潜江凹陷古近系盐间地层沉积环境与有机质富集——以潜江组三段下亚段10号韵律为例

Sedimentary environment and organic matter enrichment of Paleogene inter-salt strata in Qianjiang sag:a case study from 10th rhythmite of the Lower Member 3 of Qianjiang Formation

潜江凹陷潜江组发育多套富有机质的盐间细粒沉积岩,是潜在的页岩油勘探目标,对盐湖环境有机质富集机制的认识不清制约了页岩油成藏机理的研究。以潜江组三段(潜三段)下亚段10号韵律盐间层为研究对象,综合岩心、岩石薄片、X射线衍射矿物分析与常量、微量、稀土元素分析等地质资料,探讨了潜江凹陷盐间地层有机质的富集机理。根据物质组成和岩相类型,将潜三段下亚段10号韵律盐间层划分为3段,其中,上段、下段由层状白云质泥岩和钙芒硝岩组成,总有机碳(TOC)平均含量为0.83%;中段由纹层状白云质泥岩和纹层状灰质泥岩组成,TOC平均含量为1.35%。盐间层中段黄铁矿的晶体形态、元素地球化学特征反映热液活动参与其沉积。盐间层中段沉积期,古气候相对湿润,水体还原性较强、盐度偏低、生物生产力高;盐间层上段和下段形成于干热古气候条件,沉积水体还原性较弱、盐度高、生物生产力低。潜江凹陷盐间层有机质的富集是热液活动和气候变化共同作用的结果。其中,热液活动为表层水体带来营养物质,利于盐湖生物繁盛和生产力水平提高;古气候的干湿变化通过改变盐湖水体的氧化还原性和盐度,进而影响沉积有机质的保存。

Multiple inter-salt layers composed of organic-rich fine-grained sedimentary rocks are developed in the Qianjiang Formation,which is a potential target for shale oil exploration in Qianjiang sag.However,the unclear understanding of the mechanism of organic matter enrichment in salt lakes restricts the study of shale oil accumulation mechanism.The geological data including core,thin section,X-ray analysis and major,trace,rare earth elements were integrated to study the enrichment mechanism of organic matter in the inter-salt stratum of the 10 th rhythmite of the Lower Member 3 of Qianjiang Formation.According to the mineral composition and lithofacies types,the inter-salt stratum of the 10 th rhythmite of the Lower Member 3 of Qianjiang Formation can be divided into three sections.The upper and lower sections are composed of bedded dolomitic mudstone and glauberite,with average TOC content of 0.83%,while the middle section is composed of laminated marlstone and laminated dolomitic mudstone,with an average TOC content of 1.35%.The crystal morphology of pyrite and geochemical characteristics of elements indicates that inter-salt layers in the Qianjiang sag has recorded hydrothermal activity.During the sedimentary period of the middle section of inter-salt layers,the climate was relatively humid,with a strong reducing condition,low salinity and high biological productivity.The upper and lower sections of inter-salt layers were formed in a dry and hot climate,with a weak reducing condition,high salinity and low biological productivity.This study indicates that the enrichment of organic matter in inter-salt layers of Qianjiang sag was resulted from the hydrothermal activity and climate change.The hydrothermal activity played an important role in improving the biological productivity in salt lakes,which brought nutrients to the surface waters.The dry-wet changes of paleoclimate can affect the preservation of sedimentary organic matter by controlling the changes of redox and salinity of the salt lake.