Among the many prolific oil fields in the Middle East, the giant Ghawar field in Saudi Arabia stands out as the crown jewel.
Discovered in 1948, Ghawar is the world's biggest oil field, stretching 174 miles in length and 16 miles across to encompass 1.3 million acres.
Current estimates, according to the numerous published articles and reports on Ghawar, tag cumulative oil production from this geological giant at 55 billion barrels, and the field just keeps going gangbusters. Average production for the last 10 years has held essentially steady at five million barrels per day.
In fact, this one field accounts for more than one-half of all oil production in Saudi Arabia, according to a number of sources.
The anatomy of Ghawar was the topic of a presentation given by Abdulkader Afifi, senior geological consultant at Saudi Aramco, during his recent U.S. tour as an AAPG Distinguished Lecturer.
Ghawar is a north-trending anticlinal structure, which is expressed on the surface by outcrops of Tertiary rocks. In the field's northern part the structure actually comprises two parallel anticlines with a small low in between.
Oil was first discovered in 1948 in the northern part using structural drilling, where geologists would map structures by drilling a grid of shallow wells to the top of the Cretaceous, according to Afifi. This technique was developed by Max Steineke, chief geologist at the Arabian American Oil Co. (parent company to Saudi Aramco), who received the AAPG Sidney Powers medal in 1951.
The initial discovery in Ghawar's southern part was in 1949 at the Haradh Field, where American geologist Ernie Berg mapped the surface of the Haradh anticline using the ordinary, tried-and-true plane table method.
The northern and southern discoveries appear as separate fields on early maps prior to being connected as a single field in 1955.
'It's Basic Geology'
The Ghawar anticline is draped over a basement horst, which grew initially during the Carboniferous Hercynian deformation and was reactivated episodically, particularly during the late Cretaceous. The Paleozoic section was eroded significantly by the Hercynian unconformity.
The asymmetrical structure, which is steeper on the western flank, becomes more complex at depth where it comprises several en echelon horst blocks. Bounding reverse faults have throws as much as 3,000 feet at the Silurian level, but they die out in the Triassic section, according to Afifi.
He also noted there appears to be a minor component of right lateral strike slip.
The producing oil reservoir at Ghawar is the late Jurassic Arab-D limestone, which is about 280 feet thick and occurs 6,000-7,000 feet beneath the surface. Growth 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.
The oil was sourced from Jurassic organic-rich lime mudstones, which were laid down in intershelf basins. The integrity of the thick anhydrite top seal is enhanced by the general absence of faults in the Jurassic section.
Despite its impressive life span and colossal production volumes, there's really no mystique to Ghawar's grandiosity. Think of it as a Geology 101 scenario, i.e., a lot of geology-type happenings in the right place at the right time.
"It's basic geology," Afifi said. "You need five conditions to form a large oil accumulation, and these things came together in a beautiful manner over a very large area.
"We have the prolific Hanifa Jurassic source rock and an excellent anhydrite seal over the thick, porous Arab-D reservoir," he noted, "and we have a large structure with a favorable growth and thermal history. The upper parts of the reservoir are very clean grainstone, with porosity exceeding 30 percent in places. In fact, the Arab-D is outstanding in terms of both permeability and porosity."
The field's copious production has had help in the form of water injection, which was initiated in 1965.
Water injection volumes are included in a number of publicly available articles about Ghawar, with one of the more recent ones pegging the injection rate at seven million barrels of seawater per day. Water cut, according to other sources, has been reduced from approximately 35 percent to roughly 30 percent since vertical well drilling was shelved in favor of horizontal wellbores.
Step on the Gas
But there's more to Ghawar than voluminous oil production.
The field gives up about 2 billion cubic feet of associated gas per day, and it has the capacity to kick out as much as 5.2 billion cubic feet of non-associated gas from the deeper Paleozoic section, where it's trapped in Permian, Permo-Carboniferous and Devonian reservoirs at depths between 10,000 and 14,000 feet. This deep gas is sourced from Silurian shales, which are the main Paleozoic source rocks in the Middle East and North Africa.
The late Permian Khuff A,B and C stacked carbonate reservoirs are the main gas producing zones at depths of 10,000 to 12,000 feet. Afifi postulates the Khuff gas likely moved laterally into Ghawar from other fields to the north, whereas gas in the deeper Unayzah and Jauf sandstone reservoirs migrated vertically along faults.
The Khuff carbonates are highly cyclical, and gas and reservoir quality is variable owing to extensive diagenesis.
Most of the Khuff is non-porous and tight, according to Afifi, who noted the best reservoir facies are dolomitized peri-lagoonal mudstones.
The Permo-Carboniferous Unayzah sandstones, which onlaped the ancestral Ghawar highlands from the south, contain sweet gas at depths of 12,000 to 14,000 feet. The gas is trapped structurally and stratigraphically in a mix of eolian, fluvial and lacustrine clastics. Variable reservoir quality is attributed to quartz cementation for the most part.
Additional sweet gas was discovered in 1994 in a fault/unconformity trap in Devonian sandstones, which were truncated along Ghawar's eastern flank.
The key challenge to deep gas exploration and development at Ghawar has been porosity prediction using geologic models and 3-D seismic data, according to Afifi.
He noted that seismic imaging is challenging because of multiples and near-surface velocity variations and low-impedance contrast in the Paleozoic section.