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 for hydraulic 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.