A Study On The Reservoir Characteristics And Dominated Factors Of P₂x₈&P₁s₂ Low Permeability Gas Reservoir In Yan'an Area

Because the Neopaleozoic gas degree of exploration is low in the region of interest, so the characteristics of low permeability reservoir are not deep enough. The thesis makes a thorough research, focusing on reservoir's microscopic pore structure, based upon various analytical and measurement methods, including the techniques of casting section, SEM, XRD, cathodeluminescence, grading analysis, high pressure mercury penetration, relative permeability, rate-controlled mercury penetration and NMR. On these bases, we make a comprehensive evaluation of the reservoir; the favorable reservoir is divided from the low porosity and low permeability reservoir. And from the comprehensive angles of tectonism, deposition and diagenesis, we analyze the dominated factors of favorable reservoir. At last we get some conclusions as follows:1. The main pore types of the P2X8 reservoir in the region of interest are dissolved pores and intercrystal pores. The main throat types are lamellar throat and tube-shaped throat. Because the throat is small, the pore connectivity is poor, the contents of illite, kaolinite and other clay mineral are relatively high, causing the reservoir's bound water saturation is high with an average of 48.9%, movable fluid saturation is low, only 14.5%.The main pore types of the P1S2 reservoir in the region of interest are primary pores and dissolved pores. The main throat types are lamellar throat tube-shaped throat and dotted throat. Since the large throat, better pore connectivity, less illite content of clay minerals, the reservoir's bound water saturation is low, and movable fluid saturation is high, respectively 29.4% and 94.2%.2. P2X8&P1S2 are currently in mid B diagenetic stage. Target zone have coal strata of diagenetic features, such as strong compaction, pressolution and kiesel cementation. Compaction and cementation made the reservoir respectively lose 20% and 17% porosity. Dissolution made the reservoir increase 3.2% face porosity. So it's one of the major factors, which can improve the reservoir's physical property.3. Favorable reservoir is controlled mainly by tectonization, sedimentation and diagenesis. Favorable reservoir is general in the place, where develop underwater distributary channel microfacies, compaction and carbonate cementation are relatively weak, dissolution is strong, and it is in the relatively high position of nose. Because in this place the sand thickness is larger, hydrodynamic is strong and quartz content is high, dissolution is strong, resulting in larger throat and better pore connectivity, so the gas saturation and physical property are relatively high in the reservoir. On the contrary, the place where develop the microfacies of underwater natural levee or the edge of the underwater distributary channel, sand body is thin, water power is weak, quartz content is low, matrix and plastic debris particles are high, compaction and carbonate cementation are strong, and dissolution is relatively weak, resulting in smaller throat and worse pore connectivity, while it is usually in the lower parts of structural nose, as a result, the physical property of the reservoir is poor.