Factors controlling organic-matter accumulation in the Upper Ordovician-Lower Silurian organic-rich shale on the northeast margin of the Upper Yangtze platform: Evidence from petrographic and geochemical proxies

Abstract

Organic-rich shales are widespread in the Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formations on the northeastern margin of the Upper Yangtze platform, and these shales have been recognized to be the hydrocarbon source rocks and shale gas reservoirs with the greatest development potential in China. However, organic matter accumulation in these shales needs further research, especially in the areas which experienced an extensive upwelling and volcanism. Here, this work integrates detailed petrographic and geochemical proxies, namely redox-sensitive trace elements (RSTEs) and ratios of Si/Al & Ti/Al, from two Ordovician-Silurian sections on the northeast margin of Upper Yangtze platform, in order to understand the depositional environment and identify the major factors controlling the organic matter accumulation during the Ordovician-Silurian transition. Data of Si/Al and Ti/Al ratios indicate that the deposition of black-gray siliceous mudstones in the Lower Wufeng Formation commenced with a transgression triggered by the Boda warming event which led to a reduced detrital input. Based on RSTEs, the black carbonaceous shales of the Upper Wufeng Formation are organic-rich owing to an elevated primary productivity caused by seasonal upwelling driven by the vigorous thermohaline circulation. Abundant nutrients supplied by the post-glacial transgression and a mass biotic recovery of marine microorganisms stimulated productivity and further strengthened the depletion of oxygen in the water column, which lead to high concentrations of organic matter in Lower Longmaxi Formation. Although the water column conditions were not conducive to the accumulation of organic matter, it was possible that the increased productivity from upwelling resulted in relatively high organic matter contents in the middle Longmaxi Formation. The RSTEs (especially Cd/Mo, Co × Mn) reveal that the organic matter accumulation of the Wufeng-Longmaxi Formation during Ordovician-Silurian transition may have been primarily controlled by the preservation conditions and also influenced by the marine primary productivity and the detrital input.