Organic-rich Lower Paleozoic Qiongzhusi, Wufneg, and Longmaxi formations in South Sichuan and North Yunnan areas are the main gas prospecting strata in China. XRD analysis is used to determine the mineral composition and brittleness indexes of three shale suites in these areas. These parameters are important for evaluating shale gas resources, analyzing reservoir-forming mechanisms, and planning prospecting. The investigation results indicate that the three shale suites have a mean quartz content of more than 27% and a mean clay mineral content of less than 37.27%. Their sedimentary environment is euxinic. They passed through a telodiagenetic phase. The mean relative brittleness index of the samples from Qiongzhusi, Wufneg, and Longmaxi formations are 69.54, 67.33, and 60.44%, respectively. Such high relative brittleness indexes imply that the studied shale suites have good fracturability.