Complex Fluid Pressure Distribution, Evolution And The Its Effects On Hydrocarbon Accumulation In The Third Member Of Paleogene Shahejie Formation, Huimin Depression

The distribution,origins and evolutionary processes of abnormal pressure are of great significance for hydrocarbon generation,migration and accumulation in petroliferous basins.However,the analysis of abnormal pressure mechanism,evolution and its influence on hydrocarbon accumulation is still challengeable.This study takes the third member of Shahejie formation（Es₃ Fm）in the Huimin Depression in Bohai Bay Basin as an example to analyze abnormal pressure distribution and pressure structure by multilogging predicting methods restricted by measured pressure.Furthermore,the logging methods,quantitative calculation,laboratory experiments with microscopes and physical simulation experiments are used as a combination to analyzed abnormal pressure origins.The paleo–pressure evolutionary processes are reconstructed by fluid inclusion data and basin modelling,and the combined controlling effects by overpressure and underpressure to hydrocarbon migration and accumulation are illustrated.Overpressure,normal pressure and underpressure are all developed in the Huimin Depression.Overpressure with the pressure coefficient（pcoe）from 1.2 to 1.4 is distributed in the Linnan Sag,which is located in the center of the Depression.While underpressure is mainly distributed in the central uplift belt（CUB）and southern slope belt（SSB）which pcoe are from 0.75 to 0.9.In vertical,the Es₃ Fm ias divided into two pressure systems.One is the lower pressure system which is consist of the 1^st member of the Es₃ Fm with higher pcoe,the other is the upper pressure system which is consist of the 2^nd and 3^rd members of the Es₃ Fm with lower pcoe.The origins of the overpressure in the Linnan Sag are disequilibrium compaction and hydrocarbon generation in source rocks.The present overpressure is mainly caused by late–period hydrocarbon generation,which generated 16 to 20 MPa excess pressure in the source rocks according to quantitative calculation.Underpressure is mainly distributes in the sandstones with well physical properties.The dominated pressure decreasing process is feldspar alteration.Furthermore,elastic rebound of rocks and temperature reduction also ledd to pressure deceasing of 6 to 8 MPa.Paleo–pressure evolutionary processes in different tectonic belts shows large disparities in the Huimin Depression.Pressure evolution in the overpressured Linnan Sag showed two rising and two falling processes:Paleo–pressure firstly rose at the end of Dongying period because of disequilibrium compaction and early–period hydrocarbon generation,and then fell during uplift period.During Guantao–Minghuazhen period,paleo–pressure increased substantially owing to the reach of hydrocarbon generation peak,then after the peak period,the paleo–pressure fell again.The paleo–pressure in the CUB and SSB fell substantially to underpressure during the uplift period.Then the paleo–pressure gradually increased along the sedimentary and hydrocarbon injection processes,while the feldspar alternation effect kept the sandstone reservoir in the underpressured environment.Continuous hydrocarbon injection caused increasing tendency of paleo–pressure from Guantao–Minghuazhen period to the present.Oil and gas break the blocking of fault and seal to migrate to shallower sandstones or accumulate to form hydrocarbon reservoirs by coupling effects of fault–overpressure–seal system dominated by overpressure.The hydrocarbon migration and accumulation processes in the underpressured area is controlled by attenuating gradient of residual pressure（AGRP）.Oil and gas migrate to the area in which the residual pressure attenuates fastest and then accumulate to hydrocarbon reservoirs.Controlled by overpressure and underpressure,the hydrocarbon migration and accumulation are characterized by long–distance migration,accumulation in multiple layers and developing in multiple pressure systems in the Huimin Depression.