Reservoir Characteristics And Favorable Area Prediction Of The Paleogene Clastic Reservoirs In The No.4 Structural Belt,Nanpu Sag

Along with the increasing degree of exploration,the middle-deep layers have become the key targets of oil and gas exploration and development in mature exploration areas.Due to the long time of burial,the reservoir quality has become one of the important factors restricting the efficiency of oil exploration and development.In this paper,the reservoir characteristics of each key section in the No.4 structural belt of Nanpu Sag are studied in comparison with each other using various analytical tools such as thin section observation,scanning electron microscopy,X diffraction analysis,high pressure mercury piezometric analysis and physical property analysis to analyze the reservoir quality differences and influencing factors and predict the favorable reservoir distribution in the study area,and the results of the study show that:The Paleogene in the No.4 structural belt of Nanpu Sag is dominated by feldspathic lithic sandstone and lithic feldspar sandstone,which are well sorted.The reservoir space is rich in types,mainly remaining intergranular pores,secondary dissolution pores and micro-fractures;the pore throat has strong heterogeneity,and there are not only wellsorted and slightly coarse-skewed intergranular pores,but also intragranular dissolved pores and intergranular dissolved pores with poor sorting and fine skew.The Paleogene clastic reservoirs in the No.4 structural belt are generally low-porosity and lowpermeability reservoirs,and a small amount of medium-porosity and mediumpermeability reservoirs are locally developed.Diagenesis has a great influence on the Paleogene reservoirs in the No.4 structural belt of Nanpu Sag,and there are great differences in the characteristics of compaction,cementation and dissolution between different layers.On the whole,the deformation of clastic particles caused by compaction is the main cause of porosity loss;the cementation is mainly calcareous(calcite and ferrodolomite cementation)and siliceous,and the early cementation is conducive to improving the resistance of the reservoir.In terms of compaction ability,the late cements will block the pores and lead to the decline of the reservoir quality;while the secondary pores formed by the dissolution are the main factors to improve the physical properties of the Paleogene reservoirs in the No.4 structural belt of the Nanpu Sag.Based on the type and intensity of diagenesis,the diagenetic phases of the Paleogene reservoir in the No.4 structural belt of Nanpu Sag can be classified as strongly compacted facies(Type I),iron calcite cemented facies(Type II),iron dolomite cemented facies(Type III),moderately compacted-moderately dissolved facies(Type IV),and undercompacted-strongly dissolved facies(Type V).The longitudinal distribution of diagenetic facies in the study area is identified using natural gamma,acoustic time,density,compensated neutron,deep double lateral resistivity and other logging characteristic curves.The paleogene clastic reservoir in the No.4 structural belt of Nanpu Sag can be divided into three types by combining various reservoir factors,and the distribution of favorable reservoirs in the study area can be predicted.It is considered that the microfacies of fan delta front underwater distributary channel and mouth bar,which are developed in faultrich area,are favorable reservoir in the No.4 structural belt of Nanpu Sag because they are conducive to the migration of acidic fluid and secondary dissolution.