Relationship Between Abnormal Pressure And Hydrocarbon Accumulation From West Slope Belt To West Sub Sag Of Xihu Depression

The Xihu Depression is extensively distributed by overpressure,and abnormal pressure mainly existed in Oligocene Huagang Formation and Eocene Pinghu Formation.At present,there are few studies on abnormal pressure and hydrocarbon accumulation in the Xihu Depression,and their relationships are still unclear.In this paper,the formation pressure of single wells are predicted by optimizing the pressure prediction method.which is more suitable for the study area,the evolutions of paleopressure are simulated by using numerical simulation software,and then the causes of overpressure are clear-cut.Combined with the hydrocarbon generation and expulsion history of source rocks in key areas,the relationships between overpressure and hydrocarbon accumulation are finally clarified by analyzing the filling periods of hydrocarbon,the change rules of reservoir physical properties,and the migration characteristics of hydrocarbon.The results achieved are as follows:(1)The study of abnormal pressure prediction methods shows that the Eaton’s method,which is suitable for undercompaction and fluid expansion,is the best method for pressure prediction in study area.For the main structures in West Slope Belt,the Eaton’s index n=1.2 is more consistent with the measured pressure data,while for the main structures in West Sub Sag,the Eaton’s index n=1.5 is more suitable.(2)The overpressure of the Pinghu formation’s mudstone in West Slope Belt was caused by undercompaction,pressure transmission and fluid expansion;while it was only caused by fluid expansion in West Sub Sag.Others were all caused by pressure transmission,such as the overpressure of the Huagang formation’s mudstone and the sandstone in the study area.The overpressure top interface gradually deepens from south to north,and the formations where overpressure exists become new.There are two overpressure centers in the West Sub Sag,and the overpressure’s range in the south is larger.(3)There are three models of paleopressure evolution:(1)The evolution model of atmospheric formation pressure,which includes the upper of the Huagang formation in the study area and the lower of the Huagang formation in West Slope Belt.(2)The evolution model of extraneous formation overpressure,which includes the lower Huagang formation in the West Sub Sag and the Pinghu formation’s sandstone in the study area.(3)The ecolution model of self-source formation overpressure,which includes the Pinghu formation’s mudstone within the depression.(4)The effects between overpressure and hydrocarbon accumulation are mainly reflected in three aspects:(1)There has a very well matching relationship between the history of hydrocarbon generation,expulsion and migration,and pressure evolution.The hydrocarbon generation period of source rock is the development period of overpressure in the source rocks,while the development period of overpressure in the Huagang formation is the period of hydrocarbon migration.(2)The compaction of reservoir is delayed,while the dissolution of reservoir is promoted by overpressure.(3)Overpressure is one of the driving forces of hydrocarbon migration,hydrocarbon are mainly accumulated near the overpressure surface,and the direction of pressure relief is also the dominant direction of hydrocarbon migration and favorable reservoir forming area.