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The Geology Of Qatar.

The prospects for finding oil and natural gas in Qatar have improved considerably in the past 20 years, thanks to the geology in its territories and because IOCs have applied the most advanced technology in exploration. Additional deposits must be found before existing reserves become more expensive to recover.

The government improved E&P terms to lure IOCs for exploration. Attractive exploration and production sharing agreements (EPSAs) have been signed for IOCs to improve oil recovery in producing fields and to embark on re-exploration in various areas with the hope of finding oil in new horizons.

One interesting area lies at the southern edge of the North Field, where Atlantic Richfield (Arco - now part of BP) found 700m barrels of oil in place and 600m barrels of probable oil reserves. The area, gas-prone, was first thought to have limited oil potentials. BP in 2002 sold its Qatar interests to Anadarko of the US.

Another interesting area is the Khara'ib geological structure, where oil is trapped in tight reservoirs. The area is within offshore Block 5 operated by the Danish firm Maersk Oil Qatar, which has proved to be the most successful among IOC operators in this emirate. Said to be linked to a section of the North Field, this is about 1,066.8 metres below the sea-bed. Maersk found the oil reserves there were far bigger than originally thought, using vertical and horizontal drilling techniques. Its al-Shaheen field is a giant, having five reservoirs.

In the past, this area was thought to have limited potential. A well drilled by Maersk has a 3,984-metre-long horizontal part with a continuous inclination exceeding 86?. Maersk said this was the longest horizontal extension drilled in any well. But the reservoirs are thin and stretched - one of the firm's main geological challenges as the field's production has been falling in the past four years (see its profile in gmt10QatrFields7Sep15).

There is oil potential off the west coast including Hawar islands, which Bahrain got in a March 2001 ruling by the International Court of Justice. Qatar used to call this "Block-3".

To the north of the territorial waters, an important part of the North Dome belongs to Iran which it calls South Pars. Shell had begun to withdraw from Qatar since the early 1970s, when it found the Dome's gas-rich structure, partly because of the Iranian dimension. But Shell has returned to Qatar in a big way, developing huge gas E&P/LNG and E&P/GTL ventures with the state-owned Qatar Petroleum (QP).

Qatar is a flat limestone peninsula of 11,437 sq km extending about 180 km northwards into the Arabian Gulf (Iran calls it Persian Gulf) from a western base. This base divides the coast-lines of Saudi Arabia and Abu Dhabi. The peninsula lies along the crest of the Qatar Arch, an almost north-south trending tectonic feature extending into the Gulf and effectively dividing it into two basins.

The Qatar Arch has had a fundamental influence on the tectonic pattern of the Gulf. It has influenced the geology of the region since Palaeozoic times. New theories about prospectivity in re-exploration have been tested by IOCs and now the results are more promising.

Qatar and its surrounding territorial waters have proved to be a prolific province of hydrocarbons. To the south lies the major oilfields of Dukhan (onshore), and the offshore structures of Maydan Mahzam, Bul Hanine, and Idd al-Shargi. To the northern offshore is the North Dome including the North Field, the world's biggest accumulation of natural gas. The Dome also has enormous reserves of liquid hydrocarbons in the form of condensate as well as oil in shallower pools.

The stratigraphy of the pre-Devonian is still not well known. But the presence of salt piercements suggests the Infra-Cambrian or Lower Palaeozoic salt of the southern Pars province and Abu Dhabi extends into Qatar.

Deep wells on Dukhan and the North Field have penetrated shales and quartzitic sandstones of the Haushi and Tawil formations (Fms), which even at depths in excess of 13,000 feet exhibited good porosity.

Gas In Khuff: Overlying the Devonian is a thick Permian/Triassic Khuff Fm, of dolomites and dolomitic limestones with some well-developed anhydrites, especially in the middle of the unit. At the top level of the Khuff, gas extends to the south into three distinct structural lobes in the Fuwairat, Ras Qirtas and Matbakh areas. At Khuff's base and at intra-Palaeozoic levels, a huge deep structure extends to Matbakh. After incorporating palynological data in the 1980s, the chronostratigraphical sequence for the Palaeozoic was established as follows:

The Lower Permian Haushi, being of Hercynian uncomformity.

The Devonian/Lower Carboniferous Tawil and the Silurian Sharawra, both being of the Caledonian uncomformity.

The Late/Middle Ordovician Tabuk.

The Khuff carbonate sequence was in accord with the regional trend of thinning from north to south. There are four main gas reservoirs in Khuff:

K-1 has a relatively modest-to-fair potential for development, with one being prominent and several minor porous zones. A gas-down-to (GDT) level was established at 8,849

K-2 has the best reservoir characteristics, with test production obtained from a 20 ft thick zone.

K-3 has a relatively poor potential.

K-4, a water-bearing reservoir, has high gas saturations in poor reservoir rock and low gas saturations in the better reservoir rock.

Above the Khuff is a standard southern Gulf succession, of which - in Qatar - the economically important units are the Uwainat, Arab C and D, and Shu'aiba limestone reservoirs for oil and liquids-rich natural gas. Some of the related oilfields are being expanded.

The Hith anhydrite is well developed and is an effective regional seal. The bituminous lime mudstones of the Hanifa, or Diyab, are the main oil sources. The Hith's efficiency as a regional seal is attested by the almost complete absence of petroleum in younger rocks. The only exception is oil's occurrence in Cretaceous Nahr Umr sandstones and Mauddud limestones in shallow pools over the North Field.

The size of the Qatar Arch and the apparent lack of salt piercement structures along its axis suggest that it is not itself a halokinetic feature and that it is probably a basement uplift. The offshore fields to the east are related to salt pillows. The nearby Halul island is a salt piercement feature.

Dukhan in the west is related to a salt wall. But the deep well on the field did not reach salt, though it penetrated the Devonian. Dukhan, a super-giant, is Qatar's main oil source. Beneath its oil Fms lies a Khuff zone with considerable non-association gas. Two Khuff gas Fms found by QP in May 1997 are in the Diyab sector, in Dukhan's southern part. The 11,500 ft-deep discovery well, DK-508, tested 14 MCF/d of gas and 2,000 b/d of condensate from the Hamlah Dolomite, and 47 MCF/d of lean gas. The pay zones were inter-sected at 8,701 ft and 10,347 ft.

With the exception of Dukhan, the potential for big oil finds onshore is limited. Dry holes were drilled on the crest of the Qatar peninsula which, on geological maps, appears as a large closed anti-cline. The offshore prospects for oil are better. BP relinquished the onshore concession in Block 2, covering the whole peninsula except for the zone around Dukhan, after making a small discovery. Amoco replaced BP and took Block 2 in February 1986, when it signed an EPSA. But Amoco relinquished it in 1992 after having drilled nine dry wells. Now this block is divided into Block-2 North and Block-2 South. Amoco was in 1999 taken over by BP.
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Publication:APS Review Gas Market Trends
Geographic Code:7QATA
Date:Sep 11, 2017
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