Abstract
Oil-bearing sandstone samples were collected from the Lower Cretaceous sequence in the Kharir-2 exploration well, Kharir oilfields (Eastern Yemen). The current study integrates biomarker of the aliphatic hydrocarbon fraction of the extracted oil with a new finding from the molecular structure of the oil-asphaltene, in order to learn more about their properties, including organic matter (OM) input, depositional environment, and thermal maturity. The overall oil composition results show that the extracted oils have a high saturated hydrocarbon of up to 50% and significant levels of aromatic hydrocarbon and polar components, indicating generally paraffinic to naphthenic oil. This claim agrees with the molecular structure of the kerogen derived from the pyrolysis-gas chromatography result of the oil-asphaltene, which suggests that the extracted oils from the Lower Cretaceous sandstone reservoirs are mainly paraffinic-naphthenic-aromatic oils, exhibiting low wax content and originated from marine type II kerogen. The type II kerogen of the marine-source rock is also demonstrated by the bulk kinetic model of the oil-asphaltene for the extracted oils, with a broad range of E(a) between 38 and 62 kcal/mol and a frequency factor (A) of 1.52-1.47 × 10(13)/1 s. The biomarker characteristics of the aliphatic fraction show that the extracted oils from the Lower Cretaceous sandstone reservoirs were generated from clay-rich source rock, containing OM origin of mainly marine and terrestrial OM input and deposited under suboxic environmental conditions. Furthermore, the maturity-sensitive aliphatic biomarker parameters indicate that the extracted oils were generated from mature source rock in the range of the peak-mature stage of the oil generation window. Oil-source rock correlation of various established biomarker proxies for OM origin, depositional environment, and lithology suggests that these extracted oils were possibly generated from a single source rock, and the Madbi clay-rich formation contributed to most of the extracted oil from the Lower Cretaceous sandstone reservoir rocks.