Accumulation mechanism and model of multi-type deep coarse-grained siliciclastic reservoirs in the eastern Jiyang Depression, Bohai Bay Basin

Deep coarse-grained siliciclastic rocks in the eastern Jiyang Depression, Bohai Bay Basin hold great potential for oil and gas exploration. Given the presence of various types of deep coarse-grained siliciclastic reservoirs and the significant geological differences in hydrocarbon accumulation between the deep and the medium-to-shallow reservoirs in the depression, there is an urgent need to understand the formation mechanisms behind these differentiated geological features in order to advance the exploration and exploitation of deep oil and gas reserves. Employing techniques such as thin-section microscopy, reservoir fluid inclusion analysis, and basin simulation, we investigate pore types in the deep reservoirs and the origin of differentiated formation pressure in the Jiyang Depression. The accumulation models of various types of deep coarse-grained siliciclastic reservoirs in the depression are established thereby. The key findings are as follows: (1)The diagenetic evolution reveals the formation mechanisms of the various pore types of reservoirs. The preservation of primary intergranular pores in the deep coarse-grained siliciclastic reservoirs is attributed to the combined effects of early hydrocarbon charging and overpressured fluids. In contrast, the secondary dissolved pores in the reservoirs are developed largely due to dissolution reactions induced by acidic fluid charging during the hydrocarbon generation of source rocks. In addition, the overpressure in the reservoirs facilitates the development of microfractures. All these lead to the formation of pore-fracture reservoir spaces; (2)The evolution of fluid overpressure in the deep reservoirs is governed by hydrocarbon generation and depositional process. The formation of overpressure environment hinges on preservation conditions, and the pressure distribution in hydrocarbon reservoirs within structural-lithologic traps is regulated by the fault-sand body transport system. In deep normal pressured-weakly overpressured reservoirs, the residual pressure difference between source rocks and reservoirs serves as the primary driving force to hydrocarbon accumulation. While in overpressured reservoirs, this residual pressure difference gradually decreases over time. Furthermore, buoyancy provides the main driving force for the secondary migration and adjustment of hydrocarbons in areas with well-developed faults and in sand bodies with good vertical connectivity; (3)Based on the joint control of hydrocarbon generation-reservoir-pressure on hydrocarbon accumulation, we establish three hydrocarbon accumulation models for deep coarse-grained siliciclastic reservoirs in the eastern Jiyang Depression, including the model of vertically differentiated hydrocarbon accumulation in multi-phase superimposed nearshore subaqueous fans, the model of top-lateral joint hydrocarbon sealing by mudstone within nearshore subaqueous fans, and the model of near-source hydrocarbon accumulation within nearshore subaqueous fans-tubidite fans. The elucidation of these hydrocarbon accumulation mechanisms and models of multi-type deep coarsegrained siliciclastic reservoirs in the Jiyang Depression lays a solid foundation for subsequent in-depth exploration.