Oman's undiscovered oil and gas: a petroleum system older than the Cambrian

A large share of the world's oil was made from organisms that lived in the age of the dinosaurs. Much of Oman's is far older. The source rocks under its salt basins date from the Neoproterozoic, the era that ended when the Cambrian began, and they generated most of the oil and gas found in those basins. In 2023 the U.S. Geological Survey asked how much of that oil and gas has not yet been found.

The answer, as a mean estimate: 1.2 billion barrels of oil, 6.4 trillion cubic feet of gas and 0.3 billion barrels of natural gas liquids, undiscovered and technically recoverable, across seven assessment units.

The seven areas the USGS assessed: three salt basins, the uplifts beside them and the foredeep in front of the Oman mountains.

Oil from before the Cambrian

A petroleum system needs rock rich in organic matter, heat to cook it, a reservoir to hold what comes out and a seal to keep it there. Oman has several source rocks, of very different ages:

The silicilites are the oddity. They are both source and reservoir: rock that generated oil and then held it, sealed inside the salt. Carbonate "stringers" in the same salt may add to the supply. In the three salt basins — Ghaba, Fahud and South Oman — the Neoproterozoic rocks generated most of the oil and gas. Silurian Qusaiba shales, the Jurassic marls and the Cretaceous Natih shales contribute in the northwest and in the foredeep.

How salt built the traps

The basins opened as rifts on the eastern edge of the Arabian plate between the late Neoproterozoic and the Early Cambrian. Cut off intermittently from the ocean, they filled with several kilometres of salt, carbonate and clastic rock. Then the Cambrian and Ordovician sandstones of the Haima Supergroup buried them, and two things followed from that weight: the salt began to move, forming the structures that now trap oil and gas, and the buried source rocks were heated enough to generate it.

Later events added and removed. Mountain-building in the Carboniferous stripped away most Silurian and Devonian rock. A Triassic to Early Jurassic wedge of clastic sediment laid a regional seal over much of the country. When the neo-Tethys Ocean began to close in the Late Cretaceous, the squeeze raised the Oman thrust belt and its foredeep, and folded the rock into anticlines that are among the country's major traps.

Reading the numbers

The estimate is a range, and the range is wide:

F95 means a 95 percent chance of at least that much; F5, a 5 percent chance. "Technically recoverable" means producible with current technology — not that producing it would pay. The figures are fully risked: they already allow for the chance that some areas hold nothing at all.

Sources

Schenk, C.J., Mercier, T.J., Le, P.A., Cicero, A.D., Drake, R.M., II, Gelman, S.E., Hearon, J.S., Johnson, B.G., Lagesse, J.H., Leathers-Miller, H.M., and Timm, K.K. Assessment of undiscovered conventional oil and gas resources of Oman, 2023. U.S. Geological Survey Fact Sheet 2024–3050. The input data for each assessment unit are in Schenk (2025). Written from the fact sheet, a work of the United States government in the public domain; more at the USGS Energy Resources Program.