Early Oligocene marine transgression and organic matter accumulation recorded in the upper Eocene to lower Oligocene Enping Formation of the Baiyun Sag, Pearl River Mouth Basin, South China Sea

Abstract

The upper Eocene to lower Oligocene Enping Formation provides the primary source rocks for hydrocarbon accumulation in the Baiyun Sag, Pearl River Mouth Basin, South China Sea. Nevertheless, the depositional environment of this formation has been a subject of debate. This study confirms that the lower to middle members of the Enping Formation predominantly consist of lacustrine deposits, and a marine transgression occurred during the deposition of the Enping Formation’s upper member in the Early Oligocene period. This transgression is supported by an increase in paleosalinity proxies (B/Ga, Sr/Ba), greater negative S isotopic values of pyrite, and an increase in triaromatic dinosteroid concentrations in the upper member samples. A series of indicators (chemical alteration index, V/Cr and Ni/Co ratios) suggest that mudstones from the lower–middle member and the upper member were all deposited under warm-humid climate with the dysoxic-oxic water conditions. The mudstones from the lower–middle member and upper member were formed with low-to-moderate primary productivity under relatively strong dilution condition. The paleoredox and paleoproductivity were minimally influenced by marine transgression. Additionally, there is a weak correlation between the total organic carbon content and indicators of these depositional conditions. The input of terrigenous organic matter is a pivotal factor in regulating the accumulation of organic material within the Enping Formation. The multi-stage deltas likely played a significant role in supplying terrigenous organic matter, thereby influencing the distribution of organic-rich mudstone in the Baiyun Sag.