Formation Pressure Characteristics And Overpressure Genesis Of Pt-E₃¹ Formation In Mahaidong And Its Surrounding Areas

Using seismic,drilling,logging,testing,core analysis and other data,the petroleum geological conditions of Pt-E₃¹formations in Mahaidong and its surrounding areas in the northern margin of Qaidam Basin,such as hydrocarbon sources,reservoir-cap assemblages and the relationship between the allocation of reservoir forming elements,were analyzed.The distribution characteristics of formation pressure in the study area are analyzed according to the measured formation pressure data of drilling test.Based on the theory of geological conditions of overpressure genesis and the empirical method of overpressure origin identification based on the comprehensive analysis of well logging curves,the causes of overpressure in Mahaidong and its surrounding areas are carefully identified.The relationship between formation pressure distribution and hydrocarbon distribution is discussed in combination with the study of hydrocarbon distribution characteristics.The normal temperature and atmospheric pressure system is found in well area of MB8-MB301 in the eastern section of Mabei bulge.The normal pressure system is found in well area of MB3-SG1 in Mahaidong structural belt.E_1-2-Pt formation is an abnormal(high)pressure system,and the pressure coefficient ranges from 1.2 to 1.36.In the E_1-2formation,abnormal pressure by undercompaction and the effect of hydrocarbon filling conduction causes overlay.Hydrocarbon filling conduction is the main cause of abnormal pressure in Pt hydrocarbon reservoir,and clay mineral transformation enhances the sealing property of the overlying strata and promotes the preservation of abnormal pressure to a certain extent.the effect of structure on the influence of abnormal pressure is unknown.The study area of oil and gas mainly comes from Jurassic hydrocarbon source rocks.The target strata in the Mahaidong structural belt are mainly composed of Proterozoic bedrock fractures,weathering crust and its upper tight/unconformity,E₃¹reservoir-cap combination;Mesozoic Jurassic J₂d reservoir and top local reservoir-cap combination;and Paleogene E_1-2,E₃¹sand and mudstone interbedded reservoir-cap combination.The trap formed earlier in the study area,and a lot of oil and gas generation and filling time coincided with the period of strong fault activity.Forming the"Lower source upper circle"configuration and"Multi-storey"duplex accumulation,Multi-layer system and multi-type oil and gas reservoirs.There are three types of hydrocarbon reservoirs:structural hydrocarbon reservoir,structure-stratigraphic hydrocarbon reservoir and bedrock(fault block)hydrocarbon reservoir.The hydrocarbon accumulation models are summarized as follows:the early hydrocarbon generation period(the end of E₃²),the far-source Oil and gas original reservoir in the ancient bulge;the hydrocarbon generation peak period(N₁),the secondary charging and near-source superposition accumulation;and the far-source moisture migration and accumulation(after N₂)trap transformed accumulation model.There is a good relationship between overpressure distribution and oil and gas distribution in the study area.The overpressure is mainly distributed in the bedrock(Pt)of Well MB303 and Well MB3,as well as Well SG1 and Well SG101 Lulehe Formation(E_1-2).The measured abnormal overpressure in the bedrock(PT)of Well MB3 and Well SG1 Lulehe Formation(E_1-2)are the main reservoir in the study area.