渝东五峰组-龙马溪组页岩矿物成分与孔隙特征分析

Mineral Composition and Microscopic Pore Characteristics of Wufeng-Longmaxi Formation Shale in Eastern Chongqing City, China

页岩的矿物成分和孔隙特征对页岩气成藏和开采具有重要意义。四川盆地渝东石柱县打风坳地区五峰组-龙马溪组富有机质页岩广泛发育,应用X射线衍射技术对其矿物成分特征进行研究,发现五峰组-龙马溪组页岩矿物成分包含石英、长石、方解石、白云石、黄铁矿、伊利石、伊/蒙混层和绿泥石。脆性矿物含量（〉50%）和黏土矿物组合特征表明五峰组-龙马溪组是宜于页岩气形成与开采的有利层位。应用氩离子抛光-扫描电镜方法对页岩微纳孔隙特征展开研究,发现无机孔、微裂缝和有机孔3种孔隙类型。无机孔主要包括粒间孔、黄铁矿晶间孔、黏土矿物层间孔和溶蚀孔,孔径数百纳米至数微米;微裂缝包括构造裂缝和解理缝,缝长集中于3～10μm,缝宽数百纳米。有机孔主要发育在有机质内部,呈片麻状或蜂窝状,孔径30～200 nm,连通性较好。总体而言,五峰组页岩中最主要的孔隙类型是溶蚀孔,龙马溪组页岩中主要发育粒间孔和有机孔。

Mineral composition and microcopic pore characteristics are significant for shale gas accumulation and exploration. Organic rich shale samples of Wufeng-Longmaxi formation from Shizhu County（Eastern Chongqing City, Sichan Basin） were investigated by X-ray diffraction（XRD） and scanning electron microscopy（SEM） coupled with Ar-ion milling technique. The XRD indicates that main mineral components of Wufeng-longmaxi shale are quartz, feldspar, calcite, dolomite, pyrite, illite, illite/smectite mixed layer and chlorite. Brittle mineral content higher than 50% is favorable for fracturing and the assemblages of clay mineral indicate an idea stratum for shale gas accumulation and exploration. SEM observations suggest that there are three microscopic pore types in Wufeng-Longmaxi shale, including mineral matrix pores, micro fractures and organic matter pores. Mineral matrix pores of hundreds of nanometers to several microns are composed of interparticle pores, intercrystalline pores within pyrite, pores between clay platelets and dissolution pores; micro fractures containing tectonic fracture and foliation crack usually develop along mechanical unstable area with length to a few micrometers and width to hundreds of nanometers; most organic matter are porous and organic matter pores of 30-200 nm are recognized with good connectivity. Generally, dissolution pore is the main pore type developing in Wufeng formation while interparticle pore and organic matter pore are most widespread in Longmaxi formation.