Characteristics And Comprehensive Evaluation Of Low Permeability Reservoir Of Chang 4+5 Member,Triassic Yanchang Formation,Zhidan Area, Ordos Basin

The lithology, physical property and pore structure of reservoir in Zhidan area of Ordos Basin are relatively complex. Lithology and physical properties vary dramatically in lateral direction; Relatively high gamma sandstone develops in some areas with the advent of corresponding oil layer with relatively low resistivity. All of these bring some difficulties to the comprehensive evaluation of reservoir and the accurate identification of oil-water relationship; it also restricts the exploration and development of the oil field.For the reference of the theory of reservoir sedimentary and reservoir geology, according to core analysis, casting amella, SEM, high pressure mercury injection, NMR analyzing data as entry points, the reservoir lithologic identification, physical properties, pore structure characters, four property relationships, fluid identification especially identification of low resistivity oil layer are conducted emphatic research on the basis of characteristics of low porosity and low permeability reservoir as well as logging reseponse combined with actual production data, geological data and geophsical data of Chang 4+5 formation in Zhi Dan area,forthermore, criteria for reservoir classification and evaluation are demonstrated and the rerservoirs are classified and evaluated. The Specific knowledge is listed as follows.Chang 4+5 Member, Triassic Yanchang Formation in Zhi Dan area was essentially of delta front subfacies depositional system, provenances were mainly from the nort, The underwater distributary channel superimposed on each other and extended to the southern part of the study area, forming a thick sand body, which is the main reservoir in the region.Minerals compost high in feldspar content, feldspar sandstone and lithic feldspar sandstone are mainly composed the reservoir with good sorting characters and relatively strong compaction, dot-line contaction and line contaction are the major form under the circumstanses of underwater distributary channel sedimentary enviroment and long distance transportation. Primary porosity is the main reservoir space of the effective reservoir; pores as a result of feldspar corrosion and relavent cement play a positive role in reservoir permeability,pore types such as intergranular pores and dissolved pores are relatively developed, moreover,the effective pore space is connected under the condition of the development of micro fractures. On the basis of "four-property" relationship, in the light of relative high gamma sandstone and low resistivity reservoir, they were directly identified and quantitively calculated through effective and reasonable analysis of qualitative identification and quantitative calculation combined with overlap chart technology and crossplot technique as well as staged pattern recognition model.The reservoir evaluation criteria are established through the comprehensive study of reservoir characteristics in Zhi Dan area, at the meantime, the classifition of 4+5th formations are studied according to this standard, which leads to a certain understanding for distribution characteristics of excellent reservoirs as well as has a posotive guiding significance for exploration and production.