Evolution Characteristics Of Formation Pressure Of Yanchang Formation In Yan’an Area And Its Impact On Shale Gas Preservation

Ordos Basin is an important continental Shale gas exploration basin in China.Formation overpressure is generally developed during geological history.Research on the evolution process from overpressure during geological history to current low pressure is the key to evaluate the preservation of Shale gas in this area.In order to clarify the formation mechanism,evolution characteristics of overpressure and the impact of pressure evolution characteristics on Shale gas preservation,this paper studies the pressure evolution process and characteristics in the study area based on the measured pressure data,drilling,analysis and testing,and logging data in Yan’an area.Firstly,the comprehensive compaction curve of mudstone from typical wells in Yan’an area and the identification chart of overpressure genesis mechanism were used to identify the overpressure genesis.Then,the basin numerical method was used to quantitatively calculate the overpressure contribution of different mechanisms during the geological history of Yanchang Formation 7.The evolution process of overpressure low-pressure was simulated,and the depressurization amount of different mechanisms was quantitatively calculated.Finally,overpressure sealing,physical property sealing,and The influence of abnormal pressure evolution on Shale gas preservation is analyzed from three aspects of structure.The following understanding has been obtained:(1)Low pressure is commonly developed in the Yan’an Formation in the Yan’an area,and the formation pressure increases with the increase of burial depth.The current formation pressure coefficient of the Yanchang Formation is about 0.7-0.8,among which the formation pressure coefficient of the Chang 7 section is about 0.72.The overall distribution of the pressure coefficient is high in the west and low in the east.(2)The formation overpressure during the geological history of the Yan’an Formation in the Yan’an area of the Ordos Basin is the result of the joint action of multiple causes,and the main mechanism of overpressure formation in the mudstone series is imbalanced compaction;In addition to uneven compaction,overpressure in source rock also contributes to hydrocarbon generation and pressurization.The main factors causing the current low pressure are the temperature reduction caused by Tectonic uplift and the pore rebound caused by uplift and erosion.(3)According to the numerical simulation results,the Yanchang Formation reached its maximum burial depth at the end of the Early Cretaceous,and the excess pressure of the Chang 7 Formation reached its maximum at this time.This stage mainly experienced undercompaction pressurization and hydrocarbon generation pressurization.The simulation results of multiple single wells indicate that the excess pressure range generated by hydrocarbon generation pressurization is 11-14.2MPa,and the excess pressure range generated by undercompaction is 3.6-5MPa.Since the Early Cretaceous,the uplift of the strata has led to pore rebound and a decrease in temperature,resulting in a decrease in formation pressure.The decrease in temperature causes a decrease in formation pressure of about 9.2 to 12.8 MPa,and pore rebound causes a decrease in formation pressure of about 2.7 to 4.2 MPa.(4)In Yan’an area,the excess pressure during the maximum burial depth of Chang 7 member of Yanchang Formation is about 14-17 MPa,and the formation pressure is less than the fracture pressure.Overpressure is generally developed and no obvious fracture occurs,providing good conditions for the sealing and preservation of Shale gas in Chang 7 member.