Fluid pressure gradient characteristics and petroleum geological significance in Shahejie Formation of Bonan Sag in Jiyang Depression

Fluid pressure gradient is the rate at which residual fluid pressure changes both spatially and temporally. It serves as an important parameter for indicating oil and gas migration pathways and accumulation zones, holding great geological significance. This paper focuses on the Shahejie Formation in the Bonan Sag of the Jiyang Depression. Based on drilling, logging, mud logging, seismic and geochemical analysis data, various technical methods were integrated to characterize the spatiotemporal distribution of fluid pressure gradients by mapping both paleo and present-day fluid pressures. Additionally, the controlling factors were analyzed, and their geological significance was discussed. The results show that the horizontal pressure gradient in key layers of the Bonan Sag is significantly lower than the vertical pressure gradient. High horizontal pressure gradient zones are mainly distributed in a ring-shaped pattern around the edge of the sag. There are three high vertical pressure gradient zones, forming elongated zones in a planar distribution with a broad spatial extent. High temporal pressure gradient zones at the beginning and end of the main hydrocarbon accumulation period in the Bonan Sag occur in the southwestern and northeastern regions of the sag, mainly driven by hydrocarbon generation pressurization, whereas the remaining areas exhibit relatively low values, suggesting intense hydrocarbon expulsion and pressure release. Tight lithological layers, fault systems, and the development of high-permeability reservoirs are the main controlling factors of the formation and distribution of pressure gradients. Among them, tight lithological layers control the high vertical pressure gradient zones, and fault systems control both high horizontal and low vertical pressure gradient zones. Fault order and activity influence the magnitude of temporal pressure gradients, and the development of high permeability reservoirs tends to result in high horizontal pressure gradient zones and low temporal pressure gradient zones. The magnitude of spatial pressure gradient indicates the preferred direction of oil and gas migration, while differences in temporal pressure gradients indicate the dominant accumulation zones for shale oil and conventional oil.