Geochemical characterization of source rocks and crude oils in the Upper Cretaceous Qingshankou Formation, Changling Sag, southern Songliao Basin

Abstract

The Upper Cretaceous Qingshankou Formation is one of the main exploration target in the Changling Sag, southern Songliao Basin. However, the origin of source rock deposition and crude oils remains undiscovered. A comprehensive geochemical study, including Rock–Eval pyrolysis, gas chromatography-mass spectrometry and carbon isotope analysis on fifteen crude oils and twenty-two source rock samples from the Qingshankou Formation were performed in order to provide new insights into source organic matter, depositional environment and correlation between crude oils and source rocks.Bulk geochemical analysis on source rock samples indicates that the lower member of the Qingshankou Formation has more hydrocarbon generating potential than the middle member. These two potential source rock members were interpreted to be deposited in slightly different environments and have different sources of organic matter determined by biomarker assemblages. The lower member is organically rich (average TOC = 1.27 wt.%), oil-prone, mature and deposited under anoxic conditions with more contribution from sapropelic organic matter. The middle member is organically poor (average TOC = 0.78 wt.%), gas-prone, less mature and deposited under less anoxic conditions with more terrigenous organic matter input. Biomarker fingerprints and carbon isotope compositions suggest that all the crude oils were genetically related and could be classified into one oil family, which has significant contribution from the lower member source rocks and minor contribution from the middle member source rocks.