Study On Prediction Method And Distribution Law Of Formation Pressure Of Baikouquan Formation In Mahu Sag

The Mahu Sag is located in the northwestern margin of the Junggar Basin.The Lower Triassic Baikouquan Formation is the first“continuous oil plays”in China with a source of 100 million tons.It is often accompanied by abnormal overpressure,which causes difficulties in drilling engineering,oil and gas exploration and production.However,there is a lack of understanding of the prediction method and overall distribution of the abnormal high pressure in the Baikouquan Formation.Under-compaction caused by rapid deposition and overpressure from other sources are the main causes of abnormal high pressure in Mahu Sag.Undercompaction mainly plays a role in mudstone,and overpressure from other sources mainly plays a role in sandstone.Based on the genesis mechanism of abnormal overpressure in Baikouquan Formation,this paper selects sonic time difference logging data that is sensitive to formation pressure,and performs pressure predictions in three different methods on wells with measured formation pore pressure points in the Mahu Sag.Calculate the results and analyze the cause of the error.Finally,the Eaton method is selected to evaluate the single well pore pressure in the whole area.Based on the analysis of the abnormal high-pressure longitudinal development of the Baikouquan Formation in five typical wells in the Mahu Sag,the plane distribution maps of formation pressure and pressure coefficient of the T₁b₁,T₁b₂&T₁b₃ were drawn,and the pressure in the western slope of the Mahu Sag was selected.The superposition of the coefficient and the thickness of the stratum,the thickness of the sand body and the ratio of the sand to the plot shows that the Baikouquan Formation generally develops abnormally high pressure,the overall pressure decreases from bottom to top,and the pressure coefficient increases,it is found that the Baikouquan Formation generally develops abnormally overpressures,and the bottom-up pressure decreases.The pressure coefficient increases;a single high-pressure center develops in T₁b₁,and there are multiple high-pressure centers in T₁b₂,and there are multiple high-pressure centers in T₁b₃.The reason for this phenomenon is that the abnormally high pressure is preferentially developed in thick mudstones and relatively thin and relatively poor physical properties.The Baikouquan Formation as a whole is degenerate fan delta facies.Mudstones increase from the bottom to the top and decrease glutenite,resulting in a reduction in the range of overpressure from other sources and an increase in undercompaction caused by rapid deposition.Therefore,the overpressure contribution from other sources in T₁b₁ is large,and the dominant reservoir location becomes the center of overpressure;the whole of T₁b₂ is degraded,and the overpressure in the sedimentary center begins.The influence range of overpressure from other sources in T₁b₃ is limited,and the impact of undercompaction increases.Therefore,strong overpressure develops in the sedimentary center of the basin,and several other overpressure centers overlap the fan distribution.