Multiple-sourced features of marine oils in the Tarim Basin, NW China – Geochemical evidence from occluded hydrocarbons inside asphaltenes

The Tarim Basin, NW China, is a large composite basin with multiple sets of petroleum source rocks. The basin has undergone numerous episodes of hydrocarbon generation, migration and accumulation, making it difficult to assess the source rocks and oil–source correlation for the widespread marine oils in this area.Protected by the molecular structure of asphaltenes, occluded hydrocarbons can provide information about the early source rocks. In this work, the occluded hydrocarbons released from the asphaltenes by a mild chemical degradation method were compared with the crude oil maltenes and the adsorbed compounds from asphaltenes. Analysis of biomarker distribution and the carbon isotope composition of individual n-alkanes suggests that the widespread marine oils in the Tazhong Uplift, Tabei Lunnan Uplift and Halahatang Depression were contributed by Cambrian–Lower Ordovician source rocks at an early stage, and later mixed with hydrocarbons derived from Middle-Upper Ordovician source rocks. The marine oils in the Tarim Basin demonstrate extensive characteristics of having been derived from multiple source rocks.

► Adsorbed/occluded fractions inside asphaltenes were obtained.
► Occluded compounds inside asphaltenes suggested an earlier charging of oils.
► Different fractions from crude oils verified the mixing features of oil reservoirs.
► Both ϵ-O1 and O2+3 source rocks contributed to the mixed oils in the Tarim Basin.