Objective The period during the deposition of the Middle-Late Ordovician Kelimoli-Lashizhong formations witnessed a key transition of the tectonic framework along the western margin of the Ordos Basin. Investigating the lithofacies paleogeography of these formations can reveal the tectono-sedimentary responses during this period, thereby providing a basis for the resource potential prediction, exploration, and exploitation of natural gas in the study area.

Methods Using field section observations, core observations, and log interpretations, as well as previous research results, this study comprehensively analyzed the Middle-Late Ordovician strata in the Ordos Basin from the perspectives of stratigraphy, sedimentary system division, and sedimentary patterns. Accordingly, the lithofacies paleogeographic framework of this period was established, and the spatial evolution of the sedimentary environments was identified.

Results and Conclusions  During the deposition of the Ordovician Kelimoli-Lashizhong formations, the western margin of the Ordos Basin generally resided in an active continental margin setting, with neritic, bathyal, and abyssal sedimentary systems formed. Furthermore, three sedimentary subfacies were further identified: platform margin, continental slope, and basin. These findings reflect a paleogeographic framework characterized by shallow eastern and deep western parts, progressively deepening water bodies, and gradually increasing topographic complexity. During the deposition of the Kelimoli Formation, the sedimentary facies and lithology along the western margin of the Ordos Basin were dominated by platform-foreslope microfacies and moderately-thinly layered micritic limestones interbedded with calcareous mudstones, respectively. During the deposition of the Wulalike Formation, the sedimentary environment along the western margin of the Ordos Basin changed significantly. While the carbonate platform facies still predominated in the east, the sedimentary environment rapidly transitioned into the continental slope-basin facies westward. During the deposition of the Lashizhong Formation, the water body depths decreased slightly. Accordingly, the study area generally evolved into a continental slope environment, with the sedimentary environment transitioning into the basin facies westward and remaining as the carbonate platform facies in the east. Overall, under the joint control of regional sea-level fluctuations and tectonic subsidence, the western margin of the Ordos Basin transitioned from the predominance of platform foreslope microfacies to the extensively developed continental slope-basin facies during deposition of the Kelimoli-Lashizhong formations. This finding provides an important geological basis for understanding the tectono-sedimentary coupling relationship and natural gas accumulation along the western margin of the Ordos Basin.