The Main Controlling Factors Of High Quality Reservoir Of Paleogene In The Northwestern Bozhong Sag

In this paper,the Paleogene sand bodies of southern Shijiutuo uplift,southeastern Shaleitian uplift and Bozhong 19-6 structure in Bozhong sag of Bohai Bay Basin were taken as the research objects.Core observation,rock thin section identification,scanning electron microscopy analysis,X-ray diffraction phase analysis,cathodoluminescence,carbon and oxygen stable isotope test,fluid inclusion analysis,logging curve analysis,burial history recovery,thermal evolution history analysis and other technical methods were used.Under the guidance of the research idea of’objective phenomenon-experimental technology-representation phenomenon-research essence-summary law-guide unknown’,the similarities and differences of sedimentary characteristics and diagenetic evolution characteristics in time domain and space domain of the study area were analyzed.Finally,the development model of high quality reservoirs in the study area was established.The result shows that:（1）Difference of sedimentary environment was the basis of difference of rock composition and rock structure.In the sedimentary period of E₂s₁,E₂s₂ and E₃d₃ in the study area,fan delta facies was the main facies in the near-source steep slope zone,and the reservoir lithology was mainly gravel sandstone,coarse sandstone and medium sandstone.Gravity flow deposits developed in depressions far from provenance and the reservoir lithology was mainly fine sandstone and siltstone.Braided river delta facies were developed in the gentle slope belt.During the sedimentary periods of E₃d₂^u and E₃d₁,braided river delta facies developed in the study area.The rock composition and rock structure was also controlled by the sedimentary facies.（2）The difference of sedimentary facies was the basis of the difference of diagenesis.In the near source steep slope zone,the distributary channel of root fan of fan delta microfacies developed multistage granular support conglomerate facies,sandy support conglomerate facies,granular support fine conglomerate facies and conglomerate sandstone facies.Although the sorting was poor,the burial rate was high during diagenesis,the fracture were well developed in detrital grains,and the high quality reservoir could be developed after acidic fluid flowed along fractures and dissolved clastic particles.For the distributary channel and mouth bar in the middle fan of the fan delta,the well sorted coarse sandstone and medium sandstone developed secondary pores,which were high quality reservoir development zones.（3）Long duration of acidic fluid environment during diagenetic evolution of Paleogene reservoir,Overpressure promoted migration of acidic fluid from source rock series to adjacent clastic reservoir.The contribution of acid fluid to reservoir was as follows:a large number of secondary dissolution pores were produced,which provided occurrence space for late hydrocarbon filling.With the increasing burial depth of strata,a large number of intragranular fractures were generated in clastic particles,and many secondary dissolution pores were developed along the fractures,which slowed down the damage of compaction on reservoir porosity.Acidic fluid inhibited late carbonate cement cementation.（4）The dominant rock structure was the prerequisite for the development of high quality reservoir in shallow strata.Fractures and dominant rock structure were the basis for the development of deep reservoirs.Dissolution was the key to the development of reservoirs in Paleogene.For the clastic reservoirs of E₃d₁ and E₃d₂^u,the well sorted sandstone had high original porosity,It was favorable for acidic fluid to dissolve clastic particles and form secondary pores.The clastic rocks of E₃d₃,E₂s and E₂k buried deeply had few primary pores.The fractures provided channels for fluids,and organic acid dissolution generated a large number of secondary pores.Some areas had long been in shallow burial diagenetic environment,and the transformation of reservoirs by meteoric freshwater was quite strong.