ORIGIN OF CRUDE OILS IN OMAN

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Abstract
The petroleum geochemistry of Oman provides a picture of considerable variey, 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. This paper reviews the geological history history of the area and places the petroleum geochemistry within the geological context.The oils can be geochemically classified into five groups. Three groups can be related to god oil source rocks found in the pre‐Cambrian Huqf Group, the Silurian Safiq and the Cretaceous Natin Formation. Another group of oils probably originates from the Upper Jurassic Diyab Formations, while the fifth group of crudes (named ‘Q’) cannot be correlated to a known source rock, but is inferred to have originated from an unsampled Huqf level.The “Huqf oils” are those that have been correlated to known Infra‐Cambrian Huaf source rocks, and are characterized by a strong C29 sterane predonominance and very light carbon isotope values of around‐36.0%. In contrast, the ‘Q’ crudes, drived from the unknown source are characterized by a C27 strerane predominance and carbon istope ratios of around ‐30.5%. Both the Huaf and ‘Q’ crudes also contain a series of characteristic compounds referred to as the ‘X’ compounds (all isomers of methyle and dimethyl alkanes). Oils reasoned to originate from Silurian Safiq source rocks have a week C29 sterane precominance, a significant content of rearranged steranes and carbon isotope ratios of ‐30.5%. The oils thought to originate from 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 Naith Formation source rocks are characteirzed by steranes with an equivalent distribution of C27, C28 and C29 isomers, and carbon isotope values of around ‐26.9%.These variations in biomarker distributions and carbon isotope values are sufficiently distinct to ensure a high degree of certainty in the grouping of the crude oils.