Carbazoles in Oils, and Their Application in Identifying Oil Filling Pathways in Eocene Syn-Rift Fault Blocks in the Dongpu Depression, Bohai Bay Basin, East China.

Carbazoles and dimethyl carbazoles (DMCs) are important nitrogen heterocyclic aromatic compounds in oils and sedimentary rock extracts. Based on both migration fractionation effects and differences in the thermal stability of their isomers, carbazoles can be used to trace oil migration orientations and filling pathways. Molecular biomarker compositions indicate that all oils and oil-bearing sand extracts in the Eocene fault-blocked reservoirs of the Huzhuangji area (Western Slope Belt) of the Dongpu Depression (Bohai Bay Basin, East China) belong to a single oil population. In this study, four geochemical indicators relating to carbazoles, namely (a) 1,8-/2,7-dimethyl carbazoles (1,8-/2,7-DMC); (b) 1,8-/2,5-dimethyl carbazoles (1,8-/2,5-DMC); (c) 1,8-/N-exposed dimethyl carbazoles (1,8-/N-exposed DMC); and (d) G1 N-shielded %, were applied to trace oil migration orientations and filling pathways. The results show that these parameter values gradually increase toward the Hu-5 fault block at the structural high. The measured values from the subsurface are consistent with the results calculated from the molecular adsorption modeling. Therefore, it is concluded that the relative parameters of dimethyl carbazoles are practical molecular indicators for tracing oil migration orientations and filling pathways in syn-rift fault-blocked reservoirs.