A new discovery of Mesoproterozoic erathem oil, and oil-source correlation in the Niuyingzi area of western Liaoning Province, NE China

The Yanliao Fault-Depression Belt is an important area for Precambrian oil and gas exploration in China. Through drilling of the Niu D1 well in the Niuyingzi area, located in western Liaoning Province (NE China), oil shows were discovered in the Mesoproterozoic Gaoyuzhuang carbonate reservoirs. In this study, we discuss the geochemical characteristics of the potential source rocks and oil-bearing rocks, as well as the relationship between the crude oil in the Niu D1 well and these source rocks, on the basis of organic geochemical analyses including biomarker compounds and stable carbon isotopes. The crude oil hosted in the limestone at depths of 341.65-362.00 m in the Niu D1 well underwent moderate biodegradation and shows three characteristic features: (i) the presence of the complete C-18-C-23 13 alpha(n-alkyl)-tricyclic terpane series; (ii) high abundances of the drimane series, C-19-C-21 tricyclic terpanes, C-24 tetracyclic terpane, and diahopanes, and low abundance of gammacerane; and (iii) depleted stable carbon isotopes of saturated and aromatic hydrocarbons (less than - 30 parts per thousand). All of these features suggest that the crude oil in the limestones of the Niu D1 well originated mainly from the organic-rich Hongshuizhuang Formation shale. Considering the lack of effective Neoproterozoic source rock, the detection of diversified steranes implies a degree of contamination from Phanerozoic sources with eukaryotic biological input. This is the first time that primary crude oil of Mesoproterozoic age has been discovered from wells drilled in the Yanliao Fault-Depression Belt, and demonstrates that Mesoproterozoic carbonates in this area have good hydrocarbon exploration potential.