Abstract: Composition of carbon and oxygen isotopes of calcite cement from 12 samples in sandstone reservoirs of the Lower Part of Number 1 of Shahejie Formation, Dagang beach area, are analyzed. The δ¹³C va-lue of calcite cement varies from -9.58‰ to +3.61‰, and the average is 5.29‰; the δ¹⁸O value varies from -12.46‰ to +0.09‰, and the average is -6.28‰. Discussion of diagenetic fluid salinity, carbon source and formation cause suggest that, diagenetic fluid salinity of calcite cement in this area is high, and the calcite cement comes from intrastratal clastic carbonate or dissolution of calcite from adjacent layers. Carbon mainly originates from organic carbon.