Abstract: The study on mineralogical characteristics of shales is an integral part of shale gas reservoir research, which lays an important foundation for the research on fracture evaluation, seepage flow migration and fracturing and so on. Using X ray diffraction (XRD) analysis technique, the paper qualitatively and quantitatively analyzes the mineral compositions of core samples obtained from 2 sets of marine shales of the Lower Cambrian Qiongzhusi formation and Lower Silurian Longmaxi formation in Well W201 of Weiyuan area, southern Sichuan basin. The results show that the minerals of the 2 sets of marine shales in Weiyuan area of the southern Sichuan basin are mainly composed of clay, quartz, carbonate and a few pyrites. The quartz contents of the marine shale samples from Qiongzhusi formation and Longmaxi formation are 38.7% and 31.8%, respectively, and the clay contents of the 2 formations are 26.1% and 37.3%, respectively. The shales were formed in a marine strong-reduction depositional environment which was good for the enrichment and preservation of organic matters and could provide good conditions for shale gas generation. Brittle mineral contents of the 2 formations are relatively high with the averages of 70.5% and 58.7%, respectively and the brittleness indexes of 73.0% and 61.1%, respectively, which indicates that the 2 formations have good brittleness and fracability. Both Qiongzhusi formation and Longmaxi formation are the key target zones for shale gas exploration