Evolution of Diagenetic Fluid of the Dawsonite-Bearing Sandstone in the Jiyang Depression, Eastern China

Based on the petrology, isotope geochemistry and fluid inclusions analysis, we established the evolutionary mode of the diagenetic fluid of dawsonite-bearing sandstone in the Jiyang Depression. Dawsonite-bearing sandstone is characterized by double injection of CO₂ and oil-gas in the Jiyang Depression that have experienced a relatively complex diagenetic fluid evolution process. The diagenetic sequence of secondary minerals involves secondary enlargement of quartz, kaolinite, first-stage calcite, dawsonite, second-stage calcite, ferrocalcite, dolomite and ankerite. Hydrocarbon charging in the dawsonite-bearing sandstone occurred at around 2.6–0 Myr. The CO₂ charging event occurred during Dongying tectonism, forming the Pingfangwang CO₂ gas reservoir, which provided an abundant carbon source for dawsonite precipitation. Carbon and oxygen isotopic compositions of dawsonite demonstrate that CO₂ forming the dawsonite was of an inorganic origin derived from the mantle, and that water mediating the process during dawsonite precipitation was sequestered brine with a fluid temperature of 82°C. The evolutionary sequence of the diagenetic fluid in the dawsonite-bearing sandstone was: alkaline syngenetic fluids, weak alkaline fluids during organic acid formation, acidic fluids in the early stage of CO₂ injection, alkaline fluids in the late stage of CO₂ injection, and weak alkaline fluids during oil and gas charging. The mode indicates an increase in HCO₃⁻ because of the CO₂ injection, and the loss of Ca²⁺ and Mg²⁺ due to the precipitation of carbonate minerals. Therefore, the evolutionary mode of diagenetic fluids is in good agreement with high HCO₃⁻, low Ca²⁺ and low Mg²⁺ composition of the present formation water in the dawsonite-bearing sandstone.