Methods such as GC，GC／MS，microscopic examination of kerogen，and carbon isotope ：analysis were used for a comprehensive study on the typical fresh—water lacustrine oil shales from five basins in China．The organic—rich oil shales can be formed in small—medium—size depressions stably developed under humid climate conditions for a long period，or in anaerobic zones at the bottom of rift—subsidence lake basins；and also shales can be deposited in anoxic／slightly oxic fa— cies zones of medium—deep／deep water column of large size in—land lake．Stratification of water column，relatively high abundance of organic matter and intense activity of microorganisms such ：as bacteria are the main factors responsible for reductivity‘of sedimentary surface waters and ac— cumulation and preservation of organic matter．