Controls on deposition of aquatic and terrestrial organic matter in the lacustrine Namling–Oiyug basin (Oligocene–Miocene, southern Tibet)

Abstract

The Namling–Oiyug basin is the only Cenozoic lake basin that has hydrocarbon potential in southern Tibet, China. Coal was once exploited in the basin from the early Miocene Mangxiang Formation, which also hosts oil-prone source rock layers. Herein, a combined investigation of sedimentological features and organic geochemistry is performed on a measured outcrop section of the Mangxiang Formation to study the lake-basin types as well as the depositional history of aquatic and terrestrial organic matter (OM).There are two periods in the lake basin evolution history: the balanced-fill and overfilled stages. In the balanced-fill basin stage, oil-prone laminated shale was deposited in a relatively deep lake environment, with total organic carbon (TOC) contents around 5wt.%, and HI values up to 316mg HC/g TOC, indicating hydrogen-poor Type II kerogen with transition to Type III kerogen. Comparison between maceral composition and geochemical properties suggests that the relative low HI values are results of thermal maturity. The high value of the gammacerane index indicates water stratification of the lake. In this stage, water and sediment input approximately equaled potential accommodation, the lake was relatively stable and hosted large amounts of lacustrine algae, and accumulated OM with low terrestrial input in an anoxic environment. After the onset of the thrusting, the potential accommodation was decreased, large volumes of clastic sediments also drained into the lake from the surrounding area, resulting in an overfilled lake basin. During this stage, peat was deposited in freshwater environment, with >30wt.% TOC, low HI, and Type III OM. The high Pr/Ph (>3) implies oxic conditions.