Calcite U-Pb dating and fluid inclusions reveal late Permian deep mantle CO2 fluid activity in the East China basin and its effect on crude oil within source rock: A case study from Huangqiao area of Lower Yangtze Plate

Accumulations of large volumes of CO2 related to mantle degassing, metamorphic reactions or magmatic processes have been found in many oil-gas bearing basins around the world . The Huangqiao area of the Lower Yangtze Plate hosts the largest CO2 gas field on mainland China. Throughout geological history, a significant influx of deep mantle-derived CO2 fluid occurred in this area. Understanding the timing of these CO2 charge and their effects on crude oil reservoirs is crucial for interpreting the distribution of present-day resources. The Cenozoic was long believed to be the only period during which CO2 charging occurred in the Huangqiao area, primarily because evidence of earlier CO2 fluid charges had been scarce. To address this, a comprehensive study utilizing petrography, cathodoluminescence, fluorescence and Raman spectrum of fluid inclusions, in-situ U-Pb dating, and basin modeling was conducted to elucidate the timing and interactions between crude oil and deep mantle-derived CO2 in the Permian Qixia Formation of the Huangqiao area. Three distinct phases of calcite veins were identified and dated: 251.7 ± 1.8 Ma, 124.16 ± 1.46 Ma, and 97.68 ± 1.20 Ma to 96.75 ± 0.25 Ma. The earliest CO2 charge, occurring around 251.7 ± 1.8 Ma, corresponds to a period when supercritical CO2 extracted low molecular-weight hydrocarbons from the S1g source rock. This timing aligns with the mass extinction event (251.4 ± 0.3 Ma), a rapid rise in atmospheric CO2 levels, and volcanic activity in the Permian Gufeng and Longtan Formations of the Lower Yangtze Plate, suggesting that the CO2 influx was volcanically driven. Between 124.16 ± 1.46 Ma and 96.75 ± 0.25 Ma, significant portions of the CO2 and crude oil within the Qixia Formation escaped due to tectonic uplift and erosion associated with the collision between the Yangtze Plate and the North China Plate. This research provides the first documentation of early mantle-derived CO2 fluid charges and their role in crude oil extraction from source rocks during transport from the mantle to the Earth's crust. Additionally, the study reconstructs the processes of CO2 and oil accumulation and leakage from the Indosinian to the Yanshanian periods, offering new insights into the evolution of hydrocarbon reservoirs in the Huangqiao area of Lower Yangtze Plate.