Organic geochemical characteristics and depositional environments of the Upper Cretaceous coals in the Jiza-Qamar Basin of eastern Yemen

Abstract

This study is the first attempt which provides information regarding the organic geochemical, biomarker and petrographic characteristics of the Upper Cretaceous coals found in the in the Jiza-Qamar Basin, eastern Yemen. The geochemical and petrographic results helped us to evaluate the type of organic matter, thermal maturity, and petroleum-generation potential, as well as depositional environments of the coals. Maceral analysis shows that the coals are dominated by vitrinite, with significant amounts of liptinite, and low amounts of inertinite macerals. Liptinite present in the samples are oil-prone liptinite macerals include petroleum-like materials (exsudatinite). The Upper Cretaceous coals are high volatile B–A bituminous in rank, possessing vitrinite reflectance in the range of 0.62–0.87%Ro. This rank determination is supported by high fixed carbon and relatively low volatile matter contents, with an arithmetic mean of 54.8wt.% and 41.9wt.%, respectively. Upper Cretaceous coals with moderate to high oil-prone liptinite content have good liquid petroleum-generation potential. These coals have relatively high hydrogen index values in the range of 286–449mg HC/g TOC, consistent with Type II and mixed Type II–III kerogens.Gas chromatograms present in the coal samples are dominated by odd carbon numbered n-alkanes (n-C23 to n-C35), indicating terrestrial organic matter input. The biomarker parameters obtained from mass spectrometer data on m/z 191 and m/z 217 indicate that these coals were deposited in a fluvial to deltaic environments and preserved under relatively oxic conditions.The Tmax, mean vitrinite reflectance and biomarker maturity data show that Upper Cretaceous coals fall into the early-mature to peak oil window.