| The8th member of mid-permian Shihezi formation (Upper Paleozoic) in the east of Ordos Basin belongs to the typical low permeability reservoir. Reservoir microscopic characteristics, especially the microscopic pore structure directly influence the reservoir properties and seepage ability, and eventually determines production of the Deep Tight Sand Gas Reservoir. By utilizing such research methods as traditional mercury-injection, the experiment of gas-water two-phase displacement, relative permeability analysis, casting thin sections analysis, compute image grain size analysis, SEM analysis,compute image granularity and por-throat analysis, sensitivity analysis, constant-speed mercury injection and NMR technology. The thesis researched mineral composition of the reservoir framework and distribution features in the study area;the character of the fillings of sandstone and the variation in different area.the impact of types and produce shape of the authigenic clay minerals on the pore structure,seepage performance,sensitivity and so on; in-depth study of the microscopic pore structure characteristics and seepage performance of the study area sandstone reservoirs, pointed out the influence of diagenesis on reservoir seepage characteristics, especially the production impact factor and the reason of the heterogeneous features on Member8of Shihezi Formation by microscopic aspects.The research results indicate that diminishing of pore space and receding of reservoir poverty is mainly caused by intergranular hydromica, I/S mixed layer and carbonate cements in sandstones reservoirs of the8th member of Shihezi formation. The highest surface porosity is observed in lithic quartz-sandstones, followed by quartz sandstones and feldspathic lithic quartz-sandstones (very few). Lithic sandstones have the lowest surface porosity but more microfractures. Intercrystal micropore in kaolinite is one of the important types of reservoir space of gas. Reduction of reservoir physical property primarily results from destructive diagenesis, including compaction and cementation. The best association of pores in the8th member of Shihezi formation. includes micropores, dissolved pores and microfractures, which are mainly observed in lithic quartz-sandstones in the middle of the research area, followed by lithic quartz-sandstones and lithic sandstones in the north and the northwest, as well as quartz sandstones in the south. Lithic sandstones reservoirs in the east are characterized by well-developed microfractures and the diverse associations of pores. In the research area. The main throat types of sandstones reservoirs of the8th member of Shihezi formation are slice-shape throat, and tube-shaped throat. The throat radius are relatively different and most of them are classified to fine throat type. Permeability is extremely low, generally less than0.1×10-3μm2,as a result of the poor connectivity between pores and throats.In gas-water two phase displacement experiment test,it is mainly uniform mesh piston type of water flooding gas. Especially the water displacement gas is complete and its displacement efficiency is maximum. Intertwined shape the non-piston type of displacement efficiency is higher to medium. Non-uniform mesh type of displacement efficiency is lower. Higher irreducible water saturation(74.4%) in sandstone of the8th member of Shihezi formation is mainly distributed in the small pore throat and movable fluid is in the larger pores and thicker throat; With the increase of water saturation, water-blocking damage degrees of sandstone gradually increases and permeability of sandstone gradually reduces. As a whole, reservoir is characterized by a stronger speed-sensitive and water-sensitive, mainly because of the high sandstone illite and kaolinite content.Some samples show a moderate to moderately strong acid-sensitive, because chlorite, calcite minerals affect, while salt-sensitive and alkali-sensitive is weak. |
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