四川盆地城口地区下寒武统水井沱组有机质富集控制因素  

Controlling Factors of Organic Enrichment in the Shuijingtuo Formation in the Lower Cambrian of the Chengkou Area,Sichuan Basin

作　　者：古恒 王剑 韦恒叶 付修根 GU Heng;WANG Jian;WEI HengYe;FU XiuGen(School of Geoscience and Technology,Southwest Petroleum University,Chengdu 610500,China;Qiangtang Institute of Sedimentary Basin,Southwest Petroleum University,Chengdu 610500,China;State Key Laboratory of oil and Gas Reservoir Geology and Exploitation(Southwest Petroleum University),Chengdu 610050,China)

机构地区：[1]西南石油大学地球科学与技术学院,成都610500 [2]西南石油大学羌塘盆地研究院,成都610500 [3]油气藏地质与开发工程国家重点实验室(西南石油大学),成都610500

出　　处：《沉积学报》2024年第3期1073-1091,共19页Acta Sedimentologica Sinica

基　　金：国家自然科学基金项目(41972121,41762003);中国石油—西南石油大学创新联合体(2020CX010100)。

摘　　要：【目的】四川盆地下寒武统水井沱组黑色页岩是四川盆地重要的烃源岩和页岩气勘探层位,但目前对该黑色页岩有机质富集机理还存在不同的看法。【方法】通过对川东北城口地区下寒武统水井沱组下部黑色页岩的沉积相与沉积微相分析、环境成因草莓状黄铁矿形态与粒径变化规律研究,结合黑色页岩总有机碳含量、主量元素、微量元素等地球化学数据分析,重建了四川盆地东北部水井沱组沉积期古地理、古环境演化模式,探讨了有机质富集主控因素与形成机制。【结果】水井沱组黑色页岩可以划分出与深水陆棚—斜坡环境有关的五个沉积微相。生产力指标Ba_(XS)、P/Ti及陆源碎屑输入指标Ti、Al表明,水井沱组黑色页岩沉积环境处于较低的古生产力水平,陆源碎屑输入较稳定且与TOC没有明显相关性。因此,古海洋初级生产力及陆源碎屑输入不是水井沱组黑色页岩有机质富集的主控因素。氧化还原敏感元素指标U/Th、U_(EF)、Mo_(EF)及草莓状黄铁矿研究表明,有机质含量与古水体氧化还原指标呈明显正相关,指示下寒武统水井坨组有机质富集主要受到水体氧化还原条件变化的控制。【结论】基于底水氧化还原条件变化规律、沉积相及沉积充填演化序列等认识,认为寒武纪第二阶—第三阶(529~514 Ma)时期,水井沱组黑色页岩有机质富集受大规模海侵事件导致的底水氧化还原条件波动控制,并提出了有机质富集模式。[Objective]The Lower Cambrian Shuijingtuo Formation black shale is an important source rock and shale gas exploration target in the Sichuan Basin.However,the enrichment mechanism of organic matter in this series of black shale is still unclear.[Methods]Here,we reported total organic carbon(TOC)contents,sedimentary microfacies,major and trace elements,and pyritic framboid size distributions in the lower part of Shuijingtuo Formation in the Chengkou area,northeastern Sichuan Basin on the Upper Yangtze Platform,providing new evidence for reconstructing the paleogeography,paleoenvironment,and main controlling factors of organic matter enrichment.The results show that the black shale succession in the Shuijingtuo Formation developed five sedimentary microfacies which related to the deep-water shelf slope environment.Productivity index(Ba_(XS),P/Ti)and continental input index Ti,Al indicate that the sedimentary environment of black shale in the Shuijingtuo Formation is at a low level of paleoproductivity,and the continental input is relatively stable and has no obvious correlation with TOC.Therefore,paleoproductivity and continental input are not the main controlling factors of organic matter enrichment in the black shale of the Shuijingtuo Formation.The redox-sensitive element index(U/Th,U_(EF),Mo_(EF))and pyrite framboid size distributions show that the organic matter content is positively correlated with the redox index,indicating that the organic matter enrichment in Shuijingtuo Formation is mainly controlled by the change of redox conditions of the water column.[Conclusions]Based on the above understanding,it can be inferred that the organic matter enrichment of the black shale of the Shuijingtuo Formation was controlled by the fluctuation of redox conditions of the bottom water caused by the large-scale transgression event during the stage 2 to stage 3 Cambrian(529-514 Ma),and the organic matter enrichment model was proposed here.

关 键 词：水井沱组 下寒武统 黑色页岩 氧化还原条件 有机质富集 四川盆地 

分 类 号：P586[天文地球—岩石学]