Preliminary study of crude oils from Shabwah depression in the Central Sab'atayn Basin (Yemen): Revealing oil-oil correlation with organic matter input and maturity based on geochemical properties of maltene and asphaltene fractions

Abstract

This study was investigated eleven crude oil samples from representing Jurassic reservoir sandstones from the Al–Uqlah Oilfield in the Shabwah depression, using physical and geochemical aspects for saturated, aromatic, and asphaltene fractions to gain information about their source rock characteristics, particularly the conditions of depositional environment, thermal maturity and organic matter (OM) character. The overall results show that the examined oils have API gravity values between 38° and 43°, indicating that most crude oils are light and not biodegradable; nevertheless, oil samples, sweet oils with a low sulfur content, with S of <1 wt%. The molecular structure of the asphaltene in these oils derived from Py–GC suggests mainly paraffinic oil exhibiting high to low wax content derived from Types I and II kerogens. This claim agrees with the high saturated HC of more than 38% and significant levels of aromatic HC and polar components. The studied oil were generated from clay-rich source rocks, containing a mixed OM origin of marine and terrestrial organic matter and deposited under sub-oxic depositional environmental conditions as indicated by their biomarker of the saturated and aromatic fractions. The Py–GC structure of the asphaltene together with the environment-related biomarker indicators reveals the existence of two oil families. The first oil family represented in the majority of the studied oils and characterised as waxy paraffinic oil and generated from a mixture of organic matter, with significant terrestrial land plant input and deposited in sub-oxic environmental conditions. Other oil samples belonged the second oil family; these samples are predominantly P–N-A oils with low wax content and were derived from marine-source rock, containing significant contributions from aquatic OM and little input from terrestrial OM and deposited under more reducing environmental conditions. The biomarker and non-biomarker maturity ratios indicate that thermally mature source rocks generated the investigated crude oils of both oil families, corresponding to the peak oil window.