Microscopic Characteristics And Formation Mechanism Of Paleogene Low-permeability Sandstone Reservoir In Nanpu Sag,Bohai Bay Basin

The burial depth of the 3rd member of Dongying Formation and the 1st member of Shahejie Formation of Paleogene is generally greater than 3000m in Nanpu sag of Bohai Bay Basin,thus the sandstone reservoirs are generally of medium to low porosity and low permeability,with strong microscopic reservoir heterogeneities and great differences in reservoirs between different strata and different oil-bearing blocks.In this paper,conventional thin sections,casting thin sections and scanning electron microscopy analysis were used to study the petrological characteristics,diagenesis,pore structure and reservoir properties of deep sandstone reservoirs in Nanpu sag together with X-diffraction,conventional pore seepage and mercury pressure test and CT scanning.The commonality and difference of reservoirs with different oil-bearing structures and different strata were comparatively analyzed,the influencing factors of densification of deep sandstone reservoirs and the formation mechanism of high-quality reservoirs were analyzed,a multiple regression model of different control factors of reservoirs was established,and the control factors and distribution rules of various parameters on reservoirs were quantitatively evaluated,and then the control factors and distribution rules of high-quality reservoirs in middle and deep sandstone were summarized.The lithology of deep sandstone reservoir in Nanpu sag are mainly feldspar clastic sandstone and lithic arkose,and the clastic particles are mainly fine sand-medium sand grain grade,sorting and equal,sub-prism-sub-round,point-line contact,and pore cementation.Pore space includes six types:intergranular dissolution pores,intragranular dissolution pores and residual intergranular pores.The reservoirs of the East Third Section and the Sand Section are in the A1-A2 period as a whole,and compaction and late carbonate cementation are the main reasons for reservoir densification.Furthermore,the quantitative characterization of the micropore structure of the reservoir was carried out by combining conventional mercury pressure and CT scanning,and the deep sandstone reservoir of the Nanpu sag was divided into four categories,of which the class I and class II reservoirs were high-quality reservoirs.Class I reservoir porosity is generally greater than 15%,permeability is more than 50×10^-3μm²,lithology is mainly coarse-grained feldspar clastic sandstone,the reservoir space is mainly intergranular dissolution pores,belongs to the medium and large pores-middle throat type,mainly developed in the Block 3 and Block 4.Class II reservoirs are mainly developed in the upper 3^rd member of Dongying Formation of Block1 and Block Gaoshangpu,with a grain size of medium sand,a slightly lower degree of dissolution than that of Class I reservoirs,and a mesoporous-mid-throat type.Clay minerals and late carbonate cements are mostly in class III reservoir fillers,and the storage space is mainly dissolved in the granular,which belongs to the small-porous-fine throat type,the throat radius is between5-10μm,and the pore-throat communication is poor,mainly developed in the section of Block 1 and Block 4.Class IV reservoir has high argillaceous content,mainly siltstone,porosity below 5%,permeability less than 1×10^-3μm²,and is distributed in the 3^rd member of Dongying Formation of Block 2,Block 1 and Block 5.According to the calculation results of the correlation coefficient of the multiple linear regression equation,it is concluded that the pore reduction mechanism of sandstone reservoirs in the 3^rd member of Dongying Formation of Nanpu sag is reduced by chlorite,illite,Eamonn mixed layer,calcite and particle sorting coefficient,while the control degree of sand section is reduced by chlorite,particle sorting,illite,calcite and Eamonn mixed layer.The content of kaolinite in the sandstone reservoirs of the 3^rd member of Dongying Formation and the 1st member of Shahejie Formation indicate the degree of dissolution of feldspar and cuttings,and the mechanism of increasing the porosity of kaolinite content.