Carbon and Oxygen Isotope Enrichment Characteristics and Formation Mechanism of Carbonate Cements in Chang 8 Tight Sandstone Reservoir, Jiyuan Area, Ordos Basin

To elucidate the formation mechanisms of carbonate cement in the Chang 8 tight sandstone reservoir of the Upper Triassic Yanchang Formation (Jiyuan Area), the types, paragenetic stages, and genesis of the cement were investigated via thin-section petrography, carbon–oxygen isotope analysis, and fluid inclusion microthermometry. Results indicate two distinct generations of carbonate cement within the Chang 8 reservoir: early micritic calcite cement and late sparry to coarsely crystalline ferroan calcite cement. The late ferroan calcite cement exhibits δ13CPDB values spanning from −7.02‰ to −12.51‰ and δ18OPDB values ranging from −18.81‰ to −30.24‰. The early micritic calcite cement is interpreted to have precipitated from supersaturated alkaline pore fluids during the shallow burial stage (Stage A of early diagenesis) at the close of the Late Triassic. Conversely, the late ferroan calcite cement formed during the Early Cretaceous at temperatures between 100 °C and 110 °C, and its genesis is closely linked to organic-acid-related fluids. These findings elucidate the characteristics of carbonate cements across different stages and have significant implications for predicting the distribution of high-quality reservoirs and for providing guidance for future exploration and development.