Mirco-pore Structure Characteristics And Quantitative Evaluationof Ultra-low Permeability Sandstone Reservoir In Chang 6 Formation,M District,Ordos Basin

The study of reservoir micropore structure characteristics and its quantitative evaluation are the the core technology and critical method in the exploration and development of Chang 6 tight sandstone reservoir of M district, Ordos Basin. However, the previous studies are mostly qualitative analysises which are lack of the precise knowledge of the micropore structure characteristics and quantitative evaluation. This has caused a lot of obstacles during the exploration and development of the reservoir. Therefore, this paper conducts a deep research on the pore structures of study area based on regular physical propertis data, the particle size analysis, the casting thin sections data, the scan electron microscope data and the X-ray diffractions data. Coupled with the conventional high pressure mercury experiment, the constant-rate mercury injection experiment and nuclear magnetic resonance experiment, the classification and the quantitative evaluation have also been given for the Chang 6 reservoir in study area. The following conclusion can be reached through this study.(1) The lithology of the Chang 6 reservoir is mainly feldspar sandstone and lithic feldspar sandstone. The particle size of the sandstone ranges from fine to medium. Porous cementation has predominated the cemention type with authigenic clay cement, siliceous cement and carbonate cement. The average porosity and permeability of the reservoir are 10.19% and 0.71×10 ?m respectively which belongs to low porous, ultra low permeable reservoir.(2) The main pore types of the reservoir are intergranular pores and dissolved pore. The combination pore types are domanited by dissolved-intergranular pores as well as intergranular-dissolved pores. The flaky and curved lamellar throat as well as the control shape throat are major throat types. The pore structure types are mostly fine pore-super fine throat, following by fine pore -fine throat and medium pore-fine throat, the proporation of medium pore-medium to fine throat is the least. The compaction and presolution are main controlling factors which has made the reservoir tight vertically, Chang 6 reservoir is in A-B stage of diagenesis.(3) The high pressure mercury experiment has indicated that there are 4 types of the pore structures in the study area, say, ? (rough type),? (partial to rough type), ?(partial to thin type) and ?(thin type). The rough type is mainly developed in the middle of the distributary channel with good properties. The reservoir properties are getting worse successively with the thin type which developed in the interdistributary bay is the worst. The main pore structure of the study area are ? type and ? type.(4) The constant-rate mercury injection experiment has indicated that the distribution ranges of throat and pore in study area are from 0.2 ?m to 2.5 ?m, from 123.4 u m to 155.78 ?m respectively. The distribution range of the pore and throat radium are narrowed with the increasing of the permeability, and it has decreased with the growth of the peak frequency. The reservoir property and seepage ability are influenced profoundly by the effective throat characteristic and its connectivity.(5) The nuclear magnetic resonance experiment has shown that the distribution of the movable fluid saturation which is range from 3.53% to 67.89%. The movable fluid saturation of type ? and type? are 51.48%,46.74% respectively which are the potential reservoir of this area. It is mainly controlled by the structure and type of pore as well as clay minerals?(6) The reason which caused the tight pore structure in the study area are sedimentation and diagenesis. Sedimentary facies has taken the mian resposbility of the reservoir property. Diagenesis in study consists of compation, cenmetation and dissolution. The result of the porsity evolution calculation has suggested that the average primaty porosity is 38.67%, the reduction porosity rate of compaction and cementation are 19.19% and 11.73% respectively, dissolution had added an average porosity of 3.18% which made the final porosity increased to 10.93%. Compaction is the predominating cause of the porosity loss.