Evaluation of sedimentary features and biomarkers in the Paleogene Niubao formation in the Lunpola basin, Tibetan plateau: implications for the oil source rocks and exploration

The Paleogene Niubao formation is mainly exposed in the central part of the Lunpola basin in the hinterland of the Tibetan Plateau and has high potential for oil and gas exploration and development. The biomarkers of source rocks are systematically studied by selecting representative samples. The middle and upper Niubao formation contain rhythmically inter-bedded mudstone and sandstone that show periodic variations characteristic of the lake water environment. Additionally, the large volumes of muddy dolomite in the upper Niubao formation (UNF) are associated with high salinity in the lake environment under a hot and arid climate. The organic geochemical analysis shows that the middle Niubao formation (MNF) and UNF have high organic abundance (total organic carbon (TOC) contents of 1.48-8.96% in the MNF and 1.52-12.83% in the UNF) and type I-II kerogen. The analysis shows that the UNF was deposited in a persistent, anoxic, stratified and highly saline paleo-lake environment, which was beneficial to the preservation of organic matter. The high phytoalkane and gamma-paraffin contents in the UNF indicate a strongly reducing and highly saline lacustrine depositional environment. The high pregnane contents and ascending sterane in the source rocks in the MNF indicate high maturity, and the distribution of n-alkanes and regular sterane indicates the input of some terrestrial organic matter. The high maturity indicates that the source rock in the MNF may represent the main hydrocarbon-generating layer in the basin.