Tracking CO₂ migration and accumulation in the Subei Basin using geochronology and fluid inclusion quantitative...

In petroliferous sedimentary basins, the interplay between CO₂ and hydrocarbons exerts a notable influence on hydrocarbon generation and accumulation. This research focuses on the Huangqiao oil and gas reservoir, which is known for hosting the largest CO₂ reserves in China. U-Pb isotopic dating of calcite veins in fractures, carbon and oxygen isotope analyses, along with rare earth element (REE) analysis were applied to elucidate the chronology and origin of inorganic and organic fluids in the studied area. Petrographic observations revealed the presence of various components of fluid inclusions, including gaseous CO₂, gaseous CH₄, CH₄-CO₂ mixtures, and hydrocarbon fluids. Besides, through Raman quantitative measurements and thermodynamic simulations, the density, composition, pressure, and temperature characteristics of CH₄ and CO₂ bearing fluid inclusions were calculated. Based on the entrapment conditions of fluid inclusions and U-Pb dating results, two stages of hydrocarbon charging were identified: an early-Jurassic stage (approximately 200-185 Ma) characterized by mid-maturity oil and CH₄ and an early-Eocene stage (approximately 61-41 Ma) marked by high-maturity oil and CH₄. CO₂ accumulation events were divided into two stages: high-density CO₂ fluid activity during the early Eocene (approximately 59-39 Ma) and low-density CO₂ fluid activity during the Tertiary-Quaternary (approximately 23-4 Ma). Moreover, deep fluid influx into reservoirs led to hydrothermal alteration, as evidenced by anomalously high homogenization temperatures and vitrinite reflectance. CO₂ has an extraction effect on crude oil, where its late entry primarily results in the removal of lighter components, especially CH₄. When high-temperature hydrothermal CO₂ fluid enters the oil reservoir, it accelerates the cracking of crude oil and alters the fluid's composition. This thermal event also speeds up the source rock's thermal evolution, leading to extraction, pyrolysis, and gas displacement throughout the reservoir's development process. This study presents a comprehensive approach for quantitatively studying geological fluids in petroliferous basins of this nature.