Geochemical evidences linking the Lower Cretaceous source rocks to the biodegraded bitumen seeps and tar sands in the Gulf of Guinea Basin

Abstract

The source of biodegraded bitumen seeps and tar sands in the Nigerian axis of the Gulf of Guinea province has been a contentious issue for decades. The most prevalent opinion is that the biodegraded bitumen seeps are migration oils that were degraded by bacterial. In the present study, outcrop source rocks (Lower Cretaceous), core samples (Upper Cretaceous) from a shallow well-x, and bitumen seeps and tar sands from the eastern axis of the Dahomey Basin of Nigeria were analyzed. Rock-Eval™ pyrolysis show that the outcrop rocks are classified as having excellent-very good source quality; in contrast, those from well-x are classified as having only poor-fair quality. The cross plot of hydrogen index (HI) against Tmax indicates that samples from the outcrops contain type II/III organic matter, whereas well samples contain type III organic matter and all source rocks are thermally immature, which is supported by vitrinite reflectance (Ro: 0.20–0.48 %), and biomarker maturity parameters (C32 (S/S + R), MPI-I). In the bitumen seeps and tar sands, isoprenoids, n-alkanes, steranes and hopanes have been degraded, and tricyclic and tetracyclic terpanes are still mostly intact. In addition, C30 hopane and homohopanes have been demethylated, and C25 norhopanes are present in abundance, suggesting that the bitumen seeps and tar sands experienced heavy biodegradation in the range of PM-6 or greater. The distribution of 4,9-DMD, 4,8-DMD and 3,4-DMD diamondoids and the δ13C of saturate and aromatic fractions indicate that the bitumen seeps and tar sands were generated from Types II and III source rocks. The ratio of demethylated Ts/Tm indicates that the samples are low maturity and were generated from immature source rocks. Cross plots of tricyclic terpane ratios (C25/C26TT vs C25TT/C24TeT, C23/C21TT vs (C19+20)/C23TT, and C24TeT/C24TeT + C26TT vs (C19+20)/C23TT) also reveal that the biodegraded bitumen seeps and tar sands are closely related to source rocks in outcrop groups 1 and 2. This conclusion is supported by the ternary plots of tricyclic terpanes, 4,9DMD, 4,8DMD and 3,4DMD diamondoids, and the cross plot of EAI against DMDI-1. The occurrence of exsudatinite in the outcrop source rocks in group 1, consistent with early hydrocarbon generation, and the low ratio of demethylated Ts/Tm (<1) in the bitumen samples confirms that the biodegraded bitumen seeps and tar sands in the Nigerian Dahomey Basin were generated by the Lower Cretaceous source rocks in group 1. The study advances on the previous propositions on the source of bitumen seeps and tar sands and contributes our knowledge in the Gulf of Guinea Basin.