Extreme organic matter enrichment in the Ediacaran Doushantuo formation: New constraints from the NE Sichuan basin, China

Abstract

The Yangtze Block is a highly prospective area for Ediacaran natural gas exploration in recent years, especially the Doushantuo Formation, a vital source rock for organic-rich hydrocarbon and a shale gas reservoir in the region. However, understanding of the Ediacaran organic-rich source rocks systematically has been limited due to the heterogeneous accumulation of organic matter and confusing paleoclimatological and paleoceanographic conditions. Such a scenario is not conducive to further exploration of the deep Ediacaran natural gas resources or understanding the evolution of early Earth. In this study, based on two new well drillings in the northeastern Sichuan Basin, the integrated sedimentological and geochemical investigations were focused on the Ediacaran middle−upper Doushantuo Formation (DST) fine-grained rocks interval. Results show this set of black organic-rich shales (total organic carbon up to 13.97%) deposited under episodic warming climate during an otherwise protracted cooling trend. These frequent climatic fluctuations intensified hydrological circulation, which together with the concurrent hydrothermal activity and the Neoproterozoic Oxidation Event, led to eutrophication of surface seawater. Under these circumstances, a substantial enhancement in primary productivity driven by algae blooming, subsequently resulted in consume excess dissolved oxygen in seawater during the degradation stage and anoxic-euxinic bottom water expansion. Consequently, the high paleoproductivity controlled the extreme organic matter enrichment during the Doushantuo time and formed the DST 4 organic-rich shales in the Chengkou and Nanhua subbasins. In contrast, although the DST 3 shales still deposited under a high productivity condition, the shallower sedimentary environments with anoxic-oxic bottom water conditions resulted in greater variation in total organic carbon contents.