Study On The Diagenesis And Pore Evolution Of Beach And Bar Sandstones Of The Shahejie Formation In Boxing Sag,Dongying Depression

The third and fourth members of Shahejie Formation in Boxing Subsag of Dongying Sag are the key target intervals for oil and gas exploration,and the most representative ones are the beach and bar sandstone reservoirs.The beach and bar sandstone reservoirs in Boxing Sag are mainly lithologic reservoirs with low permeability.With the increase of exploration and development degree and the improvement of technical means,more and more remaining oil and gas are developed,and low porosity and low permeability reservoirs are paid more and more attention by scholars and experts.Diagenesis can transform reservoir physical properties,so it is important to explore the influence of diagenesis and diagenetic fluid evolution on reservoir physical properties for the evaluation of oil and gas exploration potential.In this paper,the beach and bar sandstones of the third and fourth members of Shahejie Formation in Boxing Sag are taken as the key research object,with the technical methods of core observation,physical property data analysis,thin section identification and fluid inclusion temperature measurement,the diagenetic evolution sequence of the beach and bar sandstone reservoirs in the study area is summarized,the diagenetic fluid evolution history is determined,the main controlling factors of pore evolution are identified,and the pore evolution model of reserviors is established.The petrologic researches of reservoirs show that the beach and bar sandstones of the third and fourth members of Shahejie Formation in Boxing Sag are mainly composed of very fine sandstone and siltstone,mainly lithic arkose.The original sedimentation makes them well sorted and rounded,and with a lower matrix content.The sandstone strata are relatively thin,and the horizontal and cross beddings are mostly developed.Its porosity and permeability are low,and low porosity and low permeability reservoirs are the main types.The beach and bar sandstones of the third and fourth members of Shahejie Formation in Boxing Sag have undergone various types of diagenesis.On the basis of thin section observations and analysis,the diagenetic sequence of beach and bar sandstones in the third and fourth members of Shahejie Formation is summarized as follows: compaction / early calcite and dolomite cementation ? feldspar dissolution / quartz overgrowth / authigenic kaolinite ? quartz and quartz secondary outgrowth rims dissolution / ferrodolomite and Ferrocalcite cementation.The evolution of diagenetic fluid is from weakly alkaline to acidic to alkaline again.Based on the diagenetic sequence of the beach and bar sandstones in the study area,combined with the homogenization temperature test of fluid inclusions and the analysis of burial history,the diagenetic fluid evolution history is determined as follows: about 28 Ma ago,the diagenetic environment changed from weakly alkaline to acidic,the acidic environment lasted about 23 Ma,and then changed to alkaline about5 Ma ago.The pore evolution of beach and bar sandstone reservoirs in the study area is mainly controlled by sedimentation,diagenesis,hydrocarbon charging and overpressure.Sedimentation determines the material basis and grain fabric.Compaction and cementation are the main factors affecting the pore evolution.It is found that the main pore reduction mechanism of beach and bar sandstones in the study area is cementation,followed by compaction and dissolution.Overpressure has an effect on reservoir porosity,and the higher the overpressure is,the better the preservation of porosity is,and the more late carbonate cements are.