Geochemical Characteristics and Organic Matter Enrichment of Mesoproterozoic Shales: Insights from the Cuizhuang Formation, Ordos Basin, North China Craton

The dark shale of the Cuizhuang Formation is the oldest organic-rich interval of the Mesoproterozoic Era in the Ordos Basin and also a potential deep-source rock interval in the basin. Its formation mechanism is of great significance to deep oil and gas exploration in the Ordos Basin. However, systematic studies on the sedimentary environment and organic matter enrichment of organic-rich dark rock series have been impeded by the scarcity of drilling samples. In order to address this issue, this study performed the first geochemical analyses on core samples of Cuizhuang Formation shales from below 4735 m in well PT1, combined with samples of the same formation from well JT1 and the Yongji section. These analyses were used to reconstruct the paleosedimentary environment, formation mechanism, and depositional model of the Cuizhuang Formation shales. The total organic carbon (TOC) content of the Cuizhuang Formation shales ranges from 0.04% to 1.47% (avg 0.49%), the bottom water redox indicator V/(V+Ni) ratio ranges from 0.63 to 0.87 (avg 0.77), and the V/Al ratio ranges from 7.6 to 21.6 (avg 12.4), all indicating anoxic conditions. Correlation analysis between V/Al, Ti/Al, and TOC and other indicators reveals that reducing environments and terrigenous clastic input are the main factors contributing to organic matter enrichment in the dark shales of the Cuizhuang Formation. Furthermore, by comparing the elemental geochemical characteristics of potential Proterozoic source rocks on the northern margin of the North China Plate with those of the Cuizhuang Formation shales, it is found that favorable preservation conditions are the controlling factors for the development of potential Proterozoic hydrocarbon source rocks.