Residual Carbonate Karst Reservoir Reconstructed by Karst Planation: A Case Study of the Ordovician Paleokarst Reservoir in the Ordos Basin

Based on the global typical karst characteristics reported in recent years and the latest oil and gas exploration results, it is found that the characteristics of the karst reservoirs in the Ordos Basin are significantly different from those of traditional karst reservoirs, indicating that the main contribution to the formation of the karst reservoirs in the region may not have been karst dissolution. The previous misunderstanding of the formation mechanism of karst reservoirs suggests that we should think about the formation and development processes of Ordovician karst and the reservoir pores in the Ordos Basin from another perspective. In this study, a combination of karstology, geography, sedimentology, and reservoir geology was used to investigate the formation of reservoirs. Based on the original understanding of the control of large-scale landforms on the sediments in sedimentary basins, this study focuses more on the dynamic spatial-temporal relationships between the evolutions of the landforms, karst, and reservoirs. By studying the petrological characteristics, pore filling, karst zonation, and planation model of the weathering crust reservoir in the Ordos Basin, it is determined that the main reservoir space was formed in advance rather than in the supergene dissolution period. A gypsum-bearing dolomite flat was the material basis for reservoir development and later karst planation transformation. The penecontemporaneous dissolution in the Ordos Basin dominated the spatial development of the early dolomite reservoirs. The traditional karst cycle is actually caused by the erosion of the bottom of the karst floor on the double-layer leveling surface. The karst planation in the Caledonian played a discontinuous and destructive filling role throughout the entire reservoir zone by changing the diagenetic environment, the main contribution of which was communication between the pores. Therefore, the weathered crust reservoir in the Ordos Basin is a residual reservoir controlled by karst planation rather than a traditional karst reservoir. The high-quality reservoirs are mainly distributed in the upper part of the karst slope and the top of the karst residual hillock in the gypsum-bearing dolomite flat facies belt.