Study On Pore Pressure Characteristics And Evolution Of Upper Paleozoic In Northern Tianhuan

The characteristics and evolution of pore pressure are related to the process of gas storage.This paper collects and collates the basic data of drilling and formation testing in the north section of Tianhuan.Through the vertical direction and section distribution of measured pressure,it is clear that the upper Paleozoic in the north section of the ring is negative pressure at present,and analyzes the reasons for the formation and distribution of negative pressure.In this paper,the compaction curve of mudstone is drawn,the residual pressure of maximum buried depth period is calculated by the equivalent depth method.The pressure section and the pressure closed layer are divided according to the vertical section of the remaining pressure.The pressure characteristics of different regions in the study area are understood by drawing the continuous well profile.The genetic mechanism of the overpressure in the upper Paleozoic is analyzed.Based on the information of lithology,geothermal gradient,depressive thickness and porosity-permeability of each well in the study area,the pressure evolution process of Shanxi formation was simulated by Petromod software.The pressure evolution history of Shanxi formation in the different region is finally restored by using the current pressure and the maximum buried depth pressure as the constraint point.Finally,the influence of pressure evolution on the process of gas storage is summarized.The conclusions are as follows:?1?According to the comparison between the measured pressure and the hydrostatic pressure,the upper Paleozoic reservoir in the north section of Tianhuan is now negative pressure.The residual pressure value is between-4?-10MPa.The residual pressure value decreases gradually from P₂h₈ to P₁s₁.This difference is mainly related to the buried depth.?2?The compaction law in the study area is basically consistent.Both the Cretaceous and Jurassic are normal compaction,but the Cretaceous compaction coefficient is large.The strata of Triassic and below are underpressure sections in which the overpressure of the Upper Paleozoic Shiqianfeng and the Shihezi formation is obvious today.Depressive thickness of Cretaceous is between 200-600m.Maximum erosion in the east and south.Cretaceous restoration thickness is between 1000-2000m.The largest thickness of recovery in the north-west and south.?3?The residual pressure zone in the maximum buried depth period is obvious.There are4 sections in the north of the study area:normal compaction section,pressure increment section,basic stability section and overpressure section.The basic stable section is generally missing in the southwest of the study area,and the length of pressure increasing section is longer,extending from Yanchang formation to Liujiagou formation.The average residual pressure of Shiqianfeng formation is 27 MPa,that of Shihezi formation is 31 MPa,and that of Shanxi and Taiyuan formation is between 23-30 MPa..?4?P₃q and P₂sh are mudstone seal.Undercompaction is the cause of overpressure.The overpressure of P₁s and P₁t source rocks is the superposition of undercompaction and hydrocarbon generation.Hydrocarbon generation is the main overpressure mechanism.?5?The evolution of pore pressure is closely related to burial history and hydrocarbon generation and discharge process.Triassic organic matter began to enter the hydrocarbon generation threshold.But the gas production is small,the formation of overpressure is mainly undercompation contribution.Late Jurassic-Early Cretacous peak gas production and residual pressure reached another peak.The vertical migration and accumulation of natural gas in early Cretaceous under the action of source pressure difference and diffusion.During the late Cretaceous period,the pressure of the formation decreased continuously and the natural gas migrated along high-angle fractures.