Finding Ghawar

Elephant Hid in Desert

The history of oil in the Middle East is essentially a story of giant oil fields (each containing over 500 million barrels).

Of the 932 known giants that researchers at the University of Texas in Austin have mapped, over 200 are located in the Persian Gulf region, which accounts for about 2 percent of Earth’s land area – and the “Elephant of All Elephants” (AAPG EXPLORER, January 2005) is Saudi Arabia’s Ghawar field.

Discovered in 1948, Ghawar started to produce oil in 1951 – 60 years ago – but to this day it remains the world’s richest and most productive oil field.

Today the oil industry in Saudi Arabia is in full control of a state-run company, Saudi Aramco. The company’s origin, however – as well as the history of oil discovery in the kingdom – dates back to the Arabian American Oil Company (Aramco).

The first oil field discovered in Saudi Arabia was not Ghawar but Dammam, in March 1938, followed by Abu Hadriyah (March 1940) and Abqaiq (December 1940). These fields are located in the Eastern Al Hasa region where pioneering mapping began by American geologists in the early 1930s.

Max Steineke
Max Steineke

The legendary Max Steineke and Tom Kock, both AAPG members with California Arabian Standard Oil Co. (Socal) in 1935 first spotted topographic indications of the En Nala anticline on which Ghawar sits.

In 1940, Ernie Berg, a young Aramco geologist who was mapping areas near the Abqaiq field, noted that the Wadi Sahaba, a seasonal river valley in the Haradh area, took a sudden bend from its east-west direction toward the south.

Berg related this wadi diversion to a north-south trending subsurface anticline. Steineke, his boss, supported the idea.

Shallow structural-stratigraphic drillings in the desert (a technique Steineke had developed to map the subsurface geology and collect pre-Neogene information) confirmed the existence of what came to be called the En Nala (“the Slippers”) anticline.

After World War II, exploration resumed on the Arabian Peninsula, and the information from shallow drillings were supplemented with gravity and magnetic surveys. The En Nala anticline was thus better imaged.

This“whale back” structure, about 280 kilometers long and 30 kilometers wide on average, contains six major structural culminations; from north to south, they are Fazran, Ain Dar, Shedgum, Uthmaniyah, Haradh and Hawiyah.

In 1948 Aramco drilled a test well at Ain Dar, which hit oil in June, marking the first post-war discovery in the kingdom. In a letter dated July 6, 1948, S.V. Campbell, an Aramco manager, reported the good news to the Saudi finance minister Shaikh Abdullah Sulaiman:

“In a 20-minute test at 6,685 to 6,746 feet, gas in the Ain Dar well rose to the surface in six minutes and oil rose to the surface in 11 minutes.”

More wildcats on the anticline, Haradh No. 1 (February 1949), Uthmaniyah No. 1 (April 1951), Shedgum No. 1 (August 1952) and Hawiyah No. 1 (1953) all discovered light crude (32-36° API) from the Upper Jurassic carbonate reservoir (the 100-meter Arab-D member) at depths of 2,000-2,330 meters.

By 1953, the geologists recognized that all these prospects were actually parts of a single field; they named it Ghawar, after the wide pastoral landscape that local Bedouins called Al Ghawar.

The northernmost part of the supergiant, Fazran, was discovered in 1957. By then 129 wells were producing 0.6 million barrels of oil per day. Except for Uthmaniyah No. 1, all the other discovery wells at Ghawar still produce oil.

The first detailed report about Ghawar by the Aramco geologists was presented at the AAPG convention in Los Angeles in March 1958 and published in AAPG BULLETIN in February 1959 – a report that, amazingly, remains the cornerstone of our knowledge of this field.

In the 1970s, deeper drillings in the Ghawar field discovered enormous volumes of natural gas from the Permian carbonates of the Khuf Formation.

Public knowledge about the world’s largest field ironically remains scanty. This has led to speculative debates about Ghawar’s peak production and near-future demise – for example, Matthew Simmon’s 2006 book Twilight in the Desert.

AAPG member Abdul Kader M. Al-Afifi, a senior geologist with Saudi Aramco, reported in his 2004 AAPG Distinguished Lecture that Ghawar produced 4.6-5.2 MMbopd from 1993 through 2003.

The Ghawar anticline is draped over a basement horst, which started to develop during the Late Carboniferous extensional uplift in the region, which, in turn, eroded the middle Paleozoic sediments at Ghawar. Bounding normal faults have throws as much as 3,000 feet at the Silurian level, but they die out in the Triassic section, according to Afifi, who is past president of the AAPG Middle East Region (AAPG EXPLORER, January 2005).

The Cenozoic anticline at Ghawar, called the En Nala structure, is asymmetrical with a steeper western flank, and a minor component of right lateral strike slip.

The oil at Ghawar was sourced from Jurassic organic-rich marlstone.

The producing oil reservoir is the late Jurassic Arab-D limestone, which is about 280 feet thick and occurs 6,000-7,000 feet beneath the surface. Afifi also noted that reactivation of the structure during Arab-D deposition localized grain-dominated shoals in the north, upgrading the quality of the reservoir, which improves upward as it progresses from lime mudstone to skeletal oolitic grainstone.

Fracture density increases going deeper in the section, enhancing permeability in the finer-grained mudstones and carbonates.

Overall, 50-60 percent of Saudi oil production has historically come from Ghawar. The field’s highest production was 5.7 MMbopd in 1981. Water cut (ratio of water to total liquid production) in the field was about 35 percent in 2003. Gas production was about eight billion cubic feet per day, out of which two billion was associated gas and six billion non-associated gas.

The International Energy Agency has placed Ghawar’s ultimate oil reserves at 140 billion barrels, including 66 billion barrels already produced and 74 billion barrels remaining oil.

How accurate these estimates are remains to be seen – but to put these figures in context, consider that the proven U.S. oil reserves currently stand at about 22.3 billion barrels.

It is apt to end this essay with a note on Steineke, a Stanford graduate and Aramco’s chief geologist, who contributed so much for the early oil discoveries in Saudi Arabia. He died in 1952, one year after Ghawar came onstream.

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Historical Highlights

Historical Highlights - Rasoul Sorkhabi

Rasoul Sorkhabi is a research professor at the University of Utah’s Energy and Geoscience Institute and is co-author of “Geological Excursions Around Miri, Sarawak” (Ecomedia, Malaysia, 2010). This paper was presented in the History of Petroleum Geology session at AAPG’s annual convention in New Orleans in 2010

Historical Highlights

Historical Highlights - Hans Krause

Hans Krause is an AAPG Honorary Member, Distinguished Service Award winner and former chair of the AAPG History of Petroleum Geology Committee.

Historical Highlights

A History-Based Series, Historical Highlights is an ongoing EXPLORER series that celebrates the "eureka" moments of petroleum geology, the rise of key concepts, the discoveries that made a difference, the perseverance and ingenuity of our colleagues – and/or their luck! – through stories that emphasize the anecdotes, the good yarns and the human interest side of our E&P profession. If you have such a story – and who doesn't? – and you'd like to share it with your fellow AAPG members, contact the editor.

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The presence of hydrocarbon-bearing sandstones within the Eocene of the Forties area was first documented in 1985, when a Forties field (Paleocene) development well discovered the Brimmond field. Further hydrocarbons in the Eocene were discovered in the adjacent Maule field in 2009. Reservoir geometry derived from three-dimensional seismic data has provided evidence for both a depositional and a sand injectite origin for the Eocene sandstones. The Brimmond field is located in a deep-water channel complex that extends to the southeast, whereas the Maule field sandstones have the geometry of an injection sheet on the updip margin of the Brimmond channel system with a cone-shape feature emanating from the top of the Forties Sandstone Member (Paleocene). The geometry of the Eocene sandstones in the Maule field indicates that they are intrusive and originated by the fluidization and injection of sand during burial. From seismic and borehole data, it is unclear whether the sand that was injected to form the Maule reservoir was derived from depositional Eocene sandstones or from the underlying Forties Sandstone Member. These two alternatives are tested by comparing the heavy mineral and garnet geochemical characteristics of the injectite sandstones in the Maule field with the depositional sandstones of the Brimmond field and the Forties sandstones of the Forties field.

The study revealed significant differences between the sandstones in the Forties field and those of the Maule and Brimmond fields), both in terms of heavy mineral and garnet geochemical data. The Brimmond-Maule and Forties sandstones therefore have different provenances and are genetically unrelated, indicating that the sandstones in the Maule field did not originate by the fluidization of Forties sandstones. By contrast, the provenance characteristics of the depositional Brimmond sandstones are closely comparable with sandstone intrusions in the Maule field. We conclude that the injectites in the Maule field formed by the fluidization of depositional Brimmond sandstones but do not exclude the important function of water from the huge underlying Forties Sandstone Member aquifer as the agent for developing the fluid supply and elevating pore pressure to fluidize and inject the Eocene sand. The study has demonstrated that heavy mineral provenance studies are an effective method of tracing the origin of injected sandstones, which are increasingly being recognized as an important hydrocarbon play.

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Size fractions (<4 and 0.4–1.0 μ) of Brent Group sandstones from the northern North Sea contain mostly illite-smectite mixed layers with kaolinite, whereas the same size fractions of Fulmar Formation sandstones from the south-central North Sea consist of illite-smectite mixed layers with minor chlorite. Transmission electron microscope observations show elongated illite laths or agglomerates consisting of small laths when larger individual laths are lacking.

The K-Ar data of the fractions less than 0.4 μm of Brent Group samples plot on two arrays in a 40Ar/36Ar vs. 40K/36Ar diagram that have isochron characteristics with ages of 76.5 ± 4.2 and 40.0 ± 1.5 Ma, and initial 40Ar/36Ar ratios of 253 ± 16 and 301 ± 18, respectively. For the Fulmar Formation samples, the data points of the fractions less than 0.2 and less than 0.4 μ also fit two isochrons with ages of 76.6 ± 1.4 and 47.9 ± 0.5 Ma and initial 40Ar/36Ar ratios of 359 ± 52 and 304 ± 2, respectively. Some of the coarser 0.4–1.0-μ fractions also plot on the two isochrons, but most plot above indicating the presence of detrital components more than 0.4 μ. The almost identical ages obtained from illite-type crystals of sandstones with different deposition ages that are located about 600 km (373 mi) apart record two simultaneous illitization episodes. These events were not induced by local burial conditions, but are related to episodic pressure and/or temperature increases in the studied reservoirs, probably induced by hydrocarbon injection. This interpretation is indirectly supported by notably different K-Ar illite ages from cores of a nearby reservoir at hydrostatic pressure.

Illite is not as well crystallized as expected for potential crystallization temperatures above 160°C measured by fluid-inclusion determinations. In both the northern and south-central North Sea, the two illite generations remain unaffected after crystallization despite continued burial, suggesting notably higher crystallization temperatures than those estimated from geothermal gradients. No recent illite crystallization or alteration is recorded in the K-Ar ages, despite a dramatic regional acceleration of the subsidence in the southern North Sea. ±

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A three-dimensional seismic data set and published data from exploration wells were used to reconstruct the tectonostratigraphic evolution of the Mandal High area, southern North Sea, Norway. The Mandal High is an elongated southeast-northwest–trending horst. Three fault families in the Lower Permian sequence, inherited from the basement structural grain of Caledonian origin, are interpreted: (1) a north-northwest–south-southeast–striking fault family, (2) a northeast-southwest–striking fault family, and (3) a near east-west–striking fault family. In addition, an east-southeast–west-northwest–striking fault family (4) that formed during Late Jurassic rifting and was reverse reactivated in the Late Cretaceous is interpreted. We suggest that inversion occurred because of small dextral motion along fault family 1. A final fault family (5) displays various strike orientations and is associated with salt movements.

Seven chronostratigraphic sequences defined by well data and recognized on three-dimensional seismic data are interpreted and mapped: Early Permian rifting in a continental environment; Late Permian deposition of the Zechstein salt and flooding; Triassic continental rifting; uplift and erosion in the Middle Jurassic with deposition of shallow-marine and deltaic sediments; rifting and transgression in a deep-marine environment during the Late Jurassic; a post-rift phase in a marine environment during the Early Cretaceous; and flooding and deposition of the Chalk Group in the Late Cretaceous. An eighth sequence was interpreted—Paleogene–Neogene—but has not been studied in detail. This sequence is dominated by progradation from the east and basin subsidence. Well and seismic data over the Mandal High reveal that large parts of the high were subaerially exposed from Late Permian to Late Jurassic or Early Cretaceous, providing a local source of sediments for adjacent basins.

Similar to the Utsira High, where several large hydrocarbon discoveries have been recently seen, the Mandal High might consist of a set of petroleum plays, including fractured crystalline basement and shallow-marine systems along the flanks of the high, thereby opening up future exploration opportunities.

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Participants will explore a number of classic geological sites that affected the Lewis and Clark expedition in Montana on this six-day excursion.

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