Oil origin and secondary migration pathway in the Bonan sag, Bohai Bay Basin, China

The oil origin and secondary migration pathways were investigated in the Bonan sag using basin modeling and petroleum geochemical data including biomarker assemblages of saturated hydrocarbons and pyrrolic nitrogen compounds. The modeling of secondary oil migration pathway was performed based on the reconstruction of burial, thermal maturity and oil generation histories. Modeling results are calibrated by the distribution of oil wells and oil fields as well as traced secondary oil migration pathways inferred from pyrrolic nitrogen compounds. Carbazole parameters were selected to trace the secondary oil migration pathway based on the investigation of oil origin. Three groups of oils sourced from source rocks of the third and fourth members of Shahejie Formation (Es3 and Es4) have been identified in this study. Group Ⅰ oils from Es3 source rocks mainly occurred in the center of sag, group Ⅱ oils from Es4 source rocks occurred in the margin regions. Group Ⅲ oils were interpreted to be the mixed sources from Es3 and Es4 source rocks. Three oil migration pathways can be traced by pyrrolic nitrogen compounds based on varied carbazole parameters. The major stages of oil generation of Es3 and Es4 source rocks occurred at about 30–20 Ma and 5-0 Ma, which coincided with oil charge periods. Secondary oil migration pathways have been predicted successfully by the oil migration modeling in the Bonan sag based on the calibration with oil wells, oil fields and the traced oil migration pathways by pyrrolic nitrogen compounds. Favorable zones located on the primary secondary oil migration pathways at the periods of oil charge may be the prospective regions for oil exploration in the Bonan sag.