鄂西来凤-咸丰地区五峰组-龙马溪组岩石学特征及其成因

来凤-咸丰地区位于四川盆地东部外侧相邻的鄂西地区,其奥陶系五峰组-志留系龙马溪组暗色岩系的有机质丰度较低。根据6条露头剖面和3口钻井岩心的详细观测,通过借助偏光显微镜、X-射线衍射、场发射扫描电镜和有机碳含量分析等对来凤-咸丰地区五峰组-龙马溪组的岩石学和矿物学特征进行了研究。研究区五峰组-龙马溪组矿物组分以石英和粘土矿物为主,发育大量的粉砂级碎屑颗粒,岩石类型以富有机质的粉砂岩、泥页岩为主,由底向上依次可划分为4个岩性段:炭质(含)粉砂质页岩段、含炭含泥(质)粉砂岩段、含泥粉砂岩段及含炭粉砂质页岩段。在对比四川盆地相应层系页岩气富集条件的基础上,根据矿物、岩石特征与元素地球化学的指示,认为来凤-咸丰地区五峰组-龙马溪组发育较短时期的缺氧还原环境、快速变化的海平面和较小的沉积速率,使其在较高的生物产率条件下,富有机质页岩的沉积厚度和有机质的丰度相对四川盆地明显较差。

Petrologic characteristics and genesis analysis of the Ordovician Wufeng Formation-Silurian Longmaxi Formation in Laifeng-Xianfeng area of western Hubei

As part of western Hubei province,Laifeng-Xianfeng area is adjacent to the east of Sichuan Basin,with its Upper Ordovician Wufeng-Lower Silurian Longmaxi Formation dark rock series bearing lower organic abundance.The petrological and mineralogical characteristics of Upper Ordovician Wufeng-Lower Silurian Longmaxi Formations in Laifeng-Xianfeng area, were investigated according to detailed observation of the six outcrop sections and three cores, in combination with polarizing microscope,X-ray diffraction(XRD),field emission scanning electron microscopy(SEM)and organic carbon content(TOC)analysis.Large amount of clastic particles in silt size are developed in the study area whose mineral components are mainly quartz and clay minerals.Besides,siltstones and shales rich in organic matters are the main rock types in this area,which can be divided into four lithologic sections from the bottom-up including carbonaceous and siltyshales,carbonaceous and argillaceous siltstones,argillaceous siltstones and carbonaceous siltyshales.According to instructionsby petrological and mineralogical characteristics and geochemistry of element,and comparison of shale enrichment conditions with the corresponding formation of Sichuan Basin,it can be figured out that the shorter-term anoxic and reducing environment,rapid change of sea level and smaller rate of deposition,render the thickness and abundance of organic matters of organic-rich shales in the Wufeng-Longmaxi Formations in Laifeng-Xianfeng area with higher bioproductivity to be significantly worse than those of the corresponding formation in Sichuan Basin.