基于矿物岩石学和地球化学分析的页岩古环境演化与有机质富集关系——以川西地区下寒武统筇竹寺组为例

Relationship between paleoenvironmental evolution and organic matter enrichment of shale of the Lower Cambrian Qiongzhusi Formation in Western Sichuan:evidence from mineral petrology and geochemistry

川西地区下寒武统筇竹寺组有机质含量在中上扬子地区处于较低水平,富集控制因素认识不清。利用岩心、岩石薄片、全岩X线衍射、有机地球化学和元素地球化学等分析资料,探讨研究区页岩矿物岩石学特征及形成环境,与黔北斜坡区的进行对比,分析古环境与有机质富集的关系。结果表明:川西地区筇竹寺组以硅质页岩相为主,不同沉积时期岩相特征和有机质含量差异较大,筇一段(w(TOC)平均为3.24%)及筇二段上部(w(TOC)平均为2.32%)硅质页岩有机质含量较高,同时发育陆源硅和生物硅,筇二段中下部(w(TOC)平均为1.22%)及筇三段(w(TOC)平均为0.45%)硅质页岩有机质含量较低,以陆源硅为主,有机质含量为筇一段>筇二段上部>筇二段中下部>筇三段的特征;黔北斜坡区筇一段(w(TOC)平均为5.13%)及筇二段(w(TOC)平均为3.82%)以生物成因的硅质页岩相为主,有机质含量远高于川西地区的。富有机质页岩的形成主要受陆源输入、氧化还原条件、古生产力水平及水体滞留程度的综合控制,筇一段及筇二段上部海平面处于相对较高的位置,具有低的陆源输入、缺氧的底水条件、高的古生产力水平和半滞留的水体性质等沉积环境,有机质含量较高;筇二段中下部及筇三段海平面处于相对较低的位置,陆源输入增多,底水含氧量升高,水体滞留程度增强,古生产力水平降低,有机质含量逐渐降低。较弱的上升洋流作用和高的陆源输入是导致研究区有机质含量处于较低水平的主要原因。该结果为川西地区下寒武统筇竹寺组页岩气储层评价和有利区预测提供指导。

The TOC of the Lower Cambrian Qiongzhusi Formation in Western Sichuan is at a low level in the Middle-Upper Yangtze Area,and there is relatively weak research on its enrichment control factors.Based on the data of core,rock thin section,whole-rock X-ray diffraction,organic geochemistry and element geochemistry,the petrological characteristics and formation environment of shale minerals in the study area are axplored,and compares it with the slope area in Northern Guizhou,its relationship with organic matter enrichment is clarified.The results show that the Qiongzhusi Formation in Western Sichuan is dominated by siliceous shale,but the lithofacies characteristics and TOC content vary greatly in different sedimentary periods.The organic matter content of the siliceous shale in the first member(average of w(TOC) is 3.24%) and the upper part of the second member(average of w(TOC) is 2.32%) is relatively high,and terrigenous silicon and biological silicon are developed at the same time.The organic matter content in the middle and lower part of the second member(average of w(TOC) is 1.22%) and the third member(average of w(TOC) is 0.45%) is low,and the siliceous is mainly terrigenous silicon.On the whole,the TOC content from high to low of Qiongzhusi Formation in Western Sichuan are:the first member,the upper part of the second member,the middle and lower part of the second member,the third member.In the slope area of Northern Guizhou,the first member(average of w(TOC) is5.13%) and the second member(average of w(TOC) is 3.82%) of the Qiongzhusi Formation are mainly biogenic siliceous shale,and its TOC content is much higher than that in Western Sichuan.The formation of organic-rich shale is mainly controlled by terrigenous influx,anoxic bottom water environment,paleoproductivity level and water retention degree.The sea level in the first member and the upper part of the second member of Qiongzhusi Formation is relatively high,and its TOC content is high with low terrigenous influx,anoxic environment,high paleoproductivity level,and semi-retention water degree;the sea level in the middle and lower part of the second member and the third member is relatively low,and its TOC content gradually decreases with increased terrigenous influx,increased oxygen content in bottom water,increased water retention,and decreased paleoproductivity.The weak upwelling and high terrigenous influx are the main reasons why the TOC content of the Qiongzhusi Formation in Western Sichuan is at a low level in the Middle-Upper Yangtze Area.The results provide guidance for shale gas reservoir evaluation and favorable areas of the Lower Cambrian Qiongzhusi Formation in Western Sichuan.