OMAN - The Geology.
For hydrocarbons, Oman's geology is different from that of Saudi Arabia and the UAE. Omani fields are small, deep and more complex, with many having low permeability. About 2,000 producing oil wells in Oman average about 400 b/d, compared to 3,700 b/d from 1,800 wells in Saudi Arabia.
With varied terrain, the interior is dominated by a mountain range called Al Hajr (rock). The largest mountain is Jabal Al Akhdar which rises to more than 2,000 metres. At the north-eastern end, the Musandam peninsula juts into the Straits of Hormuz and is similar to the barren fjords of Norway, with a difference in temperature. In the south-east, the fertile Batina plain inclines towards Muscat and then on to reach the edge of the Rub' Al Khali. From there one travels through mountains to the monsoon-fed plain of Salalah in the south which is near tropical.
The main petroliferous areas of Oman include (a) the Arabian Basin, linked to that of the UAE and Saudi Arabia; (b) the Oman Salt Basin in the centre and south; and (c) a narrow strip of offshore territory facing the western coast of the Musandam Peninsula. But it is in onshore areas that prospectivity has encouraged foreign developers.
More than half of Oman's proven oil reserves of over 5 bn barrels are located in the north of the country, mainly in the fields of Yibal, Natih, Fahud, Al Huwaisah, Lekhwair and Shibkah. These form a single geological structure with proven oil reserves in excess of 2 bn barrels. There are large reserves of very heavy oil in southern Oman, along the Marmul-Jalmud trend. The region's heavy oil in place is said to be 60 bn barrels.
Within onshore Oman there are four oil fairways. The most northerly lies close to the borders with Abu Dhabi and Saudi Arabia, in the extension of the Arabian Basin. The three main fields there are Lekhwair, Safah and Daleel. Their oil and gas reservoirs are of the Thamama group similar to that of eastern Abu Dhabi. Field sizes are small, compared to the great Thamama field of Abu Dhabi. A full-scale water flooding programme, which PDO launched in 1990, should have increased recoverable oil reserves at Lekhwair to 400m barrels (see Part 2).
South of the Lekhwair-Daleel trend, three productive areas in central and southern Oman are each associated with a sub-division of the main Oman Infracambrian Salt Basin. In the north of these areas are fields centred on Fahud-Natih-Shibkah (Yibal), with oil and gas mostly in Cretaceous limestone reservoirs. Field sizes range from 500m to 1 bn barrels, but are still small by Middle East standards.
In central Oman, a second group of fields centred on Saih Nihaydah is also associated with a salt basin, but there the hydrocarbons are reservoired in both Upper Paleozoic sandstones and Mesozoic limestones. Field sizes for oil are small. But since late 1989 drilling at Saih Nihayadah and nearby fields has led to the discovery of a deeper gas/condensate pool with very large reserves. In east-central Oman, the Barik and Saih Rawl fields have fairly large reserves of oil and gas in Aptian Shuaiba limestones which are dolomitic, and in Permo-Carboniferous sandstones of the Khlata and Qharif groups. Saih Rawl has a deep gas pool in Cambro-Ordovician sandstones of the Andam group. Total, a partner in PDO, came to this area in mid-1995 (see Gas Market Trends).
In southern Oman a fourth petroleum province lies along the Marmul-Nimr-Jalmud trend. Fields there are associated with the Southern Oman Salt Basin. The oil is very heavy, mostly reservoired in Lower Paleozoic clastic deposits. Marmul oil, found in 1956, was considered uneconomical because of its quality (22[degree sign] API with a very high sulphur content). Some fields in the southern trend also have natural gas.
Due to the lack of infrastructure in this area, however, companies are not interested in gas bearing blocks on offer. The reservoirs are unusual in that many of them are periglacial sandstones often sealed by glacial tillites. This is one of the few areas in the world where deposits of such type host hydrocarbon reserves in commercial quantities.
PDO found oil at Al Noor-2 in Jan. 1994 in a rare Athel formation in the south. The well, 40 km west of Nimr field, produced 100 CM/d (625 b/d) of 47[degree sign] API oil and 25,000 CM/d of gas. A PDO executive said the find was "in a unique rock from which oil and gas have never before been produced in Oman". The Athel formation had been penetrated many times in the course of drilling in the south but had "never proven to be oil- and gas-productive before". The well encountered formations which "allow very high reservoir pressure to exist". Tests on the rock showed an average microporosity of 22%, almost as good as some of Oman's most prolific fields.
The find was hooked up by pipeline to the Birba field, 65 km from the Al Noor-2 well. The executive said Al Noor-2 "may provide significant new oil and gas reserves, and open up new exploration avenues". Athel, about 4,500 metres deep, now is believed to contain more than 3 bn barrels of oil in place. This complex structure required cutting-edge technology. Shell, the technical leader in PDO, had to overcome such challenges as seismic interpretation, reservoir characterisation and well productivity.
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|Publication:||APS Review Oil Market Trends|
|Date:||Jan 31, 2000|
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