Abstract: Based on lithology,mineralogy and geochemistry,the generation environment and source for carbonate cements in sandstone reservoirs in the lower position of the Paleogene Lower Ganchaigou Formation in the north of the Mahai Uplift which located in the northern margin of the Qaidam Basin were studied systematically.Two types of calcite cements were found,including xenomorphic micrite and hypidiotopic microcrystalline.Their carbon isotope values (δ¹³C) ranged from -7.1‰ to -3.3‰ with the average of -4.9‰,and the oxygen isotope values (δ¹⁸O) ranged from -11.3‰ to -9.6‰ with the average of -10.3‰.The Z-values of paleosalinity ranged from 107.0 to 115.6 and the average value was 112.1.The generation of carbonate cements in sandstone reservoirs in the study area might be related to the decarboxylation of organic matters during the early diagenetic stage.A little organic carbon also participated in the reaction.Diagenetic fluids mainly came from clast and atmospheric freshwater.