Oman contains some of the most fascinating and noteworthy geological features to be found anywhere on the planet.
For exploration geologists, it also provides an opportunity to investigate untapped potential for future oil and gas discoveries.
Henk Droste, an expert on the petroleum geology of Oman, spent decades working for both Shell and Petroleum Development Oman. He said the country has “an exceptional dataset that allows the study of a wide variety of geological processes at different scales and times.”
“Due to the arid climate, the outcrops of the mountain belts in the north and east are of very high quality and laterally continuous, and provide textbook examples for sedimentological, stratigraphical and structural studies,” he noted.
Oman’s outcrop geology is a “one-to-one analogue for the oil and gas fields in the subsurface of interior Oman. Integration of the extensive subsurface dataset with the outcrop geology offers plenty of scope for new scientific discoveries and insights,” he added.
Droste wrote the chapter “Petroleum Geology of the Sultanate of Oman” for AAPG Memoir 106, “Petroleum Systems of the Tethyan Region.” He will present “Foredeep Development Along the Late Cretaceous Obduction Orogen in North Oman” at AAPG’s International Conference and Exhibition in Muscat, Sept. 30-Oct. 2.
Neil Hodgson is vice president-geoscience for Searcher Ltd. in the United Kingdom. Hodgson said, “The most fascinating geology in Oman is that in the Mid-Cretaceous, a slab of oceanic crust was obducted onto continental crust to form the mountains of western Oman.”
The Semail (or Samail) Ophiolite ranks as Oman’s most studied and most noted geological feature, made of ultra-mafic and volcanic mantle rocks that were overthrust onto the continental crust.
“This westward-verging obduction came out of the contracting basin between Iran and Oman, which has subsequently developed an eastward-verging subduction zone, and yet a working hydrocarbon system is envisaged, with structure, reservoir and source rocks identifiable on seismic – but as yet unexplored,” Hodgson observed.
Mike Searle, a specialist on Oman’s geology, is lecturer in the Department of Earth Sciences at Oxford University in England and a senior research fellow at Worcester College, Oxford. Searle wrote an article for a recent issue of the Geological Society of Oman’s magazine, detailing efforts to protect and preserve the country’s landmark geological features.
Three in particular have been suggested for UNESCO World Heritage Site status, he noted:
- The Semail Ophiolite at Wadi Jizzi
- The Jebel al Akhdar-Jebel Nakhl anticline
- The Musandam peninsula, the northern extension of the Oman Mountains ending at the Straits of Hormuz
What’s So Special About Oman?
Asked what makes Oman’s geology so compelling, Searle asked, “Where should I start?”
“Oman has by far and away the biggest and best-exposed and most-studied obducted slice of ocean crust and mantle exposed on the continent anywhere in the world. The Semail Ophiolite is a 15-kilometer-thick section through the entire crust and mantle which is almost 100-percent exposed in the desert climate,” he said.
“Oman also has the only exposures of the Mesozoic shelf carbonates that are buried elsewhere in Saudi Arabia, in the Gulf countries, etc. So Jebel Akhdar has been meticulously studied by stratigraphers and paleontologists and they can match outcrop to well sections in the desert. Half the world’s oil is locked in reservoirs that can be studied in the field in Oman,” he added.
The country has “a complete spectrum of metamorphic rocks – amphibolites and greenschist in the metamorphic sole of the ophiolite, subduction-related high-pressure eclogites and blueschist in northern Saih Hatat east of Muscat, and some rare, granulite high-temperature rocks in Bani Hamid in the UAE and northern Oman,” Searle said.
Overall, more than 50 locations in Oman have been identified as potential UNESCO GeoPark sites, as areas of international geological significance.
Located on the southeast edge of the Arabian plate, Oman is bounded to the east by a transform fault with the Indian plate. The Eurasian collision zone lies to the north, and the Gulf of Aden spreading center to the south.
A thick, buried stack of older sedimentary rocks below Oman’s interior plains holds most of the country’s known hydrocarbon reserves. Droste said the presence of multiple salt basins at the Precambrian/Cambrian boundary, which contain and overlie several Neoproterozoic source rock intervals, help make Oman an attractive province for hydrocarbon exploration.
“In most areas, hydrocarbon generation from these source rocks took place well before the reservoirs and traps where they are found today were formed. However, the kilometer-thick salt layers within these basins trapped these hydrocarbons for a long time,” he noted.
“Only during later tectonic tilting, salt movement and dissolution hydrocarbon migration continued to shallower levels. As a result, these source rocks charged reservoirs that range in age from Precambrian to Late Cretaceous and that encompass a wide range of lithologies, such as tropical carbonates, shallow marine and continental siliciclastics, and even glaciogenic deposits,” he said.
Oman’s Production History and Future
Improved seismic methods led to the discovery and development of large oil deposits in Oman beginning in the 1960s. Exploration in central and southern Oman in the 1980s then resulted in major oil discoveries in upper and lower Paleozoic clastics and Precambrian dolomites.
Today, Oman has 380 producing fields, about 4.9 billion barrels of crude and condensate reserves and 24 trillion cubic feet of natural gas reserves, according to its Ministry of Energy and Minerals. Oman is not a member of OPEC, although it cooperates with the OPEC+ alliance.
Several presentations at ICE ‘24 will examine potential new production possibilities, including “Late Neoproterozoic Alkaline Intrusives in Huqf Sediments: A New Reservoir Target in Southern Oman” and “Manal Basal Sands: A New Exploration Concept in the Oman Heartland.”
In Block 56 on the southern Oman coast, recent drilling showed the presence of intrusive rocks in Huqf Supergroup sediments, where porosities over 10 percent were recorded. Research found these intrusives contain a light to medium, Huqf-sourced oil.
Reprocessed seismic data and an encouraging well penetration led Shell to take a second look at Oman’s fluvial-alluvial Manal Basal sands. A reinterpretation of the geophysical model and deeper stratigraphy indicated on seismic have suggested additional and better target opportunities in the previous exploration area.
Also, in August, a Shell subsidiary issued contracts for a proposed blue hydrogen and ammonia project in Duqm, Oman. Blue hydrogen technology combines natural gas reforming, typically steam methane, with carbon capture and storage. Future hydrogen production will be supplied locally and for export to meet energy transition goals, the project reported.
And a new frontier for hydrocarbon exploration and production could be just offshore. Oman’s prospective offshore area has been called both vast and vastly underexplored. The energy ministry has announced near-term plans to auction several oil and gas blocks on the Arabian Sea shelf.
“What is intriguing about offshore of Oman is that it actually has two very different margins. The Indian Ocean margin is a transform margin which has potentially huge shallow gas prospects in quartz clastic reservoirs, in addition to potential AVO-supported deepwater oil plays,” Hodgson said.
“Between Oman and Iran, the Sea of Oman is very different. Compressional tectonics has dominated this margin through the Cretaceous, yet a recent gravity-driven fold and thrust belt dominates the slope,” he noted.
Hodgson said, “Cretaceous and Tertiary clastic and carbonate targets sit on ocean crust both below and in front of the thrust belt, sandwiched against the Makran accretionary prism. None of these plays have been explored to date.”