The Geology Of Saudi Arabia
APS Review Gas Market Trends, 28.09.2009
Saudi Arabia's sedimentary terrain is of exceptional quality. Huge oil reserves are contained in just nine field groups, where recovery costs are the lowest in the world.
There are over 100 oil and gas fields in Saudi Arabia. The main concentration of oilfields lies in the north-eastern part of the kingdom and in the offshore in the north of the Persian Gulf. The areas there are dominated by a series of generally north-south trending oil fairways. The most prominent of them lie along the super-giant Ghawar structure, which consists of several oilfields, the largest in the world.
Oil in place in the Ghawar fields is said to exceed 300 bn barrels. Oil in place in the other fields, including the offshore Safaniya and many different structures in the kingdom, is said to total a further 200-700 bn barrels. These make Saudi Arabia the biggest oil reservoir in the world. The state-owned Saudi Aramco is among the world's most conservative companies in releasing reserves figures.
Ghawar, found in 1948, is more than 250 km long. It splays out as it passes offshore. There is not a clearly defined lineation in its offshore part. A similar, parallel, north-south trend lies to the west of Ghawar and may extend from Mazalij in the south to as far as Burgan (Kuwait) in the north. In Saudi Arabia, the southern end of this trend includes Khurais, Qird, Abu Jifan and Farhah, part of a super-giant structure.
Saudi Arabia is rich in other minerals. They include the world's largest deposit of tantalum at Ghurrayeh in the north-east. Tests in 2002 confirmed 94,000 tons of tantalum pentoxide, worth $17-19bn, plus deposits of niobium, yttrium and rare-earth oxides - worth $17-18bn. (Tentalum, dense metal, is used in electronics. Its price has risen sharply over the last decade due to high demand for PCs and cell-phones).
There are major deposits of bauxite, the main ore of aluminium found in a Precambrian Arabian Shield formation (Fm) in the north. This is the largest known bauxite reserve in the Middle East. Alone the deposit at al-Zabirah contains 126m tons of ore, grading 57.5% alumina. Abdullah Dabbagh, CEO of the state-owned Saudi Arabian Mining Co. (Ma'aden), in late 2002 said: "We have a vision of becoming the largest aluminium-producing region in the world". A set of multi-billion dollar projects for this and other minerals will include a mining railway system. Companies involved in all this include a JV of Saudi Oger and Stroytransgaz of Russia. Saudi Oger is controlled by the family of the late Rafiq Hariri, a Lebanese-born Saudi billionaire close to the Saudi royal family and a former PM of Lebanon assassinated on Feb. 14, 2005, in Beirut. Stroytransgaz is the engineering and construction arm of the Kremlin-controlled Gazprom, which will be involved in gas E&P in Saudi Arabia under one of the deals signed during a Moscow visit on Sept. 2-4, 2003 by then CP Abdullah, now the king (see omt17SaudiwhoOct26-09).
The oil and gas/condensate fields and discoveries south-east of Riyadh, in the centre of the kingdom, are another northerly trending belt of fields representing the western-most petroleum occurrences.
The bulk of the kingdom's produced oil reserves occur in limestones of the Jurassic Arab A, B, C and D units, with substantial amounts in older Jurassic limestone Fms, e.g. the Dhruma and Hanifa. The latter unit is the major source rock for most of the Jurassic oil in the region and it may have sourced some of the younger Fms where the Hith anhydrite is absent or breached by faulting.
In the offshore fields, the main petroleum Fms are Cretaceous in age and comprise both sandstones and carbonates. In view of the strong development of the Hith or Gotnia anhydrites in this area, it is likely that the oil is derived from Cretaceous sources which have reached maturity in more deeply buried areas to the north-east.
Deep drilling on large Jurassic oilfields has proved up reserves of gas sometimes with condensate in Permian Khuff limestones and pre-Khuff sandstones. Free gas has been pursued since industries and utilities in Saudi Arabia, which depend heavily on gas, were deprived of supplies during the 1980s when oil output was cut and associated gas production fell. Non-associated gas in Khuff and pre-Khuff Fms has been developed to feed the Master Gas System, which is the biggest of its kind in the world.
Rich Paleozoic reservoirs have been found in central Arabia south of Riyadh since June 1989. They contain sweet oil, sweet gas and condensates. Some of the oil is sulphur-free and can be used directly as motor fuel. The Paleozoic units are older than the Khuff, mainly in a Permo-Carboniferous Fm called Unaiza. This has opened up another play, with Saudi Aramco geologists saying the potential for large pre-Khuff light oil reserves may exist beneath the known Fms. The theory has aroused interest among geologists in nearby countries.
Geologists at Saudi Aramco had begun testing new petroleum sourcing theories, including the Paleozoic potentials, since the early 1980s when several interesting developments occurred in Oman and Yemen. The discovery of deep Fms, mostly of the Paleozoic age, beneath existing fields in Oman encouraged Saudi Aramco to let geologists investigate the Paleozoic in several parts of the kingdom.
Initial attempts made south and south-east of Riyadh in the early 1980s had a negative outcome and the central region was declared barren. Geologists' persistent argument about Paleozoic potentials were not only disregarded but such theories were overtaken by a fall in oil prices, a drastic cut in Saudi Aramco exploration and staff, etc. It was only in late 1986, after Saudi Aramco received precise, high-tech equipment, that another survey of unexplored areas was encouraged. By then several fields' production capacity had dropped as a result of extensive moth-balling.
The petroleum sector was restructured, with emphasis on the upstream. This made Saudi Aramco's E&P experts more prominent. Aramco was Saudised as a NOC owned by the state and became known as Saudi Aramco. Well equipped and authorised to explore anywhere, in early 1989 it embarked on a programme for the central region to test those new sourcing theories once again. Within about 12 months, the results were impressive enough to become the subject of keen investigation by world geologists.
Four major fields were found before end-1990. The first was al-Hawtah, announced on June 7, 1989. Dilam was found on Oct. 11, 1989. Ragheeb was found on Jan. 1, 1990. Na'eem was announced on April 11, 1990. These and other discoveries made later in the central region were also referred to as Najd fields.
From then on, the programme had an unprecedented success ratio in oil well drilling anywhere in the world. It confirmed the geologists' theory that central Saudi Arabia was the most accessible region to investigate the Paleozoic in the Middle East, with depths ranging from 7,900 to 9,000 feet. The thickness of sedimentary sections is over three times that of Paleozoic layers in Oman and tend to increase northwards.
The oil zone at Dilam, one of the northern-most wildcats, is more than 140 ft thick; and the gas zone containing 65? API condensate as well as sweet gas is much thicker. A further study of these horizons has had major implications on theories related to hydrocarbon sourcing, migration, etc.
A number of equally interesting discoveries - all of sweet oil and gas - have been made since then. One of them, the Khuzami-1 well spudded in April 1997, later tested 1,550 b/d of sweet oil and 1 MCF/d of gas, 120 km south of Riyadh and 30 km north of al-Hawtah. The earlier and later finds have all been developed and the fields now are producing over 200,000 b/d of Arab Super Light crude oil.
Of the kingdom's known sedimentary terrain, less than 60,000 sq km lie under the adjacent continental shelf, where water depths are less than 200 metres. The western-most part of the land area is occupied by outcrops of the Arabian Shield to the north and west, of which lies a narrow continental shelf in the Red Sea.
The Shield is non-prospective for petroleum but rich in other minerals. The Red Sea shelf, now being explored, is believed to be highly prospective. The gas/condensate field Barqan was found in Miocene sands near the southern end of the Red Sea. Another major field found in the north of the Red Sea, Midyan, has large reserves of gas and condensates as well as oil. Still a limited number of wells have been drilled south of Barqan on the Red Sea shelf.
Midyan was first found in November 1992 on the Red Sea coast about 150 km west of Tabuk and 550 km north of Yanbu'. The Fm is a 129-metre thick Tertiary Miocene limestone at a depth of 2,212-2,343 metres. The discovery well tested 45 MCF/d of sweet gas and 1,300 b/d of 55? API condensates. A second limestone Fm of the same age later tested 55 MCF/d of sweet gas and 1,900 b/d of 58? API condensates. Midyan-3, confirmed an important oil Fm underneath the gas structure at 2,351-2,377 metres. In March 1993 Saudi Aramco tested 2,300 b/d of 39? API oil with 0.8% sulphur and 19 MCF/d of gas. Further drilling in the area led to small discoveries in similar Fms.
Barqan and Midyan on the Red Sea are to be re-explored and developed by foreign companies. They were to be taken by a consortium headed by ExxonMobil, which signed the initial agreement on this with the government in early June 2001. This was one of three core JVs resulting from the first gas opening. But the Exxon-led group abandoned the deal in 2003 (see background in Vol. 61, Gas Market Trends 13).
In the east of the Arabian Shield outcrops, the sedimentary section thickens towards the Gulf and into Rub' al-Khali (RaK). To the north-east, strata dip and thicken towards the Iraqi border but the northern and north-eastern areas are still being explored. One small oil discovery has been made at Hamza in Jordan some 20 km from the Saudi border. Deeper on the Saudi side of that frontier, however, important reservoirs were discovered by Saudi Aramco in the past 17 years.
In the RaK basin, several shallow stratigraphic wells have been drilled. But with wells spaced at 100-200 km apart, the region and its great depths remain virtually unexplored. There may be huge gas deposits deep beneath the RaK basin, said to he the source of Qatar's North Field which extends to Iran's South Pars - by far the largest accumulations of natural gas in the world.
South of the border with Abu Dhabi, several fields have been found in tectonic and stratigraphic settings similar to those of Abu Dhabi. Reservoir ages range from Jurassic Tuwaiq to Cretaceous Mishrif with the major accumulation at Shaybah occurring in reefal facies of the Shu'aiba Fm. Their origin similar to that of the big fields in Abu Dhabi, where big swells developed over salt triggered by deeper basement faulting.
With the exception of Shaybah, none of the Saudi fields there has been developed. Reserves in place at Shaybah have been estimated at 7 bn barrels of light/sweet oil. The one large gas find there has a high sulphur content. Shaybah extends into Abu Dhabi. Under a 1973 border accord, the two states agreed that any field found on the shared frontier would belong to the country in which its bigger part was located.
Since the Abu Dhabi part was smaller, Riyadh said, Shaybah's entire reservoir belonged to Saudi Arabia. But when the field was inaugurated in March 1999, a meeting there of GCC oil ministers was boycotted by their UAE counterpart - in a sign that ownership of Shaybah was yet to be settled. Tension over this remains.
In the south-west, small oil finds were made in the past 17 years in the al-Wajh region. The area did not prove to be of importance, and Saudi Aramco moved its exploration team back to the Midyan zone. In the south of the Red Sea, finds were made around Jizan. One flowed at 4,000 b/d in 1992. In late 1992 the oil flow was difficult to test because the casing partially collapsed due to unstable salt dunes. One of the wells drilled to 2,730 metres was dry. The rig was in mid-1993 moved to the north for drilling in the Qunfidah region. The Jeddah area is believed to be prospective.
