Productivity or preservation? The factors controlling the organic matter accumulation in the late Katian through Hirnantian Wufeng organic-rich shale, South China

Abstract

The Wufeng organic-rich shale is widespread in the Upper Yangtze platform, and it is one of the most advantageous shale gas producers in China. Its deposition commenced with the onset of the Boda warming event in the late Katian and terminated at the Hirnantian glacial maximum. The depositional mechanism of the Wufeng Formation was poorly understood owing to its relatively thin thickness, being interpreted as the result of organic matter (OM) accumulation in a shelf setting under anoxic conditions. This study integrated detailed geochemical analyses based on the core samples from three boreholes which are located in the outer-, middle- and inner-shelf of the Upper Yangtze platform, in order to investigate the depositional environments during the deposition of the Wufeng Formation. Geochemical redox proxies (Mo-EF, U-EF, and V/Cr) and productivity proxies (P, Baxs, Sixs) indicate that the deposition of the Wufeng Formation was associated with four intervals (WF1 to WF4), each characterized by its unique geochemical features related to sea-level fluctuations. Black siliceous mudstones deposited during WF1 and WF2 are relatively organic-lean due to low productivity, less favorable preservation conditions, and terrestrial dilution. Whereas black siliceous mudstone deposited during WF3 and WF4 is organic-rich due to high paleoproductivity and conducive preservation conditions. The geochemical proxies (Cd/Mo, Co x Mn) associated with the modern upwelling settings were replicated in the Wufeng Formation, revealing that the outer-shelf of the Upper Yangtze platform was dominated by persistent upwelling. The upwelling-driven primary productivity is considered as the first-order control for the OM enrichment at the outer-shelf of the Upper Yangtze platform. Seasonal upwelling may influence the middle-shelf of the Upper Yangtze Platform, where both primary productivity and preservation played essential roles in the OM accumulation. In contrast, the OM accumulation at the inner-shelf was fundamentally driven by good preservation conditions.