Formation Time and Material Source of Carbonate Cements in Permian Sandstone in the Ordos Basin, Western of China

Abstract

AbstractCarbonate cements are an important type of cement in Permian tight gas reservoirs in the Ordos Basin. Moreover, the distribution of carbonate cements in sandstone provides an important basis for studying the fluid mechanism and genesis of cements and a scientific basis for investigating the physical properties of reservoirs and the underground distribution of oil and gas. Based on the observations of thin sections and temperature measurements of fluid inclusions, this study concludes that the carbonate cements in the study area are dominated by calcite and ferrous calcite and divided into three phases, where the latter two phases are dominant. The material source of carbonate cements is obtained via the thermal simulations of gold tubes in source rocks, temperature of fluid inclusions in carbonate cements, composition of fluid inclusions in quartz overgrowth, and analysis of carbon and oxygen isotopes. Results show that phase I calcite may be directly precipitated from the supersaturation of pore water by Ca2+ and CO32−. The CO32− in the phase II calcite and ferrous calcite stems from the CO2 generated in the hydrocarbon generation process of organic matter, while the CO32− in phase III calcite stems from the CO2 generated in the decarboxylation of organic acids. Ca2+ is derived from the dissolution of early carbonate cements in sandstone and conversion of smectite into illite in mudstone. The Permian sandstone in the Yishan Slope at the center of the basin is close to the source rock, and the carbon isotopic value of the carbonate cements, which is dominated by organic carbon, ranges between −7.5‰ and −19.0‰. However, the Permian sandstone in the Tianhuan Depression at the edge of the basin is far away from the source rock, with its carbon isotope value ranging from −5.9‰ to −8.7‰, which is probably due to the insufficient supply of organic carbon and a small amount of early inorganic CO2 remains in the reservoir.