| Tao-2 well area is located at the southeast of Sulige Gas Field and lies in the northwest of Shanbei Slope, Ordos Basin, area of which is about 700 square kilometers. Recently, breakthrough has taken place in Lower Paleozoic of the study area?but the understanding in the palaeokarstic reservoir of Majiagou Formation is far away from the practical need of exploration and development. Based on the former research results and the analysis of logging data, well-logging data, core, cuttings and thin sections datas?and testing data, as well as some production data, a systematic research was taken on sedimentary microfacies, diagenesis types? reservoir spaces, physical property characteristics, pore texture in Ma5j-Ma54 palaeokarstic reservoir of Majiagou Formation, Ordovician, in the study area. The type of karstic palaeogeomorphology was reconstructed according to "carboniferous stratum thickness map of Tao-2 well area”?“Ma S/’s bottom-Ordovician top residual thickness map of Tao-2 well area’’?" fore-Carboniferous stratum paleogeologic map of Tao-2 well area”?as well as combining qualitative recognition method with quantitative indexes. On the basis of all above, the palaeokarstic reservoir was comprehensively evaluated.Results showed that the Ma5|-Ma54 reservoir of Majiagou Formation, Ordovician, in the study area is a typical palaeokarstic reservoir that contains a suite of carbonate rock with thin-layer gypsum rock?among which the micritic-microlite dolomite, microlite-silty dolomite and siltv dolomite are more widely distributed, while thick-layer micritic limestone can also be found in some area; the palaeokarstic reservoir experienced dolomitization, compaction and pressure-solution,dissolution and cementation, among which the capillary condensation dolomitization, reflux permeability dolomitization and syn- sedimentary, epidiagenetic, buried dissolution are the key to form high quality reservoir; The reservoir is characterized by low porosity and permeability?and among the main types of reservoir spaces, intercrystalline solution pores, gypsum mould pore, karst caves or residual holes filled with dissolved pore, nonselective solution pores, structural fractures and solution fractures are considered as the effective ones; The reservoir karst caves mainly belong to half-full filled type, and the calcite fillings account for most part, and with the big amount calcite filling the caves, the physical ABSTRACT property of reservoir is getting much worse; Horizontal fracture takes the first place in the reservoir fracture, and the second place belongs to vertical fracture, while oblique fracture is with a low development degree. Also, fractures are more developed in silt-crystalline dolomite and argillaceous-bearing dolomite, characterized by half-full filling, and the fillings are mainly calcite and quartz, for which fractures can easier promote the reservoir physical property and the migration of natural gas. According to pore structure parameters and capillary pressure curve features, the pore structure is divided into 4 class; The best favourable reservoir types are dominated by fractured-cavern type and fractured-porous type; The Ma51-Ma54 palaeokarstic reservoir of Majiagou Formation, Ordovician, in Tao-2 well area are divided into 4 Class by means of porosity, permeability, gas saturation, types of mercury injection curves, effective thickness, open-flow capacity and other indexes. It is discovered that the study area is mainly distributed with Ⅲ+Ⅳ Class reservoir, and Ⅱ Class reservoir distributes in some area, whileⅠClass reservoir distributes sporadically. Also, combining dolomitization trend area plans with Ordovician karstic palaeogeomorphology map, it is recogonized thatⅠClass and Ⅱ Class reservoir usually distribute in the area with intense dolomitization, and the middle-scale monadnocks at karst slope are considered as fine reservoir development zones. |
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