Biological markers and carbon isotope composition of organic matter in the Upper Cretaceous coals and carbonaceous shale succession (Jiza–Qamar Basin, Yemen): Origin, type and preservation

Abstract

Bituminous coals and carbonaceous shales, located in the Jiza–Qamar Basin, on the eastern Yemen, were analysed based on a combined investigation of organic geochemistry and petrology to define the palaeodepositional environment condition, organic matter inputs and their thermal maturity. Carbon isotope compositions and various biomarkers were performed on coals and carbonaceous shales within the Upper Cretaceous Mukalla Formation in the Jiza–Qamar Basin. The δ13C values range from −29.2‰ to −24.5‰ with average values around −26.7‰, whereas the organic carbon contents in most sediment samples are consistently high (>3%), indicating that the organic matter in the Mukalla coal and carbonaceous shale sediments has a predominantly terrigenous origin with slightly marine influence during burial. This has also been identified from kerogen microscopy, which is characterized by dominance of vitrinite and liptinite organic matter.The investigated biomarkers indicated that the Mukalla coals and carbonaceous shales contain high contribution of land plants with a minor aquatic organic matter input that was deposited in a fluvial to deltaic environments and preserved under relatively oxic conditions. This has been achieved from acyclic isoprenoids, terpane and sterane biomarkers of a saturated hydrocarbon and dibenzothiophene, alkylnaphthalene and methylphenanthrene biomarkers of an aromatic hydrocarbon.Based on the analysed biomarker thermal maturity indicators, it may be deduced that the Mukalla coals and carbonaceous shales in the Jiza–Qamar Basin have entered an early-mature to mature stages for hydrocarbon generation. This is also supported by vitrinite reflectance values of 0.63–0.83% Ro indicative that these sediments have reached oil window maturity.