n-Alkane bound biomarker fingerprints from asphaltenes in the bitumens of Eastern Dahomey Basin, Nigeria: Source and genetic implications

The asphaltene fractions of the bitumens of Eastern Dahomey Basin in Nigeria, were analyzed by flash pyrolysis-gas chromatography (Py-GC) method in order to unravel its geochemical history and properties. The distributions of the initial biomarkers of the original oils from the pyrolysates are related to the assessment of organic matter source, paleo-redox conditions and source environment during deposition. Also, it effectively establishes the genetic relationship of the bitumens. The n-alkane distributions in the pyrolysates reveal nC${}_9$-nC${}_{32}$ n-alkanes, maximizing at nC${}_{14}$, isoprenoids-pristane (Pr) and phytane (Ph), and some n-alkene peaks. High peaks of low to medium-weight nC${}_9$-nC${}_{20}$ n-alkanes and low peaks of nC${}_{21}$+ n-alkanes characterize the distributions. These reveal that abundant algal organic matter with some terrigenous inputs contributed to the source rock of the bitumens. The high concentration of marine organic matter inputs to the source rock is further confirmed by the nC${}_{17}$/nC${}_{27}$ ratios which range from 5.39 to 19.82 and shows the predominance of nC${}_{17}$ alkanes. The general unimodal n-alkane distributions in the bitumens indicate derivation from similar organic matter types showing that they are genetically related. ... mehrThe anoxic to suboxic environmental conditions that prevailed during the deposition of the sediments is revealed by the isoprenoids, Pr/Ph ratios (0.72–1.28). Pristane/nC${}_{17}$ and Phytane/nC${}_{18}$ range from 0.16 to 0.33 and 0.22 to 0.56, revealing that the bitumens were from predominantly marine organic matter (type II kerogen) preserved in a reducing environment with no evidence of biodegradation. However, the Ph/nC18 ratio and the cross plot of Pr+Ph/nC17+nC18 allow the classification of the bitumens into two subfamilies/groups (A and B). The bitumen samples have low wax content as indicated by the degree of waxiness ranging from 0.21 to 0.38 which confirms low terrigenous input. Based on the carbon preference index (CPI: 0.92 to 1.55) and odd-even predominance (OEP: 0.70 to 1.36), it is concluded that the bitumens are immature to marginally mature.