Organic geochemistry of Late Cretaceous Mukalla shales from the Mukalla-Sayhut Basin, Southern Yemen—terrestrial organic matter inputs and gas-resource potential

The Late Cretaceous Mukalla Formation is preserved in the Mukalla-Sayhut Basin of Southern Yemen. Using a combination of organic geochemical and petrological methods, we investigated shale intervals in the Mukalla Formation using the Masilah-1 well. We focused on identifying the organic facies and their hydrocarbon generating potential, which has relevance in terms of attracting future hydrocarbon exploration in this basin. The geochemical analysis was conducted on bulk cutting shale samples and indicated fair–good source rock potential, with moderate–high total organic carbon (TOC) contents of 0.51–3.49 wt%. Rock-Eval hydrogen index (HI) pyrolysis results suggested that the organic matter in the analyzed Mukalla shales was mainly type III kerogen; thus, it is likely to have shale-gas generation potential. The high proportions of type III kerogen are consistent with the distributions of lipid biomarkers and visual kerogen typing. Visual kerogen results showed that the analyzed Mukalla shales mainly contained vitrinite organic matter derived from land plants. The biomarker fingerprints of normal alkane, isoprenoid, sterane, and terpane in the saturated hydrocarbon fraction further suggested high contributions of terrestrial organic matter that deposited in fluvial to fluvial-deltaic environments under oxic conditions. The thermal maturation level was inferred from both chemical and optical maturity indicators and indicated that the shale samples, which were buried at relatively shallow depths in the Masilah-1 well, were of low maturity, i.e., within early-mature oil window.