| Nowadays, more and more scholars pay attention to the exploration and development of low and ultra-low permeability reservoirs. The proportion of it have take a great proportion in our nation's petroleum industry, This type of reservoir has already become a pillar of China's crude oil production and play a dominate role in national crude oil production. The typical characteristics of low and ultra-low permeability reservoirs characteristics mainly include: poor reservoir physical property, Strong inter-layer and internal heterogeneity, complex pore structure and permeability characteristics, Serious contradiction between oil and water. And the characteristics of the reservoir are mainly influenced by the diagenesis, which plays an important role in reservoir quality. All these characteristics of Low and ultra-low permeability reservoir make many oil researchers face a huge challenge on improve the degree of development of low and ultra-low permeability reservoir. So it is particularly important to carry out research on microscopic characteristics.The Triassic Chang 6 member of Yanchang formation is a typical low and ultra-low permeability reservoir as the main oil-bearing strata which wide distribution in Ordos basin, while there is an obvious disparity between different regions. In order to compare the difference of low and ultra-low permeability reservoir which belong to the different regions, Banqiao-Heshui Area, Huaqing area, Jiyuan area, Ganguyi area have been chosen as the object. A large number of core samples of casting thin sections, scanning electron microscope, mercury injection,constant pressure mercury, nuclear magnetic resonance and other experiment analysis were carried out. The paper is based on a large number of experimental data, starting from the differences of sedimentary characteristics,and petrology and diagenesis of Chang 6 reservoirs which in different regions.The differences of micro pore structure and seepage characteristics between the Chang 6 reservoirs of different regions are compared and analyzed, and the main reasons for these differences are also analyzed. This research mainly has the following understanding:(1) The study selected Banqiao-Heshui area, Huaqing area, Jiyuan area, and Ganguyi area were distributed from east to west on the plane, and has obvious difference in sedimentary characteristics. Their sedimentary facies from the east to the West show the characteristics of the change from the delta plain facies to the deep-semi deep lake facies.(2) Lithic feldspar sandstone is the main type of the four regions of Chang 6 Formation, the difference is mainly reflected in the proportion of sandstone types in different regions,Lithic feldspar sandstone accounted for 49.07%,53.39%,46.95%,75.68% in Banqiao-Heshui, Jiyuan, Huaqing and Ganguyi area respectively.(3) There is a big difference between the total content of all kinds of interstitial material in different regions, the interstitial content in Jiyuan area is highest, reaching 17.96%,followed by the Banqiao-Heshui area and Huaqing area,13.50% and 15.60% respectively. Ganguyi area is the lowest which only 10.90%.(4)In The Banqiao-Heshui area, Jiyuan area and Huaqing area, the cement has great proportion of interstitial content, which are 6.70%,11.15% and 8.74% respectively. Ganguyi area showed a completely different with the other three regional, the content of cement of Ganguyi area is relatively low, mainly carbonate cements.(5) The degree of porosity distributions of Chang 6 reservoir of Banqiao-Heshui area, Jiyuan area, Huaqing area, Ganguyi area, is similar, which is low and ultra-low porosity. There is a little differences in average porosity among four regions, at around 10%.(6) As for permeability distributions of Chang 6 reservoir of Banqiao-Heshui area, Jiyuan area, Huaqing area, Ganguyi area, the range distribution is,ranged from0.01?10-3um2to44.21×10-3um2, The average permeability is quite different in different regions. The average permeability of Banqiao-Heshui area is 0.93×10-3um2; The average permeability of Jiyuan area is 1.05×10-3um2; The average permeability of Huaqing area is 0.25×10'3um2; The average permeability of Ganguyi area is 0.68×10-3um2. Huaqing area compared to the other three regions has the lowest permeability level, its reservoir is mainly ultra-low permeability reservoir.(7) The Effect of easily dissolved minerals feldspar on the physical properties of the reservoir is positive and constructive for both porosity and permeability. The feldspar dissolution pore is used as the space of reservoirs, its content is often directly controlled reservoir quality.(8) Porosity decreased with the increasing of detritus content, The porosity of Ganguyi area and Huaqing area are sensitivity to high content of debris, Jiyuan area showed that the increased content of detritus on porosity has minimum damage degree.(9) The reservoir pore types are mainly residual intergranular pore and feldspar dissolved pore in Banqiao-Heshui area, Jiyuan area, Huaqing area, Ganguyi area. The proportion of Debris dissolution pore, intercrystalline pore and micro cracks in these four areas are low, fluctuation within a narrow range.(10) The dissolution-intergranular pores type and intercrystalline pores type is the main pore assembly types of Banqiao-Heshui area, Jiyuan area, Huaqing area, Ganguyi area, and the Throat types are mainly lamellar throat.(11) There is no much difference in the porosity development degree between the regions of Banqiao-Heshui area, Jiyuan area, Huaqing area and Ganguyi area. The difference of throat development degree is the main reason causing the regional difference of reservoir quality.(12) Compared to Banqiao-Heshui area and Huaqing area, two-phase fluid flow in Chang 6 reservoir of Jiyuan area and Ganguyi area, the oil phase flow resistance is smaller, and the seepage ability is relatively strong.(13) The chang6 reservoir movable fluid saturation of Banqiao-Heshui area, Jiyuan area, Huaqing area, Ganguyi area is low, mainly class III and IV. The movable fluid saturation is the highest compared to the other three regions in Jiyuan area.The Huaqing area has the lowest movable fluid saturation.(14) Differences in microscopic pore structure and clay mineral content of Chang 6 reservoir are the main reasons causing the movable fluid saturation difference between the regions of Banqiao-Heshui area, Jiyuan area, Huaqing area and Ganguyi area. |
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