Ordovician karst weathering crust reservoirs on the northern Jingbian buried platform in the Ordos Basin

Judged from the pre-existing exploration history of the Jingbian and Wushenqi Gas Fields, the key factors for the successful exploration of the Lower Palaeozoic natural gas depend on the dolostone reservoirs modified by karstification. It is apparently important to the refinement of the interpretation of the genesis of the dolostone reservoirs in the study area. The examination of the types and development of the weathering crust reservoirs within the Jingbian buried platform has disclosed that the porosity types in Ma-54 and Ma-55 strata contain the fissuresolution openings (pore caves) type, fissure-pore space type and pore space type. Because of the exhumation of the Ma-5₄ and Ma-5₅ strata, the source rocks in the Upper Palaeozoic coal measures became direct and effective gas source for the hydrocarbon accumulation in the dolostone reservoirs. The gullies and grooves widespread in the karst slope zone tend to penetrate into the uppermost part of the Majiagou Formation, and thus allow the alignment of the Ma-5₄ reservoirs along these gullies and grooves. The dolomitization and karstification are interpreted to be the key factors for the formation of the concealed lithologic and stratigraphic-lithologic traps. The middle and lower parts of the Ma-5 reservoirs will still be the target horizons in the future exploration.