Depositional environment changes and organic matter accumulation of Pliensbachian-Toarcian lacustrine shales in the Sichuan basin, SW China

Abstract

Pliensbachian-Toarcian lacustrine shales in the Sichuan basin is characterized by two organic matter-rich layers (Dongyuemiao and Da′anzhai members) interlayered with an organic matter-poor layer (Ma'anshan member). These are promising target layers for shale oil and gas in China, and they are also a terrestrial sedimentary record for the Toarcian Oceanic Anoxic Event (T-OAE). Previous studies have mainly focused on this lacustrine shale only in the single inheritance sub-sag. Here, we studied the depositional environmental changes and their correlation with organic matter accumulation in the Pliensbachian-Toarcian lacustrine shale on the subaqueous paleo-uplift in the Sichuan basin based on lithofacies and geochemical analysis. We concluded that lake stratification and oxygen deficiency were gradually strengthened upward from Pliensbachian Dongyuemiao member to late Toarcian Da′anzhai member. However, primary productivity is reduced. Chemical weathering was strongest during the early Toarcian stage (Ma'anshan member). The organic matter accumulation in these lacustrine shales is closely related to environmental changes caused by the Sichuan basin’s tectono-sedimentary evolution and depositional conditions. The Pliensbachian Dongyuemiao member is consistent with high primary productivity and low terrigenous detrital input. In the case of the deposition of early Toarcian Ma'anshan member, the organic matter was difficult to preserve due to the high sedimentation rate and the lack of anoxic water conditions. However, the late Toarcian Da'anzhai member was primarily composed of chemical precipitation that was closely correlated with the climate changes during T-OAE. The organic matter accumulation in this member was positively correlated with salinity-induced stratification and elevated CO2 concentration.