西湖凹陷杭州斜坡带超深层CO2成因与成藏主控因素OA

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 cla-rified 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.Res-ults show that:① The carbon isotope values(δ13CCO2)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 au-thors 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 base-ment 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 reser-voir formation,making it difficult to form high-purity CO2 gas reservoirs.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.