Study On The Reservoir Characteristics And Reservoir-forming Rule Of Tight Sandstone In Linxing Block

Linxing area is located in the central region of eastern margin of Ordos basin. In this paper, theories of sedimentary petrology, reservoir geology and logging geology is synthetically adopted as the instruction. On the basis of collected geological, logging and well logging data, by core observation and a variety of test and analysis methods, the types and characteristics of sedimentary systems in Linxing area is identified. Besides, reservoir characteristics and influence factors of sandstone interval in Shan2 and Tai2 section are studied detailedly and thoroughly, and gas reservoir forming conditions and the accumulation model is discussed.According to the marker beds of No.3+4 and No.9 coal seam, Shanxi group and Taiyuan group are divided into Shanl, Shan2 and Tai1, Tai2 respectively. The study shows that the sedimentary environment of Shan2 section is delta plain, mainly include sevaral microfacies, such as distributary channel, distributary bay, swamp and natural levee microfacies, and the sedimentary environment of Tai2 section is epicontinental sea, including barrier island, tidal flat and lagoon sedimentary. The plane distribution of sandstone is mainly controlled by sedimentary microfacies.According to the microscopic thin section identification and physical parameters analysis, the types of reservoir rock, reservoir space and diagenesis are identificated. The lithic sandstone is the main reservoir rock type in the study area, and the dominant detrital composition is quartz. Reservoir space type is featured with secondary pores, of which 70% have a porosity range of 4% to 8%, and 70% have a permeability lower than 0.05 md, so the reservoir belongs to the tight sandstone reservoir. The types of diagenesis include compaction, cementation and dissolution. The cementation is obviously seen in clay, authigenic quartz and carbonate mineral, and greatly reduces the reservoir physical property. The acidic fluids generated by coal-bearing formation have a corrosion effect on detritus, matrix and cement materials to different degrees, and effectively improve the physical properties of reservoir.Data analysis shows that there are not only a good fitting relation between the porosity parameter and the log values of density and acoustic time, but also a good correlation between permeability and porosity values in the study area. By using data fitting and multivariate regression methods, a log interpretation model of the tight sandstone reservoir petrophysical parameters is establishment. The validation shows that the model has a high reliability and can meet the demand of well logging interpretation.The reservoir forming condition of tight sandstone gas is analyzed. It revealed that the types of source rocks in the study area are mainly coal and carbonaceous mudstone, which have high organic carbon contents and result in good quality of source rocks. The organic matter types are mainly sapropelic-humus type and humic type. The large-scale mudstone of fluvial facies in the upper part of Shanxi group is the ideal regional cap rock for gas reservoirs. The main combination patterns of accumulation are two types, the first one with source rocks in the upper and reservoirs below and the second one with source rocks below and reservoirs in the upper. The migration dynamics of gas are mainly pressure difference between the source rock and the reservoir rock.