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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Fall/Winter 2000 23

Microfossils abound in Dammam Dome area.

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DAMMAM DOME

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Dhahran

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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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