Geochemical characteristics and organic matter enrichment mechanism of Permian black mudstone in the South Yellow Sea Basin, China

The middle Permian organic matter-rich mudstone in the South Yellow Sea Basin has been considered the most important source rock for Paleozoic reservoirs in the basin. However, information on the variation in depositional environment within the Qixia Formation to lower Longtan Formation interval is lacking, and the formation mechanism of these organic matter-rich mudstones is unclear. Based on a series of geochemical analyses of borehole CSDP-2, the paleoclimatic conditions, nature of the watermass, paleoproductivity and terrestrial detrital influx of the different intervals in the middle Permian of the South Yellow Sea Basin were assessed to determine the main factors controlling organic matter enrichment. All three intervals (from the Qixia Formation to the lower Longtan Formation) were deposited under a relatively warm and humid paleoclimate with moderate terrestrial detrital input. The Qixia Formation was deposited under oxic paleoredox conditions in a normal marine environment. During deposition of the Gufeng Formation, the paleoredox conditions changed to suboxic with intermittent anoxia associated with paleoproductivity higher than that in the other studied intervals. The lower Longtan Formation was characterized by a brackish watermass, suboxic paleoredox conditions and low paleoproductivity. The covariant relationships among paleoproductivity, paleoredox conditions, watermass salinity indicators and TOC content indicate that paleoproductivity was the critical factor controlling organic matter abundance. High primary productivity not only provided organic matter but also enhanced organic matter preservation by consuming oxygen in the watermass, which resulted in the formation of the organic matter-rich source rock in the Gufeng Formation.