Geochemistry of crude oils in Oman

Abstract

Abstract The petroleum geochemistry of Oman provides a picture of considerable variety since crude oils and their source rocks are found both throughout the country and throughout the stratigraphic column from the Infra-Cambrian to the Tertiary. The oils can be geochemically classified into five groups. Three of them can be related to good oil source rocks found in the Precambrian Huqf Group, the Silurian Safiq and the Cretaceous Natih Formations. Another group of oils probably originates from the Upper Jurassic Diyab Formation whilst the fifth group of crudes (name ‘Q’) cannot be correlated to a known source rock but is inferred to have originated from an unpenetrated Huqf level. The ‘Huqf oils’ are those that have been correlated to known Infra-Cambrian Iuqf source rocks and are characterized by a strong C 29 sterane predominance and very light carbon isotope values (δ C 13 ) of around −36.0‰. In contrast, the ‘Q’ crudes, derived from the unknown source are characterized by a C 27 sterane predominance and carbon isotope ratios of around −30.5‰. Both the Huqf and ‘Q’ crudes also contain a series of characteristic compounds referred to as the ‘X’ compounds (all isomers of methyl and dimethyl alkanes). Oils thought to originate from Silurian Safiq source rocks have a weak C 29 sterane predominance, a significant content of rearranged steranes and carbon isotope ratios of approx. −30.5‰. The oils correlatable to the Jurassic Diyab Formation have a similar sterane distribution but heavier carbon isotope values of around −26.5‰. Finally, the crude oils from the mid-Cretaceous Natih Formation source rocks are characterized by steranes with an equivalent distribution of C 27 , C 28 and C 29 isomers and carbon istope values of around −26.9‰.