LAUSR

Abstract Petroleum bulk properties such as API gravity are important parameters in petroleum system analysis. Three sags (the Bayindulan, Saihantala and Wuliyasitai Sags) in the Erlian rift basin of northern China provide interesting insights into the correlations between organofacies, maturity and oil fluid properties. The API gravity of oils in the three sags shows obvious differences, ranging from 17.0 to 30.9° for Bayindulan oils, 25.7–40.5° for Saihantala oils, and 32.8–41.9° for Wuliyasitai oils. Oil geochemical parameters indicate that the differences in oil fluid properties among the three sags can be attributed to the variations in organofacies and thermal maturity, consistent with the analysis of geochemistry and maceral composition of the source rocks in the three sags. Bayindulan oils were expelled from organofacies C with an extremely high initial hydrogen index (HIo) of 600–900 mg/g TOC in the early mature stage (