Chemical Composition and Isotopic Characteristics of the Carbonate Cements in Sandstone Reservoir Layer of Dongying Sinking

The chemical composition of the carbonate cements in reservoir sandstones was determined by the EPMA. The generation sequence is: calcite, dolomite, ferrous calcite, ankerite, and the latest are coarse-grained and fine-grained calcites which are veined. In the MnO-FeO-MgO triangular diagram, the samples fall clearly on the four areas. The carbon and oxygen isotope data on the carbonate cements show that δ13C are from −6.44 to +4.79, δ18O are from −4.25 to −15.21, focused on three areas in the map. The paleosalinity Z from 107 to 133 calculated by the carbon and oxygen isotope value reflects the characteristics that salinity changes are caused by fresh water filling lake water. The average temperature of calcite precipitation calculated by the oxygen isotope value is 650 °C, while dolomite precipitation is 70.6 °C. The precipitation temperature changes greatly and shows negative correlation with temperature and salinity, which reflects the characteristics of lake sediments.