Comprehensive Evaluation Of Sealing Ability Of Paleogene Caprock In The Western Part Of The Northern Margin,Qaidam Basin

The study of Paleogene caprock in the western part of the northern margin of Qaidam Basin is weak,which seriously restricts the progress of fine oil and gas exploration in this area.In this study,the core drilling,logging cuttings,logging data,two-dimensional seismic data and test analysis data are fully used to accurately identify the Paleogene caprock in the western part of the northern margin of Qaidam Basin,and the macro and micro seal characteristics of the caprock are studied.Combined with the dynamic evolutionary history of the sealing capacity of the caprock,the effectiveness of the historical evolutionary sealing capacity of the caprock is analyzed,and the evaluation parameters of the macroscopic sealing capacity of the caprock are selected(mud content,type of sedimentary facies,cumulative thickness of the effective caprock,diagenetic stage and fracture distance Etc.)and micro-sealing capacity evaluation parameters(displacement pressure,abnormal pressure coefficient and brittleness index,etc.),the classification and evaluation criteria of the caprock in the study area are established.Among them,the cover layers of the Eboliang structural belt,Lenghu structural belt and the latent structural belt have the ability to effectively seal oil and gas before 5.1Ma,and the formation period is earlier than the generation and expulsion of hydrocarbons from Jurassic source rocks,indicating that the seal is closed at time.In the Eboliang structural zone,the E₃¹ and E₃² of the Eboliang No.II,the N₁ of the Eboliang No.III,and the E₃²and N₁ of the Hulushan area all have conditions for the development of large-scale effective caprock to seal the industrial oil flow.Meanwhile in the Qianfu structural belt,the E₁₊₂and E₃¹ in the north of Pingtai uplift,E₃² and N₁ in the south of Pingtai uplift and N₁ of Sinan depression also have effective seals on large scale.In this paper,the Paleogene caprock in the western part of the northern margin of Qaidam is comprehensively evaluated and the favorable development zones and horizons of the caprock are predicted,which provides an effective exploration direction for increasing reserves and increasing production in the study area.