the many prolific oil fields in the Middle East, the giant Ghawar
field in Saudi Arabia stands out as the crown jewel.
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.
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.
this one field accounts for more than one-half of all oil production
in Saudi Arabia, according to a number of sources.
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.
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.
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.
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.
and southern discoveries appear as separate fields on early maps
prior to being connected as a single field in 1955.
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.
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.
noted there appears to be a minor component of right lateral strike
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.
density increases going deeper in the section, enhancing permeability
in the finer-grained mudstones.
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
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.
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.
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."
copious production has had help in the form of water injection,
which was initiated in 1965.
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.
more to Ghawar than voluminous oil production.
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.
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.
carbonates are highly cyclical, and gas and reservoir quality is
variable owing to extensive diagenesis.
the Khuff is non-porous and tight, according to Afifi, who noted
the best reservoir facies are dolomitized peri-lagoonal mudstones.
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.
sweet gas was discovered in 1994 in a fault/unconformity trap in
Devonian sandstones, which were truncated along Ghawar's eastern
challenge to deep gas exploration and development at Ghawar has
been porosity prediction using geologic models and 3-D seismic data,
according to Afifi.
that seismic imaging is challenging because of multiples and near-surface
velocity variations and low-impedance contrast in the Paleozoic