Genesis and main controlling factors of ultra-deep CO₂ accumulation in the Hangzhou Slope Zone of the Xihu Sag

Under traditional understanding, the CO₂ content in organically generated natural gas in the western slope of the Xihu Sag, East China Sea shelf basin is relatively low, ranging mostly from 2% to 3%. This CO₂ mainly originates from kerogen thermal degradation or microbial activities. However, in the practice of oil and gas exploration and development in the Hangzhou Slope Zone, the CO₂ content in some natural gas reservoirs reached 5% to 7%. The proportion of this non-hydrocarbon gas requires focused consideration in its economic impact on reserve resource evaluation. To clarify the genesis and distribution patterns of different types of CO₂ and reduce exploration and development risks, in this study, we demonstrated the main sources of CO₂ in natural gas and clarified the typical characteristics and distribution rules of CO₂ of different genesis by comparing factors such as the regional geological background of key oil and gas fields and the geochemical characteristics of natural gas. Results show that: ① The carbon isotope values (\textδ^13 \mathrmC_\mathrmCO_2 ) of the small amount of inorganic CO₂ in the gas fields were mainly between −5.7‰ and −3.4‰. This CO₂ is of crust-derived inorganic origin and formed mixed-genesis gas reservoirs when mixed with organically generated methane. ② Most areas in the deep to ultra-deep gas reservoirs where the CO₂ content exceeds 5% coincide with the scope of intra-crustal volcanic magmatic activities. The authors innovatively constructed a "transfer station" stepped migration model for intra-crustal volcanic rocks related to CO₂ in the Hangzhou Slope Zone. Non-hydrocarbon gases such as CO₂ migrate upward through major basement faults connecting volcanic rock "transfer stations" and accumulate to form reservoirs at the intersection of faults and reservoirs. This model reveals the "inefficient transfer" role of intra-crustal volcanic rocks in CO₂ reservoir formation, making it difficult to form high-purity CO₂ gas reservoirs. ③ The discovery of inorganic CO₂ in the Hangzhou Slope Zone deepened our understanding of CO₂ genesis in oil and gas reservoirs of the East China Sea, clarified the key identification indicators for CO₂ of different genesis in the East China Sea Shelf Basin, and provided an important basis for the exploration in ultra-deep areas such as buried hills in the Xihu Sag.