Formation conditions and hydrocarbon accumulation model of deep low-ultra-low permeability gas reservoirs of Y Structure in Central Anticline Belt, X Sag, East China Sea

Oil and gas exploration has confirmed that the deep low-permeability to ultra-low permeability sandstone reservoirs in the central anticline of the X Sag in the East China Sea contain abundant natural gas resources. However, the geological understanding of oil and gas resources in low-permeability to ultra-low permeability reservoirs is not yet deep, which greatly responds to and restricts the increase of oil and gas reserves and production in the East China Sea. The Y structure is the largest inversion anticline structure in the X Sag. Through in-depth analysis and research on the characteristics of source rocks, reservoir properties, and diagenesis, combined with its thermal burial history, this study focuses on elucidating the evolution process of low-permeability ultra-low permeability sandstone in the main production layer of the gas reservoir, the influencing factors of low-permeability ultra-low permeability, and the coupling relationship between reservoir diagenesis and reservoir formation. Research has shown that during the reversal period of the Longjing Movement, the erosion amount of the Y structure exceeded 1 200 m, indicating that the sandstone reservoir of the Huagang Formation experienced the historical maximum burial depth and highest temperature at 13 Ma, making it easy to form low-permeability to ultra-low permeability sandstone reservoirs. According to the degree of densification and evolutionary characteristics of sandstone reservoirs in the research area, gas reservoirs can be divided into three types: the H3 sand layer of the Huagang Formation composed of low-permeability gas reservoirs, the H4-H5 reservoir formed while forming extremely low-permeability gas reservoirs, and the H8-H12 reservoir formed first and then extremely low-permeability gas reservoirs. On this basis, combined with the analysis of the process of oil and gas reservoir formation, a reservoir formation model of "vertical two-stage filling of Huagang Formation and adjustment and migration of Longjing Formation" was established in the central anticline belt.