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Reservoir Characteristics, Formation and Gas Accumulation of Ordovician Ultra-Tight Paleokarst Carbonates in Eastern Ordos Basin, China


Ordos Basin is one of the most important hydrocarbon producing basins in China. The Ordovician Palaeokarst carbonates, which are tight reservoirs, are important hydrocarbon-bearing reservoirs and have been believed to develop in the central part of the basin only. However, recent discovery of commercial gas flow from a few wells in eastern basin have attracted much attention to reevaluate the carbonate reservoir potential. In this study, we use cores, scanning electron microscope (SEM), and pore-throat image analysis of casting sections to study pore structure, karst caves and fracture systems of Ordovician Palaeokarst carbonates reservoirs. Two kinds of pores are identified, including solution and dolomite intercrystal pores. Solution pores, which were initially formed during epigenic karstification and partly filled during burial diagenesis, are the most important storage spaces. The pore sizes are usually micrometer to millimeter scale, with pore diameter from 1μm to 2mm and throat diameter less than 15μm. The carbonates with pores size less than 30 μm and throats radius less than 5μm are not considered as effective reservoirs. Because karst cave and fracture systems are mostly filled during burial diagenesis process, Paleokarst reservoirs are featured by low to ultra-low porosity and low permeability to tight. Porosity of 3% and permeability of 0.05 mD are regarded as lower limit of reservoir physical property. Depositional environment, karstification, and burial diagenesis are major factors controlling the formation of high-quality carbonate paleokarst reservoirs. The dolomite with anhydrite concretions, which were dissolved during epigenic karstification and incompletely filled during burial diagenesis, can form high-quality reservoirs. Palaeokarst landform, especially upland and slope, have important effect on gas accumulation. Palaeokarst upland and slope mainly distribute in higher karst intensity area where high-quality reservoirs develop well, and palaeokarst unconformity as migration pathway can easily connect overlying coal-bearing source rock and underlying reservoirs in this area. This study explores the reservoir characteristics of Ordovician Paleokarst carbonates in eastern Ordos Basin, which could be beneficial to understand hydrocarbon accumulation mechanism for both ultra-tight carbonate reservoir in China and other basins in the world.