Influence of depositional environment on organic matter accumulation in lacustrine shales: the Jurassic Dongyuemiao Member, Sichuan Basin, China

Changing depositional environments easily influence organic matter accumulation in continental lacustrine basins. This paper examines the shales from the Jurassic Dongyuemiao Member in the Sichuan Basin. Based on the petrological and geochemical analyses of the shale samples from coring wells XT1, ZX1 and YT1, the lacustrine depositional environment during the Dongyuemiao sedimentary period is reconstructed. The primary controlling factors of organic matter accumulation are described, and an organic matter accumulation model is established. The results indicate that the kerogen type in the shale is predominately of type II₂, and the organic matter is primarily derived from terrigenous higher plants and phytoplankton. The paleoenvironmental indicators show that the Sichuan Basin spanned freshwater to brackish conditions with a semi-dry to humid climate. Under the influence of paleoclimate, the lake maintained an overall moderately productive and oxic bottom water environment, with moderate-high terrigenous influx. Until the late Dongyuemiao period, the Sinemurian–Pliensbachian boundary event led to a semi-dry climate in the Sichuan Basin. This resulted in decreased nutrient availability and a decline in phytoplankton. The productivity of the lacustrine basin ranged from low to medium. Organic matter was preserved in the shallow and brackish bottom water. This research, along with previous studies, highlights the need to pay special attention to the impact of paleoclimate and geological events on various processes of lacustrine organic matter accumulation.