The Formation Pressure Characteristics And Its Hydrocarbon Accumulation Control In The Kongqueting Area,Xihu Depression

Overpressure is widely developed in the Paleogene Eocene Pinghu Formation and the Oligocene Huagang Formation in the Kongqueting area of Xihu Depression.The distribution characteristics and genetic mechanism of abnormal pressure are of great significance to reveal the hydrocarbon accumulation of the study area,and can also provide guidance for the prediction of favorable areas.However,there are still some problems in the study area,such as unclear distribution characteristics of abnormal pressure,disunified genetic mechanism and unclear reservoir control function.Therefore,based on formation testing,logging,logging interpretation,geochemical and seismic data,this study first revealed the pressure distribution characteristics of single well,plane and profile in the Pinghu Formation and the Huagang Formation by combining logging and seismic methods.Then,Bowers method and velocity density crossplot method were used to identify the genetic mechanism of overpressure in combination with actual geological conditions.Finally,the control effect of overpressure on hydrocarbon migration and accumulation was analyzed through the distribution characteristics of excess pressure,so as to establish the overpressure hydrocarbon accumulation pattern.The main conclusions can be summarized as follows:(1)The overpressure is mainly distributed in the Eocene Pinghu Formation and the Oligocene Huagang Formation.The lower member of Huagang Formation is characterized by hydrostatic pressure,and the maximum pressure characterized by the medium overpressure level is distributed in the lower member of Pinghu Formation.Horizontally,overpressure mainly develops in the lower and middle zones of study area,and the pressure shows a decreasing trend from east to west.The overpressure top interface deepens gradually from high zone to low zone,and is distributed almost along the layer.(2)The overpressure in the study area is caused by undercompaction,hydrocarbon generation and pressure transfer.The overpressure in high zone is mainly due to undercompaction and pressure transfer,the overpressure in middle zone is mainly comes from hydrocarbon generation and pressure transfer,and the overpressure in low zone is mainly produced by undercompaction,hydrocarbon generation and pressure transfer.From the perspective of stratum,pressure transfer is dominant in the upper part of Huagang Formation and upper member of Pinghu Formation,undercompaction and pressure transfer play the main roles in the middle member of Pinghu Formation,and undercompaction and hydrocarbon generation contribute significantly to the lower part of Pinghu Formation.(3)The oil and gas shows in the study area are mainly located in the Pinghu Formation,with the overall characteristic of "more gas and less oil".Vertically,it shows "upper oil and lower gas",and the reservoir types are fault anticline and fault block.The low value zones formed by excess pressure are diverse.The upper and lower member of Huagang Formation are located in middle and high zone,and the upper,middle and lower member of Pinghu Formation are located in high zone.(4)Local source rocks of the Eocene Baoshi Formation and the Pinghu Formation and external source rock in deep depression provide hydrocarbons to the study area.The hydrocarbon charging and migration occurred in the early Miocene and early Pliocene,and the excess pressure is the main driving force of hydrocarbon migration in the study area.The vertical conductivity of faults is excellent in the Pinghu Formation and the low and high zones of Huagang Formation.The connectivity of multiple sets of sand bodies in the lower and middle member of Pinghu Formation is superior,which provide favorable channels for long-distance lateral migration of oil and gas.The overpressure sealing is the main oil and gas reservoir preservation condition in the Pinghu Formation and the lower zone of Huagang Formation,while the capillary sealing is the main hydrocarbon reservoir preservation condition in the high zone of Huagang Formation.Therefore,an accumulation model was finally established,characterized by dual sources as hydrocarbon kitchens,faults and sand bodies as migration channels,and pressure-controlled migration and accumulation.