| The exploration shows that the natural gas resources in the Ordovician weathered crust at the eastern part of Ordos Basin is rich. An in-depth analysis for the pore-filling characteristics of the reservoirs is of great significance to the prediction of reservoir distribution and to the improvement of drilling success. Located in a stable carbonate platform in Majiagou period(Early Ordovician), there developed a wide range of carbonate rocks, among which anhydrite concretion?-bearing dolomite flat developed in Majiagou Submember-51-2 . Under the weathering and erosion(about 130 Ma) in the Late Ordovician, weathered crust karst reservoir developed mainly with anhydrite moldic pores. Based on cores, thin sections, and microstructure analysis by many experiments including carbon and oxygen isotopes, electron probe, and cathodoluminescence, there are 12 kinds of minerals such as calcite, dolomite, quartz, fluorite, pyrite, etc. filled in the pores, which usually formed 8 types of mineral assemblages. According to the diagenetic evolution of the Ordovician weathered crust karst reservoir, there are 4 filling stages including sea water, meteoric water, shallow burial, and deep burial. Mainly controlled by Ordovician paleokarst geomorphology, the anhydrite moldic pores are weakly filled at the platforms, mounds, slopes, and both sides of ancient trenches in the eastern part of the basin, with relatively high porosity. |
