Petrographic and geochemical characteristics of the Middle Devonian Eifelian Nabiao Formation in the Youjiang Basin, South China: Implications for paleoenvironment and organic matter accumulation mechanism

The Nabiao Formation of the Middle Devonian Eifelian period has been identified as a promising site for shale gas exploration in the Youjiang Basin, South China. And yet, little is known about the accumulation mechanism of organic matter under transgression in the archipelago paleogeography. In this study, the core samples were analyzed by thin section microscopy, field emission-scanning electron microscopy, pyrite morphology, X-ray diffraction, and geochemical analysis (total organic carbon (TOC) content, sulfur content, organic petrography, inorganic carbon isotope and major and trace elements) to understand changes in and controls on the mineralogy, lithofacies, depositional environments, as well as the organic matter accumulation mechanism of the Nabiao Formation. The study indicates that organic-rich sediments were deposited during the two transgressions of the Chotec Event and the Kacak Event. The deposition of carbonaceous mudstone occurred in water columns with weak upwelling, while siliceous mudstone and siliceous shale were developed in a strong upwelling water column. The correlations between multiple geochemical proxies (paleoredox, paleoproductivity, and terrestrial detrital influx) and TOC content indicate that the Chotec Event is more conducive to organic matter enrichment of the Nabiao Formation than the Kacak Event. In stage 1, TOC content is low due to oxic water columns and low productivity. In stage 2, the Chotec Event causes the relative sea-level of Member 2 to rise rapidly and the basin bottom water to quickly change to dysoxic conditions. This is the key to organic matter enrichment. In stage 3, the oceanic upwelling is crucial in organic matter enrichment by increasing paleoproductivity. Under the protection of dysoxic condition, organic matter is accumulated in stage 4. Organic matter of the Nabiao Formation is not significantly affected by the terrigenous detrital flux. By taking into account the changes in paleo-productivity, redox conditions, upwelling events, and terrigenous detrital flux, an evolution model is proposed in this paper to elucidate the paleoenvironmental change and the organic matter enrichment under transgression in the archipelago paleogeography.