Overpressure Distribution,Evolution And Its Effects On Hydrocarbon Accumulation In The Deep Stratum In The North Belt Of Dongying Depression

Overpressure is commonly related to the distribution of oil and gas reservoirs and can affect the formation,migration and accumulation of oil and gas.The deep oil and gas reservoirs in the Yanjia area of the north of Dongying Depression is dominated by normal pressure,while oil and gas reservoirs in the Shengtuo area is generally overpressured,suggesting that there are differences in the distribution,origin,evolution of formation pressure and controls on oil and gas accumulation.In addition,the formation mechanism of abnormal pressure has not been revealed.In this paper,the characteristics of deep pressure distribution in the north Dongying Depression were analyzed and the pressure systems were determined through logging data and fluid inclusion.Further combined with geological background,logging data and quantitative calculation,the cause of overpressure was identified.The inclusion homogenization temperature-salinity method and basin simulation method were used to recover the paleopressure evolution.Finally,the influence of overpressure on hydrocarbon generation,reservoir and hydrocarbon migration were analyzed.The deep oil and gas accumulation models under pressure control were summarized.In this study,we have obtained the following understandings:（1）Deep overpressure is well developed in mudstone formations in the north of Dongying Depression,in which the reservoir is normal pressure-weak overpressure because it is an open system.The r sandstone lens reservoirs in the Shengtuo area are overpressuered and the overpressure amplitude is large than that in the Yanjia area.Oil and gas reservoirs developed in the uplifted wall of Shengbei fault are dominated by normal pressure and overpressure.Two sets of pressure systems are developed in the deep formation of the north Dongying Depression,and the gypsum-salt layer is the main sealing layer at the top of the lower submember of the fourth member of the Shahejie Formation.（2）The origins of deep overpressure are different in the north Dongying Depression.The overpressure in the Es₄^s is mainly derived from hydrocarbon generation in source rocks and the increased pressurs is 12MPa～29MPa.The overpressure in the Es₄^x caused by disequilibrium compaction and hydrocarbon generation,and hydrocarbon generation induced formation pressure increases of 22MPa～28MPa.The overpressure preservation and pressurization amplitude are affected by the high deposition rate,high quality source rock and gypsum-salt rock.The gypsum-salt layers at the top of the lower submember of the fourth member of the Shahejie Formation play a sealing role and can preserve abnormal pressure resulted from undercompaction.（3）There were two deep paleopressure increases in the northern Dongying Depression,and a pressure drop occurred between the two rises.Overpressure formation time and amplitude are related to hydrocarbon charging time,depositional process and preservation conditions.The periods of palepressure increase of the Shengtuo and Yanjia areas were basically the same.The first formation pressure increase occurred during the first period of oil and gas charging.Due to the sealing effect of gypsum-salt layer,the overpressure induced by undercompaction was well preserved in the Es₄^x.However,the overpressure amplitude of the upper submember of the fourth member of the Shahejie Formation was low.The second pressurization was at the end of Guantao Formation deposition.This time,the overpressured mudstones were generally developed and caused by hydrocarbon generation.The overpressure amplitude of the Es₄^x was greater than that of the Es₄^s.（4）The accumulation effect of overpressure is reflected in that the early overpressure and closed environment will delay the hydrocarbon generation threshold of deep source rocks.Meanwhile,the micro-fractures induced by overpressure can be used as the migration pathway of oil and gas.Furthermore,oil and gas can migrate into the sand body and accumulation forced by overpressure.In addition,the overpressure in reservoirs can resist the compaction and promote dissolution,and inhibit cementation.Two oil and gas accumulation models were established in the Yanjia and Shengtuo areas.For the Yanjia area,overpressure inhibits hydrocarbon generation and promotes dissolution,and the gypsum-salt rock maintains overpressure.Oil and gas charge the near-source reservoirs driven by overpressure and buoyancy,and are sealed by the tight proximal fan.For the Shengtuo area,overpressure inhibits hydrocarbon generation and promotes dissolution,and the overpressure was preserved by gypsum-salt layer.Overpressure drives oil and gas near-source charging,and lithology-fault jointly seals reservoirs in the downthrown side.Oil and gas accumulation in the upthrown side depends on buoyancy driving force and the sealing of tight proximal fan.