Organic geochemical characteristics of oils from the offshore Jiza-Qamar Basin, Eastern Yemen: New insight on coal/coaly shale source rocks

Abstract

Coals, coaly shales and oil show samples from Late Cretaceous sections in the offshore Jiza-Qamar Basin (Yemen) were analysed using organic geochemical analyses to investigate the source of organic matter input and the genetic link between oils and potential source rocks in the basin. The coals and coaly shales have high TOC contents between 6.90 and 77.62wt% and Hydrogen Index (HI) values are largely above 300mg HC/g TOC, indicating that the Late Cretaceous coaly shales and coals as expected are organically rich, and predominantly contain Type III-II kerogens.The oil shows are classified as paraffinic–naphthenic–aromatic (P–N–A) oil with high wax contents, suggesting that the oils were generated from coal and coaly source rocks that were deposited in a deltaic–coastal environment. This is supported by bulk geochemical characteristics and biomarker compositions. A variety of biomarkers n-alkanes, regular isoprenoids, terpanes and steranes indicate that the oil shows were derived predominantly from source rock containing higher plant derived organic matter and were deposited in fluvial to deltaic environments under relatively oxic conditions. The biomarker characteristics and stable isotope compositions of the oils are similar to those of the Late Cretaceous coal and coaly shale rocks. The new data presented in this paper suggests genetic link between oil shows and coal/coaly shale rocks in the offshore Jiza-Qamar Basin and provides new evidence for the oil generative potential of the Cretaceous coals and coaly shales in the basin.