Organic geochemical characteristics of Paleocene-Eocene source rocks in the Avengco Basin, Tibet

The Tibetan Plateau is located in the eastern part of Tethyan realm, which contains over tow-thirds of the global petroleum reserves. Crude oil or high-quality source rocks have been found in a series of Cenozoic continental... [ view full abstract ]

The Tibetan Plateau is located in the eastern part of Tethyan realm, which contains over tow-thirds of the global petroleum reserves. Crude oil or high-quality source rocks have been found in a series of Cenozoic continental basins where were located in central Tibet. These basins developed along the Bangong-Nujiang suture (BNS) zone, which is located in central Tibet and separates the Qiangtang and Lhasa terrances, and its length in an east-west direction exceeds 1500 km. Avengco basin is located in the western part of the BNS. It is similar to the Nima and Lunpola basins in the eastern part in terms of sedimentary characteristics and tectonic settings, which are well known to provide a good hydrocarbon potential. The Paleocene-Eocene Niubao Formation, which is divided into the upper member and lower member, dominates the sedimentary succession and they consist primarily of conglomerate, pebby sandstones, medium-fine sandstones, siltstones, mudstones and marls. Depositional environments were mainly fan delta and shallow-deep lake. The organic geochemical characteristics of the Paleogene strata in the Avengco basin are still unclear and more comprehensive studies are strongly needed. A total of thirty-four marl and mudstone outcrop samples of the Niubao Formations were collected and subjected to the following analyses: total organic carbon (TOC), Rock-Eval pyrolysis, stable carbon isotopes of kerogen, gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). Here, we present the results indicating the organic matter of the upper Niubao Formation is mainly composed of Type II kerogen with a mixed source, which is dominated by algae. However, the lower Niubao Formation has the less oil-prone Type II-III kerogen, and the sources of the organic matter are mainly terrestrial plants with less plankton. In addition, the samples are thermally immature to marginally mature. The Niubao Formation was deposited in an anoxic-oxic environment, which was brackish with an imperceptible stratified water column. The upper Niubao Formation has a medium to good hydrocarbon-generating potential. However, the lower Niubao Formation has a zero to poor hydrocarbon-generating potential.