Main controlling factors of organic matter enrichment of the Mesoproterozoic source rocks from the North China Craton

Despite the discovery of dozens of primary Mesoproterozoic petroleum fields with a large amount of petroleum reservoirs, which were not considered to be exploration targets previously, little information is available on the main controlling factors of the organic matter enrichment of the Mesoproterozoic source rocks. In this study, based on geochemical element and total organic carbon (TOC) analyses of samples collected from four sets of the Mesoproterozoic source rocks in the North China Craton (NCC), namely, the Cuizhuan, Chuanlinggou, Hongshuizhuang, and Xiamaling formations, an attempt was made to determine the primary productivity and redox conditions during the deposition of these stratigraphic units. The quantitative primary productivity (PP) indicated that the palaeo-productivity was moderate to high during the deposition of the Xiamaling, Chuanlinggou, and Hongshuizhuang formations, all of which coincide with upwelling areas, and the palaeo-productivity was low during the deposition of the Cuizhuan Formation, which correspond to continental shelf or open ocean. The vanadium versus chromium, the nickel versus chromium, the enrichment factor of molybdenum versus that of uranium, and the pristane versus phytane ratios indicate that the source rocks of the Hongshuizhuang and Xiamaling formations were accumulated in anoxic-sulphidic environments; however, most source rocks of the Cuizhuan and Chuanlinggou formations were accumulated in oxic-suboxic settings. High TOC values are only exhibited in the source rocks from the Hongshuizhuang and Xiamaling formations, suggesting that the organic matter enrichment degree in the Mesoproterozoic source rocks was likely controlled by both the PP and consequent bottom-water redox conditions.