A distinct oil group in the Dongying Depression, Bohai Bay Basin, China: New insights from norcholestane and triaromatic steroid analyses.

• A distinct oil group was discovered in the Dongying Depression. • Related source rock of the distinct oil group has abundant bacteria/algae inputs. • Norcholestanes and triaromatic steroids are effectively used in oil classification. • This oil group has a higher abundance of nordiacholestanes than norcholestanes. • This oil group contains extremely high abundances of 4-methyltriaromatic steroids. The lower third and upper fourth members of the Eocene Shahejie Formation (lower Es3 and upper Es4) are known as two sets of effective source rocks in the Dongying Depression, Bohai Bay Basin. In this study, the saturated and aromatic steroids of 20 crude oil samples from the Dongying Depression were systematically investigated using gas chromatography–mass spectrometry (GC–MS) and GC–MS metastable reaction monitoring. Based on the distribution patterns of the steranes, norcholestanes, and triaromatic steroids, the Group I and II oils were clearly identified and shown to be derived from the lower Es3 and upper Es4 source rocks, respectively. Significantly, it was discovered that three oil samples collected from the Eocene Kongdian Formation and the Ordovician reservoirs exhibit completely different steroid and isotopic compositions from those of the Group I and II oils. These oils, as a distinct oil group (Group III), have the characteristics of abnormally high concentrations of C 29 steranes, 24-nordiacholestane, 27-nordiacholestane, C 28 triaromatic steroids, C 27 4α-methyltriaromatic steroids, and C 29 4α-methyl-24-ethyltriaromatic steroids, and low concentrations of 24-norcholestane, 27-norcholestane and triaromatic dinosteroids, and 13C depletion. Although the related source rock of Group III oils needs to be further clarified in the study area, the 13C depletion, abundant C 29 steranes, and low 24-norcholestane content may suggest greater bacteria/algae inputs (e.g., green algae rather than dinoflagellates) than both the lower Es3 and upper Es4 source rocks. Identification of the Group III oils indicates that the norcholestanes and triaromatic steroids may be more effective molecular fossils for the classification of crude oils in complicated petroleum exploration fields. [ABSTRACT FROM AUTHOR]