Paleoenvironmental conditions and organic matter enrichment of the Late Paleoproterozoic Cuizhuang Formation dark shale in the Ordos Basin, North China

Recent years' research advance on the life evolution, oxygen content, and paleo-oceanic conditions of the Mesoproterozoic all indicate that during the Mesoproterozoic era there were suitable environments for the formation and preservation of high-quality hydrocarbon source rocks. The discoveries of the Proterozoic oil and gas resources in recent years, also suggests a need to enhance our understanding of the China's Mesoproterozoic black organic shale depositional environment and its potential for generating hydrocarbons. The Paleoproterozoic dark shale of the Cuizhuang Formation is the potential petroleum source rock in the Ordos Basin. Mineralogy, organic and inorganic geochemical analysis were applied in this study to evaluate the sedimentary paleoenvironments and its factors that control the enrichment of organic matter and to reconstruct the organic matter accumulation model of the Cuizhuang Formation dark shale. Quartz and clay minerals are the most abundant minerals in the Cuizhuang Formation dark shale. The TOC content of the dark shale is relatively fair, ranging from 0.47% to 1.47% and averaging of 1.00%, with the organic matter having a high thermal maturity (mean of 2.24 Ro%) . The Cuizhuang Formation shale was deposited in a passive continental margin setting, and the climate was under moderate warm and humid conditions with low salinity of seawater as evidenced by the proxies of chemical alteration index (CIA) and Sr/Ba. The paleoproductivity proxies Babio, Ba%, Ba/Al, P2O5 and P/Ti indicate low productivity. The low productivity may have significantly played a role in limiting the organic matter formation resulting in relatively low TOC content. The redox parameter U/Th, V/(V+Ni) and other indicators indicate that the anoxic redox environment of the dark shale is indicated. There is a significant positive correlations of V and V/Al with TOC in black shale. The anoxic redox environment in deeper waters and a higher sedimentation rate than the grey and red shale played the main role of enriching the organic matter of the dark shale. Given the widely development of the black shale, favorite depositional environment, along with the high shale TOC and suitable thermal evolution degree, the Cuizhuang Formation dark shale in the Ordos Basin is regarded as a potential source rock, holding promise for petroleum exploration.