上扬子地区下寒武统筇竹寺组页岩沉积环境

Sedimentary environment of the Lower Cambrian Qiongzhusi Formation shale in the Upper Yangtze region

上扬子地区下寒武统页岩展现出了良好的页岩气勘探前景,是油气资源的重要战略接替领域。通过露头观测、岩心描述、薄片观察、扫描电镜分析以及主微量元素地球化学分析,对该套页岩沉积环境进行了综合研究。根据矿物学、岩石组构及沉积构造的不同,在筇竹寺组识别出5大类12小类岩相类型,进一步总结了浅水陆棚、深水陆棚、斜坡-盆地沉积体系中发育的6类典型的沉积序列,建立了相应的沉积模式。浅水陆棚沉积处于氧化、相对低的古海洋生产力条件下,发育磷质-云质-泥质沉积序列和砂质-泥质-灰质沉积序列,有机质含量普遍偏低。深水陆棚以及斜坡-盆地沉积处于贫氧-缺氧、高海洋生产力的条件下,发育富含有机质的泥质-磷质-云质沉积序列、硅质-泥质沉积序列和泥质沉积序列。在四川盆地资阳-长宁一线陆内裂陷槽内部以及四川盆地之外东南部黔中、黔北及鄂西-渝东等大部分深水陆棚沉积区是富有机质页岩发育的有利区域。此外,热液活动为黔西北地区下寒武统牛蹄塘组黑色页岩提供了必要的物质基础和保存条件,有利于形成富有机质的页岩。

The lower Cambrian shale shows a bright potential for shale gas exploration and is an important strategic successor play for oil and gas exploration in the Upper Yangtze region.The sedimentary environment of the Qiongzhusi Formation shale was comprehensively studied through outcrop observation,core description,thin section observation,SEM analysis,and geochemical analysis of major and trace elements.According to mineralogy,rock fabrics and sedimentary structures,five major-and 12 sub-lithofacies types were identified in the Qiongzhusi Formation shale,six typical sedimentary sequences developed in shallow-water shelf,deep-water shelf,and slope-basin sedimentary systems were summarized,and the corresponding sedimentary models were established.Under the paleoceanographic conditions of oxidation and relatively low productivity,phosphorus-dolomitic-argillaceous sedimentary sequences and sandy-argillaceous-calcareous sedimentary sequences were developed in shallow shelf facies with low TOC.In contrast,organic-rich argillaceous-phosphorus-dolomitic,siliceous-argillaceous,and argillaceous sedimentary sequences were developed in deep shelf and slope-basin facies with sediments deposited in oxygen-poor,anoxic oceans with relatively high productivity.The majority of sedimentary zones in deep shelf facies is favorable for the development of organic-rich shales in the interior of intracratonic rift along Ziyang-Changning trend within Sichuan Basin,central and northern Guizhou and western Hubei-Eastern Chongqing areas in southeastern Sichuan Basin.In addition,hydrothermal activities provide necessary materials and preservation conditions for the black shales of the Lower Cambrian Niutitang Formation in northwestern Guizhou,which in turn may generate organic-rich shales.