Reservoir Characteristics Of The Upper Fourth Member Of Shahejie Formation In Yi176 Block Of Bonan Sag

The exploration and development of the middle and shallow layers in all major oil fields in China are relatively high,and stable production faces certain difficulties.Therefore,deep oil and gas exploration has increasingly become the focus of attention.Bonan sag,located in the middle of Jiyang depression,is the mainly target area of deep exploration in Jiyang depression.In this study,cast thin section identification,scanning electron microscopy and energy spectrum analysis,X-ray diffraction,cathode luminescence,fluid inclusions,petro-physical property testing and other analysis and test data,with combination of core observation and logging data,and previous research results,systematically study the petrological characteristics,physical properties,and diagenesis of Upper Fourth Member of Shahejie Formation in Yi176 Block,Bonan Sag,and discuss the main controlling factors of the reservoir.Comprehensive research shows:The reservoir sandstones in Upper Fourth Member of Shahejie Formation in Yi176 Block are mainly medium-fine-grained lithic arkose and feldspathic litharenite.with sorting being medium to-good,and the roundness is subangular to subround.The whole of debris particles are point-to-point,and point-line contact,both compositional maturity and textural maturity are generally low.The interstitial fillings of the sandstone are matrix and cements,matrix include clay matrix and stucco matrix.The cement is mainly comprised of carbonate minerals（calcite,dolomite,ferrocalcite,ankerite）,followed by siliceous,and with a small amount of gypsum and zeolite as cements.The reservoir space types of the upper fourth member of Shahejie Formation in Yi176 block are mainly classified into primary intergranular pores,intergranular dissolved pores,intergranular dissolved expansion pores,followed by intragranular dissolved pores,mold pores,and a small amount of intercrystalline pores and hetero-based micropores.The porosity of the sandstone is between 10%and 12%,the permeability ranges from 0.01×10^-3μm²to 0.1×10^-3μm².This is characterized as the ultra-low porosity and low permeability reservoir.The diagenesis of sandstone reservoir in the upper part of the fourth member of the Shahejie Formation in Yi176 block includes compaction,cementation,metasomatism and dissolution,among which carbonate cementation is the primary destructive diagenesis,while the dissolution improves the physical quality of the reservoir and therefore plays a positive role.The diagenetic stage of the reservoir sandstone in the study area belongs to stage A of middle diagenesis phase,and few belong to stage B of middle diagenesis phase.The diagenetic evolution sequence is:compaction→early quartz overgrowth→early carbonate minerals filling and replacement→carbonate minerals and particle dissolution→autogenous quartz filling dissolved pores、ferrocalcite and ankerite（later）filling and replacement,autogenous quartz filling dissolved pores.The reservoir in the study area is controlled by sedimentation and diagenesis.The impacts of petrological characteristics on the reservoir is as follows:the content of plastic debris is negatively correlated with the surface porosity;when the content of the matrix is greater than 5%,the surface porosity decreases significantly,primary intergranular pores and intergranular dissolved pores;when the cement content is greater than 10%,the surface porosity decreases continuously with the increment of the cement content.The impact of the sedimentary environment on the reservoir is as follows:the sedimentation settings control the initial conditions of the sandstone formation in the reservoir.The detrital composition,particle size,and roundness have certain effects on the physical properties of the reservoir.In the sedimentary microfacies such as underwater distributary channels and estuary dams with strong hydrodynamics,the content of matrix is small and the primary intergranular pores are more developed,the reservoir physical properties are better.The effect of diagenesis on the reservoir is as follows:the plastic debris are distorted and deformed by compaction,which reduces the physical properties of the reservoir to a certain extent;the early carbonate cement protects the reservoir,while the siliceous cement and the late carbonate cement block the pores and throat and play a destructive role.The dissolution is mainly intergranular dissolution,which is mainly manifested in the formation of intergranular dissolved pores and the expansion of dissolved of primary intergranular pores.After the dissolution,a large number of secondary pores are developed in the sandstone,and the physical properties of the reservoir is improved.