Organic Matter Enrichment Mechanisms in the Lower Cambrian Shale: A Case Study from Xiangandi #1 Well

Abstract

In order to investigate the effect of primary productivity, organic matter dilution, and preservation on the accumulation of organic matter, geochemical data, and proxies of primary productivity, clastic influx, and redox conditions were obtained for organic-rich shales in the Cambrian Niutitang Formation. The primary productivity (total organic carbon [TOC], Mo, P, Ba, and Babio) and redox (Ni/Co, V/Cr, U/Al, and Th/U) proxies suggest the organic-rich shales were deposited in anoxic-euxinic conditions during periods of high primary productivity. Pyrite in the Niutitang Formation comprises spherical framboids, which also indicate that anoxic bottom waters were present during organic matter deposition. High primary productivity enhanced the organic C flux into the thermocline layer and bottom waters, which lead to the development of anoxic bottom waters owing to O2 consumption by microorganisms and organic matter degradation. The anoxic bottom waters were beneficial for the preservation of organic matter. In addition, Ti/Al ratios correlate well with TOC contents throughout the Niutitang Formation, indicating that clastic input increased the burial rate and prevented organic matter degradation during deposition. Therefore, the accumulation of organic matter in the Niutitang Formation was controlled mainly by primary productivity rather than bottom-water redox conditions.