The evolution of abnormal pressure of Yanchang Formation in Zhenjing area of Ordos basin and its reservoir-forming significance

The Chang 8 group of Upper Triassic Yanchang Formation is the main oil layer in Zhenjing area of southern Ordos basin.The reservoir rock of Chang 8 group is mainly the delta front distributary channel sandstone and its reservoir type is lithology.Low formation pressure, low oil saturation and wide variation range constitute the characteristics of the Chang 8 group reservoir.Research on the evolution of formation pressure is very important for understanding the dynamic mechanism of hydrocarbon accumulation and enrichment.Logging acoustic data calculation and basin numerical simulation were combined to recover the evolution of Chang 7 source rock formation pressure, and the dynamic significance of abnormal pressure in hydrocarbon accumulation was analyzed.The research result shows that the present formation pressure of Mesozoic strata in Zhenjing area is distributed in 17.99 MPa to 22.46 MPa, and the pressure coefficients range from 0.8 to 1.05.The formation is generally in the atmospheric pressure to weak negative pressure state.In Early Cretaceous, the Chang 7 source rock was quickly buried along with the rapid subsidence of basin.Due to the effect of under-compaction and hydrocarbon generation, the mudstone was commonly subjected to overpressure.In the last phase of Early Cretaceous, the overpressure was increased to 4 MPa.The pressure coefficient ranges from 1.1 to 1.3, which means the strata were subjected to small overpressure.Since the Late Cretaceous, with the uplifting of the formation, the pressure has gradually released and evolved into present pressure state.The buoyancy was not the main motive power for hydrocarbon migration after the formation of tight clastic reservoir, and abnormal pressure played an important role in the hydrocarbon accumulation in Zhenjing area of southern Ordos basin.