Geochemical characteristics of the lower Cambrian Qiongzhusi Formation in Huize area, east Yunnan: implications for paleo-ocean environment and the origin of black rock series

Abstract

The lower Cambrian black rock series in eastern Yunnan province is a high-quality shale gas reservoir with great prospect for oil-gas exploration. However, the depositional conditions and mechanisms of organic matter enrichment are not well understood. In this study, geochemical characteristics of total organic carbon (TOC) and major and trace elements of fourteen black rock series samples from Dahai outcrop in eastern Yunnan province are systematically investigated to analyze the paleoenvironment and their effects on organic matter enrichment and established the accumulation model of organic matter. Results show that TOC contents of the four lithologic units of Qiongzhusi Formation black rock series are 0.70, 0.34, 1.77, and 0.27, respectively, which suggests that TOC content is generally low (mean = 0.51%). The main elements show that P2O5, CaO, and MgO are relatively rich, whereas other elements depleted. The enrichment factors (EFs) of trace elements showed that Pb and Zn are highly enriched in all samples, and Cu, Sr, Ba, U, Mo, V, Be, and Mo are also enriched in different lithofacies units. The elemental anomalies may be related to hydrothermal activities, which may lead to the distortion of some paleoenvironmental indicators to some extent. The Rb/Sr and Sr/Cu values show that the palaeoclimate gradually changed from dry to humid. Al concentrations indicate a gradual increase in terrigenous detritus supply. The hydrodynamic conditions and sedimentation rates implied by Zr/Al and Ti/Al values correspond well with lithology and grain size. Based on the comprehensive analysis of B/Ga and Sr/Ba ratios, it is considered that the black rock series in Qiongzhusi Formation was mainly formed in brachial water. Multiple paleoredox indicators (e.g., Ni/Co, V/Cr, U/Th, and V/(V+Ni) ratios) suggest that the sedimentary environment is mainly oxidized, which is usually considered unfavorable for organic matter preservation, and it also forms carbonaceous shale (unit 3) in a dysoxic environment. The Mo/TOC ratios suggest that the sedimentation during Qiongzhusi Formation period was under restricted conditions. The paleoproductivity indicated by P/Ti, Ni/Al and Babio values decreased gradually from medium level to low level. In summary, relatively high paleoproductivity is a prerequisite for organic matter accumulation and less terrigenous debris supply, and relatively arid climate and reductive conditions are favorable for the preservation of organic matter.