勘探的主要目标层位。通过岩心及薄片观察、二维XRF元素扫描、扫描电镜、微区QEMSCAN分析等手段,结合全岩XRD以及地球化学分析测试等资料,对川南地区五峰组—龙马溪组页岩岩相特征、沉积环境演化以及沉积作用进行研究。结果表明: (1)根据“TOC含量-矿物含量-成因-沉积构造”多端元岩相划分方案,将五峰组—龙马溪组页岩划分为高TOC块状生物硅质页岩、极高TOC薄纹层生物硅质页岩、高TOC极薄纹层生物-碎屑硅质页岩、中TOC纹层状碎屑硅质页岩、高TOC纹层状化学-生物钙硅混积页岩、高TOC极薄纹层生物-碎屑硅泥混积页岩、中TOC纹层状碎屑硅泥混积页岩、中TOC纹层状碎屑三元混积页岩共8种岩相类型,并对其岩相特征、垂向分布特征以及空间展布特征进行了刻画。(2)沉积环境自下而上经历了5个阶段的变化,古气候经历了从五峰组底部温湿到中部干热再到顶部干冷、随后从龙马溪组下部的湿热到中上部的温湿的演变。在五峰组下部以及龙马溪组下部经历了2次大规模的海侵,海平面较高时,水体封闭性弱、还原性强、盐度较低、古生产力较高; 在五峰组顶部发生冰川事件导致海平面下降,海平面较低时,水体封闭性强、还原性弱、盐度较高、古生产力较低。(3)沉积环境的高频变化对页岩岩相发育及有机质富集具有明显的控制作用,不同岩相中有机质含量与古生产力和水体还原性均呈良好的正相关,在古生产力高、水体还原性较强的阶段主要沉积极高TOC薄纹层生物硅质页岩和高TOC极薄纹层生物-碎屑硅质页岩。研究成果不仅有助于对有机质富集和页岩沉积作用的理解,而且可为该区域下一步油气勘探的有利层段预测、勘探及开发提供理论依据。

关键词: 岩相特征, 古环境, 沉积模式, 五峰组, 龙马溪组, 四川盆地

CLC Number: 

• P512.2