SAUDI ARABIA - The Geology.
The main concentration of oilfields lies in the north-eastern part of the kingdom and in the offshore of the northern Gulf. The areas there are dominated by a series of generally north-south trending oil fairways. The most prominent of these lie along the super-giant Ghawar structure, which consists of several oil reservoirs, by far the largest in the world.
Oil in place in the Ghawar set of fields is said to exceed 300 billion 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 billion barrels. These make Saudi Arabia by far the biggest oil reservoir in the world, with Saudi Aramco being among the world's most conservative companies in releasing reserves figures.
Ghawar, discovered in 1948, is more than 250 km long. The Ghawar trend splays out as it passes offshore. And there is not a clearly defined lineation in the offshore areas. 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. Within Saudi Arabia, the southern end of this trend includes Khurais, Qird, Abu Jifan and Farhah, which may be part of a super-giant accumulation.
It is tempting to extrapolate the more recent discoveries south-east of Riyadh, in the centre of the kingdom, into yet another northerly trending belt of fields representing the westernmost oil occurrences in the country.
The bulk of the kingdom's currently produced oil reserves occur in the limestones of the Jurassic Arab A, B, C and D units, with substantial amounts in older Jurassic limestone members, e.g. the Dhruma and Hanifa. The latter unit is also believed to be the major source rock for most of the Jurassic oil in the region and it may have sourced some of the younger reservoirs where the Hith anhydrite is absent or breached by faulting.
In the offshore fields, the main reservoirs 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 several of the large Jurassic oilfields has proved up reserves of gas sometimes with condensate in Permian Khuff limestones and in pre-Khuff sandstones. Deposits of free gas have been pursued since industries and utilities in Saudi Arabia, which depend heavily on gas for electric power generation or industrial feedstock, were deprived of supplies during the 1980s when oil production was cut back and associated gas production fell. In the more recent years, non-associated natural gas in Khuff and pre-Khuff reservoirs has been developed to feed the kingdom's Master Gas System (MGS), which is the biggest of its kind in the world.
Rich Paleozoic reservoirs have been discovered in central Arabia south of Riyadh since June 1989. They contain sweet oil as well as sweet natural gas and condensates. Some of the oil is sulphur-free and can be used directly as a motor fuel. The Paleozoic units are older than the Khuff, mainly in a Permo-Carboniferous formation called Unaiza (or Unayzah). This has opened up yet another play, with Saudi Aramco geologists saying the potential for large pre-Khuff light oil reserves may exist beneath the known reservoirs. The new theory has aroused interest among geologists elsewhere in the Arabian Peninsula and in nearby regions.
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 formations, mostly of the Paleozoic age, beneath existing fields in Oman encouraged Aramco executives to let geologists investigate the Paleozoic in several parts of the kingdom.
Initial attempts made south and south-east of Riyadh during 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 negative events like the fall in oil prices, a drastic cut in Aramco exploration and staff, etc.
It was only in late 1986, after Aramco received very 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 mothballing.
In effect the entire petroleum sector was to be restructured, with emphasis on the upstream. Eventually restructuring made Aramco's upstream experts more prominent. Aramco was Saudised as a company wholly owned by the state and became known as Saudi Aramco. Well equipped and authorised to explore areas outside the former Aramco concessions, in early 1989 the company 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 discovered before end-1990. The first field was Al Hawtah, announced on June 7, 1989. The second was Dilam, discovered on Oct. 11, 1989. Ragheeb was discovered on Jan. 1, 1990. Naeem 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 proved to have an unprecedented success ratio in oil well drilling anywhere in the world. It also 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 about 9,000 feet. The thickness of sedimentary sections is said to be more than three times that of Paleozoic layers found in Oman, to the south, and tend to increase northward.
The oil zone at Dilam, one of the northernmost wildcats, is more than 140 feet thick; and the gas zone that contains 65 deg. API condensate as well as sweet gas is said to be 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 the discoveries, the Khuzami-1 well spudded in April 1997, later tested 1,550 b/d of sweet oil and 1 MCF/day of gas, 120 km south of Riyadh and 30 km north of Al Hawtah. The earlier and later discoveries have all been developed and the fields now are producing over 200,000 b/d of Arab Super Light 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 westernmost part of the land area is occupied by outcrops of the Precambrian Arabian Shield to the West, of which lies a narrow continental shelf in the Red Sea.
The Shield is non-prospective for hydrocarbons. But the Red Sea shelf is yet to be properly explored and is believed to be highly prospective. One gas and condensate field was discovered, Barqan, in Miocene sands near the southern end of the Red Sea. Another major field discovered in the north of the Red Sea, Midyan, is believed to contain large reserves of natural gas and condensates as well as oil. Only a limited number of wells have been drilled south of Barqan on the Red Sea shelf.
Midyan was first discovered in November 1992 on the Red Sea coast about 150 km west of Tabuk and 550 km north of Yanbu'. The reservoir is a 129-metre thick Tertiary Miocene limestone at a depth of 2,212-2,343 metres. The discovery well tested 45 MCF/day of sweet gas and 1,300 b/d of 55 deg. API condensates. A second limestone reservoir of the same age was found later and tested 55 MCF/day of sweet gas and 1,900 b/d of 58 deg. API condensates. A third well, Midyan-3, confirmed an important oil reservoir underneath the gas structure at a depth of 2,351-2,377 metres.
In March 1993 Saudi Aramco tested 2,300 b/d of 39 deg. API oil with 0.8% sulphur and 19 MCF/day of gas. Further drilling in the area led to small discoveries in similar formations.
Barqan and Midyan on the Red Sea are to be explored and developed by a consortium headed by ExxonMobil, which signed the initial agreement on this with the Saudi government early June, 2001. This is one of three core projects resulting from the gas opening.
East of the Arabian Shield outcrops, the sedimentary section thickens towards the Gulf and into Rub' Al Khali. To the north-east, strata dip and thicken towards the Iraqi border but the northern and north-eastern areas are relatively unexplored. 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 nine years.
In the Rub' Al Khali basin, several shallow stratigraphic wells have been drilled. But with wells spaced at 100-200 km apart, the region must be considered to be virtually unexplored. South of the border with Abu Dhabi several fields have been discovered 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 Shuaiba formation.
The origin of these fields is believed to be similar to that of the big fields in Abu Dhabi, where large swells developed over salt pillows possibly triggered by deeper basement faulting. With the exception of Shaybah which is now on stream, none of the Saudi fields there has been developed. Discovered reserves in place at Shaybah have been estimated at 7 billion barrels of light/sweet oil. The one large gas discovery there has a high sulphur content. Shaybah lies on the border and extends into Abu Dhabi. Under the 1973 border accord, it is said, the two states agreed that any field found on the shared frontier would belong to the country in which its bigger part is located. Riyadh then said that, since the Abu Dhabi part is smaller, Shaybah's entire reservoir belongs to Saudi Arabia. But when the field was officially inaugurated in March 1999, a meeting there of GCC oil ministers was boycotted by their UAE counterpart - in an apparent sign that ownership of Shaybah is yet to be settled.
In the south-west, small oil discoveries were made in the past nine years in the Al Wajh region. The area did not prove to be of importance, and Aramco moved its exploration team and equipment back to the Midyan zone.
At the southern end of the Red Sea, discoveries were made around Jizan. One discovery flowed at 4,000 b/d in 1992. Saudi Aramco said 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 a depth of 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.
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|Publication:||APS Review Oil Market Trends|
|Date:||Oct 15, 2001|
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