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

Abstract

The Paleoproterozoic dark shale of the Cuizhuang Formation is the potential petroleum source rock in the Yuncheng 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 0.13% to 1.09% and averaging of 0.51%, with the organic matter having a high thermal maturity (mean of 2.24 Ro%) and type I kerogen. 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 Ba bio , Ba%, Ba/Al, P 2 O 5 and P/Ti indicate low productivity with negative and no correlations with TOC content and thus didn't control the organic matter enrichment. The low productivity may have significantly played a role in limiting the organic matter formation resulting in relatively low TOC content. The anoxic redox environment of the dark shale is indicated by the proxies of V/(V + Ni) and V/Sc, with strong positive correlations of V and V/Al with TOC. 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. Therefore, the Cuizhuang Formation dark shale in the Yuncheng Basin is regarded as a potential source rock, holding promise for petroleum exploration. • Paleoenvironment and its relationship with organic matter enrichment was revealed. • Low productivity resulted in low TOC content of the Cuizhuang Formation shale. • Reducing environment and higher sedimentation rate contributed to the organic matter enrichment of the dark shale. • The accumulation model of the dark shale organic matter in the Cuizhuang Formation was established. .