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The Dammam Dome, site of the first oil field discovered in Saudi Arabia. By Robert Lebling, in Saudi Aramco Dimensions magazine, Fall/Winter 2000.
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22 Saudi Aramco Dimensions
A vertical section across the center of the Dammam Dome,showing how the Hormuz Salt pillow underlies the dome andthe various formations. Midra al-Janubi (next page), one of theprominent jabals or peaks of the Dammam Dome, is composedof the Dam Formation. It has near its base two stromatolitebeds, formed of blue-green algae from the Middle Miocene(about 15 million years ago). The jabal is kept from erodingaway by an iron-hard cap-rock, made of travertine, probably the infill of an ancient cave which itself has eroded.
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Tiny Fossils Point the Way to
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Fall/Winter 2000 23
Microfossils abound in Dammam Dome area.
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DAMMAM DOME
N
Dhahran
S A U D IA R A B I A
B A H R A I N
HALFMOON
BAY
Dammam
Al-Khobar
A R A B I A N G U L F
0 5 10 15
Km
It’s the only place in the Eastern Province where you can find layers of exposed
sedimentary rockfrom the Tertiary or “post-dinosaur” period some
65 million years ago.24 Saudi Aramco Dimensions
A simplified geological/location map of the DammamDome, centered at Dhahran inSaudi Arabia’s Eastern Province.
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Dhahran, the heart of the world’s largest petroleum
enterprise, Saudi Aramco, lies in the middle and right
on the crest of the Dammam Dome, an oval-shaped
swelling of the earth’s surface that extends about 9 miles
along the major northwest-southeast axis and covers an area
of about 60 square miles.
The high point of the structure is Jabal Umm er Rus,
the familiar “mountain” just north of Saudi Aramco’s core
area headquarters, that many years ago caught the eye of
geologists in Bahrain who were scanning the horizon for
signs of oil-bearing formations similar to what they had
found on the island.
As we now know, some 4,000 to 5,000 feet below the
Dammam Dome lies pay dirt: four oil-bearing reservoirs—
Arab A, B, C and D—and one shallow, sweet gas reservoir
in the Mishrif Formation.
The Dammam Dome, which rises to about 150 meters
above sea level at its highest point, got its uplift from the
swelling of a “pillow” of salt—part of an underlying structure
known as the Hormuz Salt—thousands of feet below the
oil-bearing zone. The process has been going on for millions
of years, tempered only by erosion and weathering, and it
continues to this day. Current estimates are that the dome is
rising at a rate of 5.6 to 7.5 meters per million years, or a little
more than half to three-quarters of a millimeter per century.
The Dammam Dome is a recent surface feature—
recent, that is, in geological terms. It’s the only place in
the Eastern Province where you can find layers of exposed
sedimentary rock from the Tertiary or “post-dinosaur”
period some 65 million years ago. The oldest rocks are
exposed near the center of the dome, near Jabal Umm er
Rus within Saudi Aramco’s camp, while the younger rocks
are exposed successively as one retreats from the center.
The exposed sediments include deep marine shales,
carbonate sands from ancient lagoons and sabkha-like
deposits of evaporated minerals like gypsum. Sharks’ teeth
can be found in deep marine shales, but they’re not seen very
often in the Dhahran area, and in fact are much commoner
in the Khurais oil field area, about halfway to Riyadh.
The lagoonal sands inside Saudi Aramco’s camp contain
microfossils called Nummulites, the largest of the single-
celled organisms. These fossils, which look like tiny coins
(hence their name, from the Latin “nummulus” or “little
coin”), show up in exposures of the Khobar Limestone
Foram fossils help give scientists a better idea of the shape and size of formations beneath the surface. This foram is a bottom-dwellingrotalid, with a segmented shell or test, from the Dam Formation of the Dammam Dome.
This thin-section reveals a nummulite foram fossil fromthe Khobar Limestone Member of the Dammam Dome.The limestone blocks of the Giza Pyramids teem withthese tiny coin-like fossils.
Fall/Winter 2000 25
Benthonic rotalid foraminifera
Benthonic foraminiferaTaberina malabarica <<<
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47990araD5R1 2/17/01 11:04 AM Page 25
Member of the Dammam Formation, a layer of sediment
formed in “shallow, clear, warm marine conditions”
about 45 million years ago, according to Saudi Aramco
micropaleontologist Dr. Geraint Wyn Hughes.
Limestone teeming with nummulites, just like this
exposure, was quarried by the ancient Egyptians to help
build the Pyramids of Giza.
The Dammam Dome provides Saudi Aramco with an
additional bonus—a local site for teaching young Saudi
Arabian geologists the principles of field geology, stratigraphy
and the use of fossils to the geologist. The large time gap
between the Eocene and Miocene rocks of the Dome makes
such unconformities in the geological record “come to
life,” and can be applied when interpreting seismic and
geological models in the office.
Hughes recently took a group of curious employees
and dependents to the edge of the Dhahran main camp,
and showed them surface fossils “hidden in plain view.”
After touring various exposed rock locations within
the camp, showing various levels of the Rus and Dammam
Formations, Hughes heads for Dammam Well No. 7 (now
called Prosperity Well), the Kingdom’s first commercially
viable well, located in the shadow of Jabal Umm er Rus.
Hughes explains how after 15 months of drilling and
a string of operational setbacks—stuck pipe, drill bits lost
down the hole, cave-ins, etc.—the company finally struck
oil there on March 3, 1938, tapping into the oil-rich
Arab Formation at a depth of 4,727 feet (1,441 meters).
Dammam-8 and Dammam-9 were completed later in the
year, and the Government was able to declare the
Dammam Field a commercial producer.
And what about the unsuccessful wells 1 through 6?
There’s no trace of them today, other than a plaque to
mark the general location. There was plenty of crude oil
beneath these wells—and still is, according to the geologists.
Exposed FossilsThe microfossils that interest scientists aren’t just found
below ground. As Hughes explains, in the Dhahran area
they sometimes lie exposed to the blazing sun, embedded
in the faces of craggy bluffs not far from where employees
live and work.
Hughes’ group scrabbled up a slope and gathered
around a cut in a familiar rock face not far from a major
26 Saudi Aramco Dimensions
Micropaleontologist Dr. Geraint Wyn Hughes,on a field trip in the Dammam Dome area,explains what conditions in the area were like millions of years ago.
The large time gap between The large time gap between The large time gap between the Eocene and Miocene rocks of the Eocene and Miocene rocks of the Eocene and Miocene rocks of
the Dome makes such the Dome makes such the Dome makes such
unconformitiesin the geological record
“come to life,”and can be applied when interpreting
seismic and geological models in the office. seismic and geological models in the office. seismic and geological models in the office.
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Fall/Winter 2000 27
company road. Hughes held a magnifying loupe against
the rock and one by one the fascinated onlookers peered
through the glass at a tiny white Alveolina, a delicate-
looking, spindle-shaped fossil the size of a grain of rice.
They were looking at the remains of a sea creature
that lived in a very different world on that same spot some
49 million years ago. That world was the Middle Eocene,
and the area we know as Dhahran and the surrounding
Dammam Dome was submerged beneath a deep sea in
which sharks and rays swam.
At other times, other environments prevailed, such as
shallow lagoons, or salt flats. Different aquatic environments
left behind distinctive layers of deposits and, within each
layer, tiny characteristic fossils, the remnants of ancient life.
Hughes works in the Exploration organization’s
Geological Research and Development Division, and
concentrates on microfossils. These tiny fossils are important
to geologists because they evolve rapidly and can be used
as index fossils for age and environment determination. If
geologists know which species are present in a given sediment,
they can date the rock fairly precisely. Hughes generally
deals with sediments younger than 300 million years.
In the early days of Saudi Arabia’s hydrocarbon
enterprise, bigger fossils—such as hand-sized gastropods,
corals, bivalves and ammonites—played an important role
as company experts geologically mapped the surface
sediments. Today the focus is on microfossils, as sediments
below the surface, associated with the search for oil and
gas, are dated with the help of cuttings and core samples
containing fossils not easily seen with the naked eye.
Saudi Aramco and other major petroleum companies
employ paleontologists—or micropaleontologists, who
specialize in tiny organisms—to examine the fossil record
at drilling sites, in an effort to better define the rock layers
beneath. This helps to delineate known reservoirs and
fields and improves the chances of striking new oil and gas.
Among the organisms that intrigue the micropaleontolo-
gist are foraminifera—also called foraminifers or forams—
one-celled amoeba-like marine creatures whose presence
in rock layers or strata creates boundaries or markers
separating one ancient time period from another.
Forams, which come in many varied species, inhabit
the seas today as they did millions of years ago. They can
be floaters (planktonic) or bottom dwellers (benthonic).
A typical hilltop on the Dammam Dome, where young geologistscan study ancient formations and find an array of microfossils.
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28 Saudi Aramco Dimensions
when no deposits are laid down, due to dry periods, erosion,
earth movements and other factors.
When an oil or gas well is being drilled, the paleontologist
receives core samples (cylinder-shaped plugs of rock
extracted from the bore hole) and cuttings (pieces of drilled
rock swept up with the circulating drilling fluid or mud)
for examination under the microscope. Ultra-thin sections
reveal the microfossils that formed the carbonate rock.
The scientist can then draw conclusions about the
configuration of the rock layers or stratigraphy below.
The sequence of fossilized organisms laid down over
time in the layers of sedimentary rock make up what the
paleontologist calls the biostratigraphy of a given formation.
The sequence of ancient environments in which these
organisms lived—called bioecostratigraphy—can be
determined by studying the fossils and their relationships
within the layers in which they are found.
They build hard little shells (or tests) for themselves out
of organic matter, sand-grains, calcium carbonate, or some
combination of these. The shells, ranging in size from 8 cm
(3 in) in diameter down to 0.05 cm (0.02 in), often survive
in layers of sediment laid down over the ages, giving
paleontologists the clues they need to pin down probable
locations of oil and gas reservoirs.
Forams and other fossils help Saudi Aramco define
the ancient (or paleo-) environment in which the deposits
were laid down. They allow experts to make correlations
between wells and between reservoirs. They also help
define regional unconformities, i.e., breaks in the sequence
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The administration building of the residential/industrial area at Shaybah. Thousands of feet beneath the sabkhas and dunes of this area lies the oil-bearing Shu‘aibah Formation, whose contours are being defined with the help of tiny creatures fossilized in the rock layers. <<
47990araD5R2 2/23/01 4:15 PM Page 28
Hughes recently published such a study in the petroleum
geoscience journal GeoArabia. He described, for the first time
ever, the bioecostratigraphy of the petroleum-bearing Shu‘aiba
Formation in the Shaybah Field. Shaybah, a 700-square-mile
elongated oil field in the Kingdom’s Rub‘ al-Khali or Empty
Quarter, went on-stream in 1998 and produces about
500,000 barrels of Arabian Extra Light crude per day.
The oil comes from about 4,900 feet down, in the porous
rock of the Shu‘aiba Formation, a carbonate reservoir with
an average thickness of about 400 feet. The formation is
almost entirely from the early Aptian Age, in the Cretaceous
Period, more than 100 million years ago. Shu‘aiba, according
to Hughes, is made up of carbonates that mostly accumulated
Fall/Winter 2000 29
as shallow marine platforms rimmed with rudists—large,
rather odd-looking bivalve mollusks of the Cretaceous
Period that had one valve shaped like a funnel or a flower
vase and another like a flattened cap.
The fossil evidence—rudists, forams and other organisms
—shows that the formation evolved from a wide, moderately
deep platform dominated by planktonic foraminifera
(“lower Shu‘aiba”) into a rudist-rimmed platform with
a well-developed lagoon (“middle Shu‘aiba”) and finally
into an extensive, deep lagoon whose banks are rimmed
narrowly with rudists (“upper Shu‘aiba”).
These findings are helping geologists to define and
delineate one of the Kingdom’s most important oil-bearing
formations. The more our experts know about a formation,
the better the chances of future drilling successes.
Fossils may be tiny critters, but nowadays they can
hold the key to potential petroleum bonanzas. �
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Fossils may be tiny critters, but nowadays they can hold
the key to potential
petroleum bonanzas. <<
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