上扬子区下寒武统海相页岩岩相类型及沉积模式

摘要

上扬子区下寒武统牛蹄塘组页岩是我国南方地区页岩气重要勘探层系之一.通过露头、岩心、薄片、实验测试等资料综合分析,识别了该套页岩岩相类型及相关沉积作用方式,在水体氧化还原条件演变规律分析基础上总结了其沉积演化模式.牛蹄塘组页岩共包含6类岩相:富有机质硅质页岩、含粉砂硅质页岩、粉砂质页岩、黏土质页岩、泥质钙质页岩及粉砂质钙质页岩,主要体现为5种沉积作用方式:砂质机械沉积、钙质化学沉积、硅质生物沉积、泥质絮凝沉积及底流改造沉积.从底到顶,该套页岩表现为一套水体变浅的进积型演化序列,水体还原程度减弱,有机质保存条件变差.牛蹄塘组下段沉积期,盆地相区和陆棚相区分别以硅质生物沉积和钙质化学沉积为主,主体岩相类型为硅质页岩和钙质页岩;上段沉积期,盆地相区和陆棚相区分别以泥质絮凝沉积和砂质机械沉积为主,主体岩相类型为黏土质页岩和粉砂质混合质页岩.提出的牛蹄塘组页岩岩相发育模式及沉积演化规律能为研究区优质页岩的分布预测及页岩气勘探选区评价提供理论依据.%On the basis of comprehensive analysis of outcrops,cores,thin-section,mineral X-ray diffraction,and drilling material,the lithfacies and sedimentary processes for the Lower Cambrian Niutitang shale in the upper Yangtze Platform are identified.Furthermore,the depositional conditions and sedimentary evolutional patterns for this shale are also analyzed.According to organic matter content,mineral composition,mineral texture and rock fabric,six lithofacies are identified,including organic-rich siliceous shale,silty siliceous shale,argillaceous shale,argillaceous calcareous shale,silty calcareous shale and silty mixed shale.These lithofacies are mainly deposited in the ways of clastic mechanical deposition,carbonate chemical deposition,silica biological deposition,argillaceous flocculation deposition,and bottom current rework deposition.Trace elements analysis reveals that the predominated redox conditions for the Niutitang shale deposition are anoxic to dysoxic environments.The degree of anoxia gradually decreases with the sea level fall upwards.During the lower member shale depositional period,the basinal environment is occupied by siliceous shale,and the shelf region is occupied by calcareous shale.During the upper member shale depositional period,the basinal environment is covered by argillaceous shale,and the shelf region is covered by silty mixed shale.The Niutitang shale generally shows a shallowing-upward prograding depositional sequence.From bottom to top,the organic matter content and anoxic degree decreases largely with the sea level fall.The average content of quartz decreases,but the average content of clay mineral increases.