Paleogeomorphy evolution of the Ordovician weathering crust and its implication for reservoir development, eastern Ordos Basin

Gas reservoir development of the Ordovician weathering crust in the Ordos Basin is closely controlled by the pre-Carboniferous paleogeomorphy. Previous studies show that the paleogeomorphy is high in the west and low in the east, and the karst highland, karst slope and karst basin are developed from west to east. With further exploration in recent years, many karst breccia that represent strong karstification, are found in the east area which previously is considered to be the Ordovician karst basin. Thus, it is necessary to revaluate controlling factors of karst paleogeomorphy development from a viewpoint of the dynamic paleogeomorphy evolution, to investigate the paleogeomorphy evolution of the Ordovician weathering crust in geological history and guide further research and prediction of development law of reservoir spaces. In order to reconstruct the paleogeomorphy of the weathering crust in the top of Ordovician in the east Ordos Basin, paleogeography, thickness of residual strata and paleokarst characteristics are well studied. The result shows that a wide range of paleokarst highland occurred in the central to east part of Ordos Basin in the early period, and the karst reservoir spaces were formed; but in the late period, the east part of the basin gradually evolved into the paleokarst basin, the current pre-Carboniferous paleogeomorphy was thus formed, and the dissolution reservoir spaces formed in the early period were mostly filled, accordingly the reservoirs were tight. However fracture networks formed by cave collapse connect intercrystalline pores of dolomite, therefore, the reservoirs can still be well preserved locally and are worthy targets for hydrocarbon exploration. Keywords: The Ordovician weathering crust, Paleogeomorphy, Karst highland, Karst slope, Karst basin, Karst reservoir, Majiagou Formation, The eastern Ordos Basin