Petrography and facies distribution of Middle Ordovician Ma 51+2 tight dolomite reservoirs in the Ordos Basin, Central China

The Middle Ordovician Majiagou Formation in the eastern Ordos Basin, central China, is an important area in the exploration for tight carbonate gas, especially within weathering crust layers in the first and second submembers of the fifth member of the formation (herein referred to as Ma 5(1 + 2)). However, karstification prevents a clear understanding of the petrological characteristics and facies distribution of these layers, which hinders exploration. Based on cores, thin sections, and cathodoluminescence analysis, we investigate the petrological characteristics of Ma 5(1 + 2), determine the nature of lateral lithological variations in the eastern and central parts of the Ordos Basin, and constrain facies distribution in the region. In addition to karst breccias with unrecognizable parent rocks, Ma 5(1 + 2) comprises four lithologies: gypsum/halite mold-bearing micritic dolomite, micritic dolomite, grain dolomite, and microbial dolomite. We recognize three main sedimentary subfacies: restricted lagoon, grain shoal, and mound-shoal complex. Ma 5(1 + 2)records a complete transgression-regression cycle. The Ma 5(2)(2) layer was deposited during a transgression associated with enhanced water circulation and abundant mound-shoal complexes, for which their frequency is positively correlated with the thickness of the unit. The Ma 5(1)(2) layer and overlying deposits correspond to a regression cycle, and the abundance of mound-shoal complexes in these units is negatively correlated with layer thickness. The Ma 5(1)(3) period represents the timing of maximum regression, when a gypsum-bearing dolomitic lagoon was dominant, associated with a restricted water body. The overall facies distribution is one of a restricted evaporite lagoon environment, similar to the central basin. Therefore, reservoir tightness is unlikely to be related to the sedimentary facies. The next phase of exploration should focus on sweet spots resulting from differential diagenesis or hydrocarbon accumulation. Our results provide guidance for research on tight carbonate reservoirs and hydrocarbon accumulation in other regions that experienced similar geological conditions.