Mineral composition of the Lower Cambrian black shale in the Upper Yangtze region and its significance in oil and gas exploration

Abstract

The Lower Cambrian is one of the important exploration strata of marine shale gas reservoirs in the Upper Yangtze region, but its exploration is geologically impacted by multiple factors such as deep burial depth, fewer wells and unsatisfactory exploration results. In this paper, the mineral composition of the Lower Cambrian black shale in the Upper Yangtze region was investigated using the data of X-ray diffraction, rock thin section, scanning electron microscope (SEM) and argon ion polishing-electron microscope, and its significance in oil and gas exploration was also analyzed. The following findings were made. First, shale in this region is mainly composed of clay minerals and quartz, and secondarily of carbonate minerals and feldspar. The average content of quartz in shale is 44.6%, and two types of quartz are developed, i.e. terrigenous quartz and biogenic silica. The average content of clay minerals is 33.32%, with illite (I) content being the highest, illite/smectite (I/S) layer content uneven, the mixed layer ratio lower, generally 5%, and chlorite (C) and kaolinite (K) locally distributed. Second, four types of clay mineral assemblages are identified, including I + I/S + C, transition from I + I/S + C to I + C, I + C, and I, indicating that diagenetic environments of shale are different in different areas. Third, the disordered arrangement of mineral grains is favorable for the survival of intergranular pores, indicating that in the diagenetic alteration process of feldspar into clay minerals, linear pores can be generated. It is concluded that the brittleness index of shale in this region is generally above 40%, indicating a good fragility. The fragility of shale in SE Guizhou is better than that in SW Sichuan Basin.