Supergene karstification: The dominant process in forming Ordovician weathering crust reservoirs, Ordos Basin?

As a crucial oil and gas exploration target, the formation mechanism of the Ordovician weathering crust reservoir in the Ordos Basin has long been considered to be mainly controlled by supergene karstification. However, new exploration discoveries have posed a challenge to the traditional theory of karst paleogeomorphology controlling hydrocarbon accumulation. Through the analysis of the basic characteristics of the weathering crust reservoir, the controlling factors of reservoir development, and the reservoir formation mechanism, the key geological factors for the formation of the weathering crust reservoir are re-discussed. Studies have found that the main reservoir space of the weathering crust reservoir is gypsum mold pores, which are controlled by sedimentary microfacies, and early selective dissolution is the key to the formation of reservoir space. Early supergene karstification formed an interconnected fracture-vug system. The karst planation process not only controls the preservation degree of effective reservoirs but also regulates the type and filling degree of gypsum mold pore fillings. Late supergene filling and burial-stage dedolomitization damaged the reservoir, leading to its densification. Therefore, the formation process of the Ordovician weathering crust reservoir in the Ordos Basin is the result of the coupling of complex multi-stage geological processes. The karst reservoir is a facies-controlled karst planation residual reservoir, and supergene karstification is not the most important geological factor for reservoir formation. The high-topography units within the gypsum-bearing dolomite flat microfacies zone are the favorable zones of high-quality reservoirs.