四川盆地南缘上奥陶统五峰组—下志留统龙马溪组页岩矿物组成与脆性特征

Mineralogy and brittleness features of the shale in the upper Ordovician Wufeng Formation and the lower Silurian Longmaxi Formation in southern Sichuan Basin

通过X射线全岩衍射、光学薄片鉴定及扫描电镜等技术手段,对四川盆地南缘上奥陶统五峰组-下志留统龙马溪组页岩野外露头及新钻井岩心样品进行测试分析.结果表明,五峰组-龙马溪组页岩矿物组分以石英和粘土矿物为主,其次为方解石和白云石,还有少量的钾长石、斜长石和黄铁矿.其中石英以陆源碎屑来源为主,部分为生物成因;伊利石与绿泥石组合是粘土矿物的特征组合类型;方解石和白云石多以亮晶胶结物和裂缝填充物的形式存在.以矿物组分为依据,将五峰组-龙马溪组页岩划分为富石英页岩、富粘土页岩及富碳酸盐页岩3类端元岩相,每类岩相在矿物组成、结构、构造及沉积背景等方面均表现出不同特征.经计算该页岩脆性指数为0.36-0.74,平均为0.51,表明该套页岩具备良好的脆性与可压裂性,有利于页岩气的开采.其中,底部页岩石英含量高,脆性指数大且富含有机质,有利于页岩气富集与储层压裂改造,是研究区页岩气勘探开发的重点层位.

A detailed study about the mineral composition of the upper Ordovician Wufeng shale and the lower Silurian Longmaxi shale in southern Sichuan Basin was conducted by the measurements on core and outcrop samples including XRD(X-ray diffraction),thin slices observation,SEM(Scanning Electron Microscope)and QEMSCAN(Quantitatively Electron Microscope Scanning). The results obtained show that quartz and clay minerals are the dominant minerals in this shale,followed by calcite and dolomite,with a small amount of potassium feldspar,plagioclase and pyrite. The quartz is mainly from terrigenous clastic sources while some of them is biogenous. The clay minerals are characterized by a combination of illite and chlorite;and the calcite and dolomite mainly act as cement and fracture filling in the shale. Based on mineral composition,the Wufeng-Longmaxi shale is classified into three lithofacies:quartz-rich shale,clay-rich shale and carbonate-rich shale,which have significant differences in the mineral composition,texture,fabric and depositional background. The calculated brittle index of 0.36-0.74(0.51 on average)for the Wufeng-Longmaxi shale indicates that this shale is favorable forhydraulic fracture to produce shale gas. The shale at the bottom has higher quartz content,larger brittleness index and richer in organic matter,which is favorable for shale gas enrichment and reservoir fracturing.