Research On Low Permeability And Tight Reservoir Diagenesis Of The Upper Fourth Member Of Shahejie Formation In Tuo 764 Area,Dongying Sag

The reservoir of the upper Fourth Member of the Shahejie Formation in the northern steep slope of the Dongying Sag is a typical deep-water turbidite fan-shaped channel deposit.There are abundant oil resources and huge exploration potential here.However,the reservoir has strong heterogeneity,deep burial,large thickness,few well points and complicated diagenesis.Under the control of sedimentation and tectonic action,the key factor leading to reservoir densification is diagenesis.Therefore,the paper from the perspective of diagenesis,through core observation,thin section observation,cathodoluminescence analysis,inclusion analysis,scanning electron microscopy,physical property analysis,X-ray diffraction,carbon and oxygen isotope,etc,based on the logging data,systematic research reservoir characteristics,diagenesis characteristics,diagenesis mechanism,diagenetic facies type and distribution characteristics,favorable reservoir prediction.The main results are as follows:(1)Study reservoir characteristics.Through core observation,thin section analysis,X-ray diffraction,carbon and oxygen isotope analysis,etc,study the petrological characteristics and petrographic characteristics of the reservoir.The lithic feldspar sandstone and feldspar lithic sandstone are mainly developed in the upper member of the Shahejie Formation in Tuo 764 Area.The lithology is dominated by coarse sandstone and medium sandstone.The reservoir has low compositional maturity and structural maturity.There are five types of lithofacies in the study area: medium coarse conglomerate facies,fine conglomerate facies,pebbly sandstone facies,sandstone facies,and mud(page)facies.Using experimental methods such as sheet analysis,mercury intrusion data analysis,and physical property testing,the reservoir characteristics,pore structure characteristics and reservoir physical properties of reservoirs are studied.The study area mainly develops secondary dissolved pores,followed by primary pores.The pore structure type is complex,and the overall physical properties of the reservoir are poor,which belongs to the ultra-low porosity ultra-low permeability reservoir.(2)Study the characteristics of reservoir diagenesis.Using rock core observation,casting thin section analysis,scanning electron microscopy,carbon and oxygen isotope analysis,cathodoluminescence,X-ray diffraction and other rock and mineral testing techniques,study the diagenesis of reservoirs.Compaction and pressure dissolution,carbonate and silicic cementation,and dissolution of feldspar particles and cuttings particles in the upper part of Shahejie Formation in Tuo 764 Area.The reservoirs in the study area are mainly in the stage A-B of the Mesozoic Formation.(3)Analysis diagenesis mechanism.The compaction effect in the study area is strongly heterogeneous,which is mainly affected by the static rock pressure of the overlying stratum,the regional structural compression stress and the compressive strength of the fluid.The calcium ions that form the early calcite cementation are mainly derived from the adjacent gray mudstone.The formation of early dolomite cementation is mainly derived from carbonate rock cuttings.Late carbonate cement is closely related to the dissolution of cuttings and feldspar.Iron carbonate cement is affected by iron-containing fluid.Silica cementation is mainly the result of pressure dissolution,and is also related to the dissolution of cuttings and feldspar.The dissolution is mainly developed in the horizontal bedding sandstone facies and tectonic deformation sandstone facies near the mudstone.It is mainly the dissolution of cuttings and feldspar particles,and it is also found that a small amount of quartz and calcite cement is dissolved.Under the alternate control of acid-base diagenetic environment,the reservoir experienced diagenetic evolution of multi-stage cementation,complex metasomatism and multi-stage dissolution.(4)Study the reservoir diagenetic facies.According to the type and intensity of the main diagenesis,the Upper Fourth Member of the Shahejie Formation is divided into five major types of diagenetic facies.And analyze the distribution law of diagenetic facies from the genesis.Based on the study of diagenetic facies response characteristics,Fisher's discriminant is used to identify the single well diagenetic facies,and the distribution characteristics are integrated.The diagenetic facies distribution characteristics of the study area are clarified.(5)Reservoir classification evaluation.Comprehensive analysis of sedimentary factors,diagenetic factors and the influence of pore structure on reservoir physical properties,classification and evaluation of reservoirs,and classification of four types of reservoirs.It is also clear that the favorable reservoirs of the upper sect of the upper member of the Shahejie Formation in Tuo 764 Area are mainly developed in the middle-pressure compaction diagenetic facies,strong compaction and strong dissolution diagenetic facies,and medium compaction in the upper middle part of the sandy debris flow ridges of deep-water turbidite fans.Middle dissolution diagenetic facies,mainly class a and b pore structures.Based on the previous studies of sedimentary simulation and sedimentary facies,the paper analyzes the reservoir from the perspective of diagenesis.The basic characteristics of the reservoir and the diagenesis characteristics of the reservoir are clarified.At the same time,the mechanism of diagenesis is discussed.Revealing the spatial distribution of diagenetic facies,The factors affecting the physical properties of the reservoir are analyzed.Finally,favorable reservoir distribution predictions are achieved.The research results have certain guiding significance for the exploration and development of deep tight oil in Dongying Sag,and also provide reference for reservoir geological research of other deepwater turbidite reservoirs.