Catalytic hydropyrolysis of asphaltenes in marine oil from the Tarim Basin, NW China: Implications to complicated oil charging histories in an old composite basin

Abstract Oil asphaltenes have been used to characterize the source(s) and timing of expulsion of early charged oils. However, the application on asphaltene in a complex petroleum basin have been limited. Marine oils from the Tarim Basin, NW China, were selected to release asphaltene-bound molecules by the catalytic hydropyrolysis technique (Hypy). The analyzed samples were 10 heavy oils from two major oil-production regions (the Tabei and Tazhong uplifts) in the basin. The asphaltenes may be mixed products from multiple hydrocarbon charging stages, if we consider the main oil filling events and the regional geothermal histories of corresponding reservoirs. However, the similar molecular and isotopic characteristics of the hydropyrolyzates indicate contributions from one major source rock (possibly lower Cambrian mudstone) during relatively early periods, based on the presently known source characteristics. The preservation of asphaltenes probably reflects a low reservoir temperature after charging. The asphaltenes were derived from marine algae deposited in a strongly reducing environment, given the biomarker data (e.g., relatively abundant aryl isoprenoids and low Pr/Ph) and carbon isotopic data (−35.5‰ to −34.4‰). Asphaltenes were expelled around the oil generation peak. Moreover, molecular ratios indicative of maturity (e.g., tricyclic terpanes/hopanes), derived by Hypy of asphaltenes, suggest that the asphaltenes of the Tazhong Uplift are generally more mature than those of the Tabei Uplift. Moreover, this is mainly related to different maturities when the oil was expelled. The free hydrocarbons in the Tazhong Uplift show greater molecular and isotopic variations than those in the Tabei Uplift. This might be attributed to more complex accumulation processes in the former. Late oils with a range of maturities, mixing proportions with early biodegraded oils, and degrees of alteration by thermochemical sulfate reduction could all have played a role in generating this variability. The results indicate that the hydrocarbons released from oil asphaltenes by Hypy can provide insights into multiple charging processes in an old composite petroleum basin.