Origin of Crude Oils in Oman

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.

Origin of petroleum in the Neoproterozoic–Cambrian South Oman Salt Basin

“…Two major oil types, the 'Huqf' and the 'Q' oils, were recognized over 20 years ago and are clearly distinguished on the basis of carbon isotopic composition and sterane carbon number distributions (Grantham, 1986;Grantham et al, 1987). The Huqf oils have abnormally low carbon isotope ratio values [typically À36‰ VPDB (Vienna Pee Dee Belemnite)] and a strong predominance of C 29 steranes, in contrast to the Q oils characterized by carbon isotope values near À30‰ and C 27 -dominated steranes.…”

“…Both oil types show conspicuous mid-chain monomethyl alkanes, trivially named ''X compounds", considered characteristic of Precambrian-Cambrian source rocks (Fowler and Douglas, 1987;Summons et al, 1988a,b;Bazhenova and Arefiev, 1997). As the organic matter (OM) within the Huqf source rocks displays similar characteristics to the Huqf oils, a straightforward correlation has been established between these oils and the Huqf Supergroup (Grantham, 1986;Grantham et al, 1987;Terken et al, 2001). On the other hand, the origin of the Q oils, clustered to the north of the SOSB and in the basins to the north, is a matter of speculation as the source rock remains unproven.…”

“…On the other hand, the origin of the Q oils, clustered to the north of the SOSB and in the basins to the north, is a matter of speculation as the source rock remains unproven. A Precambrian source for the latter was proposed on the basis of the mid-chain monomethyl alkane presence and of their spatial and stratigraphic distributions (Grantham et al, 1987).…”

Effect of Permo-Carboniferous Climate on Illite-Smectite, Haushi Group, Sultanate of Oman

“…Input of organic material during hydrocarbon migration in the subsurface, a second source for organic acid anions, is similar for both formations (Grantham et al, 1987).…”

Molecular biogeochemistry of modern and ancient marine microbes

“…In contrast to the scarcity of suitable organic-rich rocks in the Paleoproterozoic and Mesoproterozoic successions, the Neoproterozoic is replete with well-characterized organic matter in low maturity sections from Australia, North America, Groan (e.g., Grantham et al 1987) and eastern Siberia (Summons and Powell 1992;see Summons and Walter 1990;, for reviews). Of particular note are the oldest commercial petroleum accumulations in Siberia and Oman.…”