The mechanism of organic matter accumulation in the archipelago marine sediments: Insights from the Middle Devonian Givetian mudstone with low TOC in the Youjiang Basin, South China

Despite the fact that the mean values of total organic carbon (TOC) content in the Luofu Formation are relatively low, at or below 1.0 wt%, it still has the potential to be a veritable source rock. To elucidate the influences of various paleogeographic factors on sedimentary environments, 25 samples from Well A on the slope and 32 samples from Well B in the basin centre were selected for study. The samples were analyzed by thin section microscopy and field emission-scanning electron microscopy, along with X-ray diffraction and geochemical analyses (TOC content, sulfur content, organic petrography, inorganic carbon isotope and major and trace elements) in order to understand variations in and controls on the mineralogy, lithofacies, depositional environments, as well as the organic matter accumulation mechanism. The results of our study indicate that mudstone of the Luofu Formation was deposited into 4 lithofacies groups, namely as the siliceous lithofacies, mixed lithofacies, argillaceous lithofacies and calcareous lithofacies. The siliceous lithofacies mudstone was developed in the deep-water columns, while the mixed lithofacies mudstone was deposited on shallow-water columns. The organic matter accumulation model of the Luofu Formation can be further divided into the following four stages, each with its own unique characteristics: (1) The organic matter accumulation was caused by the relatively high productivity and dysoxic basin bottom water columns; (2) The organo-mineral complexes may promote the organic matter accumulation to some extent; (3) The organic matter enrichment was facilitated by the dysoxic/ anoxic conditions and relatively high productivity from the basin to the slope; (4) The organic matter was preserved at shortened exposure duration of oxic conditions on the slope. The correlations between various geochemical indicators (redox, paleoproductivity and terrestrial debris flux) and TOC content suggest that oxic water columns and low productivity are the main reasons for the widespread mudstone with low TOC. The organic-rich mudstone, as the target stratigraphic section for exploration, may be deposited in the basin bottom water columns away from upwelling and close to hydrothermal activity in stage 3.