Sedimentary and karst-controlled natural gas accumulation in a restricted carbonate platform of the Lower Paleozoic Majiagou Formation, southeastern Ordos Basin

The weathering crust gas reservoirs of the Lower Paleozoic Majiagou Formation in the southeastern Ordos Basin represent an important yet insufficiently understood target for natural gas exploration, particularly with respect to the multi-scale controls on hydrocarbon accumulation. Based on integrated analyses of core, well logging, gas testing, and geochemical data from 68 wells, this study systematically constrains the relationships among depositional systems, karst paleogeomorphology, reservoir development, and gas accumulation mechanisms within the Ma5 1 –Ma5 5 sub-members. The results indicate that the gas accumulation is demonstrated to be controlled by the dynamic coupling of four key geological factors: source rock distribution, karst paleogeomorphology, structural configuration, and sedimentary reservoir characteristics. A semi-quantitative multi-factor evaluation model is further developed to predict favorable exploration zones, highlighting that gas enrichment is preferentially associated with the spatial superposition of thick coal-measure source rocks, terrace–platform geomorphic units, structurally elevated positions, and dolomite-dominated reservoir facies. These findings provide new insights into the integrated sedimentary–karst–structural controls on natural gas accumulation in deeply buried karstified carbonate reservoirs, with broader implications for exploration in analogous cratonic basins worldwide.