Research paperRecognition of a novel Precambrian petroleum system based on isotopic and biomarker evidence in Yangtze platform, South China

•Carbon isotopic and biomarker compositions are distinct from Phanerozoic oils.•A novel Precambrian petroleum system is recognized based on geochemical data.•The petroleum was derived from a carbonate source rock horizon.

The Precambrian Doushantuo Formation is regarded as a possible petroleum source rock horizon with TOC values of 0.85-1.85% and type I kerogen in northwestern Sichuan Basin, Yangtze platform, South China, while showing high-to post-mature thermal maturity. A set of outcropping bitumen and oil sands from various reservoirs display overall depleted values of δ13C from −36 to −32‰ (average −35‰), typical of a Precambrian source. In addition, the sterane distributions in these bitumens and oils are unusual compared to marine Phanerozoic oils with a predominance of C29 members, as well as their triaromatic counterparts. High amounts of 20-n-alkylpregnane with very low diasteranes indicate restricted, sulfur-rich conditions, typical of an anoxic carbonate source rock. This interpretation is supported by some other specific biomarkers such as abundant 30-norhopane and C24-tetracyclicterpane, and elevated C35 hopane. Furthermore, the presence of gammacerane indicates a saline, stratified water column. Hydrocarbons extracted from the solid bitumen and oil sands have many compositional characteristics in common with other oils derived from Precambrian carbonate source rocks from elsewhere. This implies the existence of a Precambrian marine petroleum system in the Yangtze platform, with the Doushantuo carbonate as the likely source. It is supposed that the reduction of temperature and pressure following basin uplift resulted in petroleum fractionation (deasphalting) in Lower Paleozoic reservoirs with the fractionated oils having migrated along the faults becoming trapped in stratigraphically younger reservoirs, where the petroleum underwent heavy biodegradation.