| Ordos basin is a large one of a multi-tectonic systems, multicycle evolutions and multi-sedimental typies. The foults and folds are very developed in the edge, but inter the basin, the structure are relatively simple. Based on the basement nature, geological evolution and tectonic characteristics, the regional tectonic can be divided into six secondary tectonic units includes Yimeng uplift, Weibei uplift, Jinxi fault-fold belt, Yishan Slope, Tianhuan Depression and western margin thrust belts. The Heshui area is located in the southwest of Ordos Basin, the Chang6—Chang8has already been found with potential oil and gas storagy, Xifeng, Baibao oilfield obtain high-yield oil and gas resources in the layers too. The Chang6—Chang8reservoir lithology characteristics is mainly affected by the influence of the nature of the depositional environment and source and it is the typical low porosity and low permeability reservoirs.Based on China’s Situation of oil and gas exploration, the research on low-permeability reservoirs common in the Paleozoic, Mesozoic and Cenozoic will be the main field of oil exploration in the next period of time of China, and with the degree of future oil explorationgradually deepened, its proportion will continue to increase. The complex study of sandstone, fine grain size, diagenesis, strong heterogeneity is of significance to strengthen the understanding the low permeability reservoir characteristics, the formation mechanism of low permeability reservoir, looking for low permeability reservoirs for oilfield stable to determine the favorable block and enhanced oil recovery.Based on the analyse of core observation, sedimentary structure, petrological characteristics, logging facies and single well facies, it implies that Heshui area develops3kinds of sedimentary systems in the sedimentary period of Chang6—Chang8. They are lacustrine, braided river and braided delta. Among them, lacustrine can be further divided into2kinds of subphases include shore-shallow lake and shallow lake; the braided delta develops the plain and front of braided delta. The plain can be further divided into the microfacies of distributary channel, interdistributary bay, the front mainly developes the microfacies of underwater distributary channel, channel mouth bar, underwater interdistributary bay, distal bar, sheet sand. Among the microfacies, the (underwater) distributary channel is the most favorable abundance area of oil and gas.Previous reaserch implies that the study area can be divided into10oil layer besed on the oil distribution. They are Changl, Chang2, Chang3, Chang4+5, Chang6, Chang7, Chang8, Chang9, Chang10. Chang6—Chang8are the main oil layer the paper studied on.Feldspathic sandstone is the main composition of sandstones reservoir in Heshui area, followed by sub-lithic arkose, sub-feldspathic lithic sandstone. Argillaceous matrix, calcite, anhydrite, quartz overgrowth, dolomite, feldspar, increase the cement. The Chang6—Chang8reservoir sandstone is of low structure maturity, middle-good sorting, middle-fine granularity and high matrix contents.The research on diagenesis implies that the main types of diagenesis in Chang6—Chang8oil member are compaction, pressolution, cementation, metasomatose, recrystallization, dissolution and etc, in which compaction and cementation cause the loss of primary porosity is the destroyed diagenesis. The dissolution can dissolute the debris like arkosic and carbonate cements, so it produces a large amount of secondary solution porosity thus it is the constructive diagenesis. Based on the analysis of reflectivity (Ro), homogenization temperatures of fluid inclusions, compaction level, authigenous mineral, porosity type of sandstone, it is believed that Chang6—Chang8oil member with the stong diagenesis belongs to the stage of middle diagenesis A period.According to the thin section data, pore size, shape, porosity of the sandstone, the pore of reservoirs can be classfied as intergranular porosity, intergranular dissolution porosity, pore component, fracture porosity. It is belived that the type of the study area are pore throat sheet throat, bend sheet throat, bundle-like throat, the contraction portion of the variable cross-section is the throat and the throat is the porosity of the narrow part of the five kinds.In the study area, the physical properties is particullaly bad, the reservoir is of special low porosity and low permeability, the Chang8sandstone reservoir properties is relatively good, average permeability is0.4165MD, mainly located between the0.01-0.5MD. Chang6and Chang7sandstone reservoir permeability is less than the0.1MD. Contrast to the change trends of the por and perm, it shows that porosity and permeability has a good correlation. In Heshui area, Chang6sandstone reservoir with bad properties is the special low porosity and low permeability reservior, in the back groud of poor por and perm, some well may appear relative high value area, with good properties conditions.Based on the research of rock-type, sedimentary facies, diagenesis, diagenesis facies, physical distribution, reservoir lithology, grain size and pore characters, the reservoir of the study area is classified as three categories:type I, type II and type III. Type I and type II are mainly developed in the main channel activity area or the junction of branch river, where have a better permeability condition. Type III is mainly developed in the the edge of channel mouth bar, having a poor permeability condition. On the basis of these, combining with distribution of oil and gas, the favorable exploration blocks are predicted that can be used for exploration and development. |
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