The core of well Pu-115 was systematically sampled to investigate the sedimentary and geochemical characteristics of the three types of rocks of the upper section of Mb Ⅲ of the Shahejie Formation: siliciclastics, carbonates and mixed rocks. The result indicates that the carbonate components of the target formation are proven to be authigenic. Carbon and oxygen isotopic compositions of the carbonates are significantly different from that of the carbonate components of clastic rocks. When siliciclastics are the dominant deposition, δ¹⁸O is depleted and does not covariate with δ¹³C, which suggests a relatively open system with minor evaporation and probably abundant inflow of fresh water. When carbonates deposite, oxygen isotope becomes heavier and carbon isotope excurses negatively. The positive trend of oxygen isotope may indicate a heavy evaporation or less inflow of fresh water, and the depletion of carbon isotope may denote the oxic condition of the bottom water. The frequent change of the carbon and oxygen isotopic composition may reflect a rapid shift of the redox state of the lake bottom, and the decrease of clastic components may indicate a fall of the lake water level and the lake was relatively isolated and closed during the deposition of the target formation.