Characteristics And Controlling Factors Of Low Permeability Sandstone Reservoirs In The Yanchang Formation Of The Zhidan Area,Ordos Basin

The Ordos Basin is an important oil-bearing basin in China,with huge resources,of which more than 90%are in low permeability sandstone reservoirs.The Zhidan Oilfield is located in the middle of the Ordos Basin which has significant amount of oil and gas produced in low permeability reservoirs.Based on core observation,log data analysis,thin-section observation,scanning electron microscopy,X-ray diffraction,petrophysical properties measurement,mercury injection pressure analysis,nuclear magnetic resonance test,carbon-oxygen isotope analysis and fluid inclusion analysis,the study of the characteristics of layer,sedimentary facies distribution,reservoir petrography,pore-throat structure,diagenesis and petrophysical evolution of the Yanchang Formation in the Zhidan Oilfield were carried out.The gray correlation method was used to evaluate the reservoir and new criteria of reservoir classification in the study area are put forward in this dissertation.The regional structure is stable and the terrain is flat during the Chang-6 to Chang-3members.Shallow water delta was deposited in the study area and the sedimentary environment is changed gradually from shallow water delta plain subfacies to delta front subfacies which follw the direction from north to south.The grain size of the bottom sediment is fine and the upper grain size is relatively coarse.The sand body of distributary channel and estuarine sand bar are the main sand type.The sand body size of distributary channel increases gradually from bottom to top,and the sand body was connected well in the direction of material source.The thickness of estuarine sand bar is thin,and it often evolves into sheet sand deposit transversely.The sand body of estuarine bar does not have the typical characteristics of lower fine and upper coarse due to the erosion of wave or the erision of distributary channel.The rock types of reservoir in the study area are mainly feldspar sandstone and lithic feldspar sandstone.The value of the feldspar content is up to 50%,higher than the quartz which about 20%,the grain size of clastic particles is fine,and developed the relatively poor gradation and psephicity of grain size.There are many types of sandstone cementation,including siliceous,carbonate and clay minerals,which developed all in the study area.The main clay minerals are chlorite,kaolinite,illite/montmorillonite mixture layer and so on.Chlorite is the most abundant clay mineral that can reach to about 60%.The pore types of the reservoir are mainly intergranular dissolution pore,as well as in-granular dissolution pore,residual primary pore,casting mold pore,intergranular pore and micro fractures which indicating the well pore connectivity.According to the pore connectivity relationship,the types of pore combination in reservoir can be divided into residual intergranular pore-solution pore combination,micropore-micro-fracture combination,residual intergranular pore-micro-fracture combination,dissolution and residual intergranular pore-micro fracture combination.The diagenesis in the study area mainly includes compaction,metasomatism,dissolution and cementation.Compaction mainly results in the deformation of plastic minerals（e.g.mica）and the crush of rigid minerals（e.g.feldspar）.The metasomatism result in the replacement of feldspar by carbonate and clay mineral.The cementation is mainly composed of clay minerals carbonate and siliceous cementation of clastic particles.Calcite and dolomite formed early diagenetically,and the conversion of smectite,microbial methanogenesis and decarboxylation of organic matter may provide the necessary cations for these carbonate cements.Ferroan calcite and ankerite formed at a relatively late diagenetic stage,decarboxylation of organic matter is the main source of these phases,which reflected the synergetic evolution of sand and mudstone.Volcanic tuffaceous dust could easily transformed to smectite,and they can also converted to semi-automorphic kaolinite and random mixed-layer I/S.The kaolinite mainly comes from the transformation of montmorillonite minerals and the dissolution of feldspar;Illite and Imon mixture mainly come from the transformation of montmorillonite and kaolinite and the dissolution of feldspar.The chlorite is mainly comes from the dissolution of pyroclastic substances,the transformation of kaolinite-montmorillonite-imon mixed clay minerals and the alteration of biotite.The carbonate cementation is mainly composed of mudstone at the contact interface of sand and mudstone Ca²⁺and CO3^2-migration and enrichment to adjacent sandstone driven by concentration difference.Siliceous cementation mainly shows secondary increase of quartz,Si O2 mainly comes from dissolution of feldspar and transformation of clay minerals.It can be seen that quartz and feldspar were metasomatized by calcite or ferrocalcite.The formation temperature of calcite/dolomite is between 47℃and 74.4℃and that of ferrocalcite/ferrodolomite is between 90.6℃and 102℃according to carbonate C-O isotope analysis.The results of temperature measurement analysis of quartz-extended inclusions show that the homogenization temperature is between 80℃and 120℃.Considering the clay mineral mixing ratio and the quartz increasing grade data,it is concluded that the reservoir in the study area is currently in the diagenetic stage of the middle diagenetic A stage.The porosity of the reservoir in the study area is between 0.045%and 15.6%,and the permeability is between 0.02×10^-3μm² and 14.1×10^-3μm²,which indicates that a low porosity and low permeability reservoir developed in the study area.Petrophysical properties were controlled by sedimentation and diagenesis.Average pore throat ratio of reservoir in the study area is between 146.8 and 575.35.The pore throat ratio varies greatly and is unevenly distributed which was characterized by large pore and small throat,indicating that the degree of heterogeneity of pore structure has an important impact on reservoir permeability.Based on the results of microscopic thin slice observation and SEM analysis,the pore evolution process of reservoir in the study area was quantitatively characterized by the inversion and stripping method.The results show that the value of porosity loss caused by compaction is 16.6%,caused by cementation is 9.9%,and caused by chlorite,quartz and late carbonate cementation is 4.1%,2.5%and 3.3%separately.It can be seen that compaction and cementation are the main reasons for decreasing reservoir porosity.Based on the analysis of reservoir influencing factors,some parameters were selected as indicators for comprehensive reservoir evaluation in the study area.The grey correlation evaluation was carried out for the reservoir in the study area and the parameters including porosity（Φ）,permeability（K）,particle size median（M）,displacement pressure（Pd）,median radius（R）,coefficient of variation（V）and mercury reduction efficiency（E）.Considering the analysis of diagenesis and pore-throat structure in the study area,the evaluation score was divided into four intervals:1.0～0.7,0.7～0.5,0.5～0.3 and 0.3～0 and the reservoir was divided into I～IV types.Type I reservoir is the high quality reservoirs in the study area,it is mainly developed in the Chang-6 member,which mainly distributed in block well Zheng51,well 3014,well 3029,well 3020 and well 3031.Type II reservoir is relatively good reservoir,which widely development in the study area.Type III is poor reservoir,which mainly distributed in block well2003,well 3291 and well 3328 in the Chang-3 member,distributed in block well Zheng 73,well 3228 and well Zheng 446 in the Chang-4+5 member,and distributed in block well Zheng50 and well 3432 in the Chang-6 member.Type IV is bad reservoir,which distributed only in small parts of the Chang-4+5 and Chang-6 members.