| The Chang 6 and Chang 8 reservoirs in the Hujianshan area of the Ordos Basin are one of the main oil-producing horizons of the Yanchang Formation.Both of them belong to typical low-permeability reservoirs.In recent years,the oil-bearing area has been expanding and the proven reserves have been increasing,showing good potential for increasing reserves.However,affected by the sedimentary environment and diagenesis,it has the uniqueness and complexity of typical low-permeability reservoirs,which are mainly reflected in large unused reserves,poor physical properties,strong heterogeneity,complex micropore structure,and oil-water heterogeneity.Inferiority has severely restricted the development of further oil and gas exploration in this area.Therefore,the application of core physical properties,cast thin films,scanning electron microscopy,cathode luminescence,image grain size,X-ray diffraction,high-pressure mercury injection,nuclear magnetic resonance,and real sandstone micro-water flooding were applied.Experiments were carried out to systematically study reservoir characteristics such as reservoir petrology and physical properties of Chang 6 and Chang 8 reservoirs in Hujianshan area,micro-pore structure characteristics,diagenesis and diagenetic facies,and micro-percolation characteristics.The main reasons affecting the physical properties of the reservoir,understand the formation mechanism and the main control factors,in order to find the development mechanism of relatively high permeability reservoirs And then a comprehensive classification and evaluation of the reservoir,combined with characteristics of hydrocarbon source rock,exploration and development of the current situation and oil test results,ultimately favorable regional study area is preferred.The main knowledge obtained is as follows:1.The Chang 6 member of the Hujianshan area mainly develops the delta front subfacies,and the channel sand bodies are distributed in the north-south,north-east,and south-west directions on the plane.The Chang 8 member mainly develops the delta fronts and delta plain subfacies.The sand body shows a north-east-south stripe distribution on the plane.Among them,the Chang 61 and Chang 82 sand bodies are relatively well-developed,with average thicknesses of 19.7 m and 14.7 m,respectively.2.The Chang 6 reservoir in the study area is mainly dark gray fine-grained feldspar sandstone and lithic feldspar sandstone,with high interstitial content,average 13.4%,average porosity 10.67%,and average permeability 0.53×10-3?m2;Chang 8 reservoirs are mainly gray and dark gray fine-grained debris feldspar sandstone and feldspar debris sandstone,followed by a small amount of debris sandstone and feldspar sandstone,with high interstitial content,14.2%,and the average porosity is 8.73%,with an average permeability of 0.53×10-3?m2.Both belong to low porosity-ultra-low permeability reservoirs.3.The typical diagenesis types of the reservoirs in the study area mainly include mechanical compaction,clay mineral cementation,iron calcite cementation,and dissolution.Among them,compaction has the greatest damage to the porosity of Chang 6 and Chang 8reservoirs,causing the porosity loss rates to be:50.62%and 52.86%,the porosity loss rates during the early cementation-metathesis process were 3.96%and 4.35%,the porosity loss rates during the middle-late stage cementation-metathesis process were 6.42%and 7.81%,and the porosity increase during the dissolution process was 2.26%.And 3.13%,the effect of corrosion and pore-increasing is relatively limited.4.The diagenetic stage of the reservoir is mainly in the stage A of the diagenetic stage,and part of it enters the stage B of the diagenetic stage.The reservoir is mainly divided into 4types of diagenetic facies,which are the remaining intergranular pore phase of chlorite film,feldspar dissolution phase,clay mineral cemented microporous phase,and carbonate cemented dense phase.The physical properties of the reservoir decrease in turn.5.The types of pores are mainly intergranular pores,followed by the development of feldspar dissolved pores.The types of larynx are mainly lamellar,curved,and constricted.The types of pore structure are mainly type III.The pore structure heterogeneity is strong,pore throat radius is the main factor controlling reservoir permeability.6.NMR experiments show that the mobile fluid saturation of Chang 6 and Chang 8reservoirs in the study area is generally low,mainly for type III and IV reservoirs.The shape of the T2 spectrum curve before centrifugation is mainly bimodal,and some unimodal;The shape of the T2 spectrum curve is mainly unimodal.The saturation of the mobile fluid is closely related to its pore structure,especially the size of the pore throat has the greatest influence on the saturation of the mobile fluid.The face rate is the main parameter affecting the saturation of the mobile fluid.7.Permeability and oil displacement efficiency have a good correlation,which is a key factor controlling oil displacement efficiency and then affecting oil recovery.Heterogeneity caused by microscopic pore throat structure restricts oil displacement efficiency,and pore throat radius The size and distribution characteristics are the key reasons that restrict the reservoir seepage capacity.8.The formation mechanism of the low-porosity and low-permeability reservoirs in the study area is mainly the result of the combined effects of sedimentation,diagenesis,and micro-pore structure.Cementation and erosion are the main controlling factors for the development of relatively high permeability reservoirs.In accordance with the reservoir characteristics of the Yanchang Formation in the study area,classification and evaluation standards applicable to Chang 6 and Chang 8 reservoirs in the Hujianshan area were established,and the study area reservoirs were divided into four types:I,II,III,and?.9.Based on the comprehensive evaluation of the reservoirs,the first and second types of reservoirs are mainly combined with the characteristics of the source rocks and the results of the test oil.The three types of favorable areas provide certain theoretical support for further oil and gas exploration. |
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