Constraints on the Accumulation of Organic Matter in the Upper Permian Dalong Formation from the Lower Yangtze Region, South China

Abstract

AbstractThe Late Permian was marked by a series of important geological events and widespread organic‐rich black shale deposition, acting as important unconventional hydrocarbon source rocks. However, the mechanism of its organic matter (OM) enrichment is still controversial. Based on geochemical data, we studied the marine redox conditions, paleogeographic and hydrographic environment, primary productivity, volcanism, terrigenous input during the Late Permian in the Lower Yangtze region, and provided new insights into the OM accumulation. Five Phases were distinguished based on the TOC and environmental variations. In Phase I, anoxic conditions driven by water restriction enhanced the OM preservation. In Phase II, euxinic and hydrological cycling environments were the two most substantial controlling factors for the massive OM deposition. During Phase III, intensified terrestrial input potentially diluted the OM in sediment and presence of oxygen in bottom water weakened the preservation condition. The Phase IV was characterized by relatively higher abundance of mercury (Hg) and TOC (peak at 16.98 wt%), indicating that enhanced volcanism potentially stimulated higher productivity and euxinic environment. In Phase V, extremely lean OM was preserved as a result of terrestrial dilutions and the decreasing primary productivity. Eventually, Phase I, Phase II and Phase IV were characterized as the most prominent OM‐rich zones due to the effective interactions of the controlling factors, namely paleogeographic, hydrographic environment, volcanism, and redox conditions.