Differential Organic Matter Enrichment in Middle Ordovician Shales of the Wulalike Formation, Western Margin of the Ordos Basin

Marine shales of the Middle Ordovician Wulalike Formation in the Ordos Basin constitute a key target for shale gas exploration. However, the sedimentary environments associated with different shale lithofacies vary significantly, and the mechanisms controlling the organic matter enrichment remain incompletely understood. To address this issue, this study conducts a comprehensive analysis of the factors influencing organic matter enrichment in distinct lithofacies. Based on mineralogical and elemental geochemical data, Wulalike Formation shales can be grouped into three lithofacies associations (i.e., siliceous shales, calcareous shales, and mixed shales). Results indicate that the siliceous shale, with the highest TOC (avg. 1.05%), was deposited under a warm and humid climate with relatively high productivity and anoxic to euxinic bottom waters, promoting preservation. In contrast, the calcareous and mixed shales, formed under semihumid to semiarid climates with lower productivity and variable terrigenous input, exhibit lower TOC. Comprehensive analysis shows that, against the background of generally low organic matter abundance, organic matter enrichment were primarily associated with redox conditions and paleoclimate, with terrigenous input playing a dual role, while paleoproductivity had a limited effect. The overall lower TOC, compared to typical marine shales, may be attributed to deposition in a nutrient-limited, restricted, stagnant basin distal from volcanic sources. This study provides new insights into the organic matter enrichment mechanisms of Ordovician source rocks in the western Ordos Basin.