Accumulation characteristics and differential enrichment modes of natural gas in different regions of Dongfang 1-1 diapir, Yinggehai Basin

As one of the most important gas-bearing structures in the central depression of the Yinggehai Basin, the diapir-associated gas field group has proven geological natural gas reserves of about 180 billion m³. However, the natural gas accumulation characteristics differ significantly across different regions of the diapir, which seriously affects natural gas exploration process in the central depression. Therefore, taking gas reservoirs in different regions (core, affected region, and unaffected region) of the Dongfang 1-1 diapir as research objects, the study employed gas component analysis, carbon isotope analysis, sandstone physical property tests, and inclusion tests to investigate the natural gas composition and source, reservoir characteristics, migration systems, charging stages, and caprock sealing capacity. The results indicated that the natural gas in the second member of the Pliocene Yinggehai Formation (Y2) in the diapiric core and the first member of the Miocene Huangliu Formation (H1) in the unaffected zone was mainly CH₄, with an average content of 69.98%. In contrast, CO₂ content in the H1 reservoirs of the affected zone and diapiric core was higher, reaching up to 65.99%. The diapiric core and unaffected zone used diapiric faults and associated micro-fractures, respectively, as the main migration systems. The H1 and Y2 gas reservoirs corresponded to the second and third stages of natural gas charging, respectively. The physical properties of the H1 reservoirs improved progressively from the diapiric core towards the affected and unaffected zones. Both the H1 and Y2 reservoirs showed strong overpressure and overpressure characteristics, with formation pressure coefficients of approximately 1.8 and 1.5, respectively. The overlying mudstone caprock of the H1 reservoir in the unaffected zone demonstrated strong physical sealing capacity, with a fracture pressure coefficient less than 0.9, followed by the affected zone, and poorest in the diapiric core. Based on these results, three natural gas migration-accumulation modes were established for the different regions of the Dongfang 1-1 diapir: (1) diapiric core: early hydrocarbon gas charging, late non-hydrocarbon gas charging, overpressure sealing in medium-deep low-permeability reservoirs, and conventional physical sealing in shallow layers; (2) diapir-affected zone: early hydrocarbon gas charging accompanied by mudstone-controlled physical sealing and late non-hydrocarbon gas charging accompanied by overpressure sealing in low-permeability reservoirs; (3) diapir-unaffected zone: hydrocarbon gas charging, micro-fracture migration, and mudstone-controlled physical sealing.