Saudi Arabia factfile
Underground Geology: Oil, Aquifers and Caves
Sandy wilderness stretches into the distance, as far as the eye can see. Bare rocks jut up from
gravel plains. Indeed, many parts of Saudi Arabia appear stark and lifeless, but underfoot lies the evidence
of millions of years of history and geological riches.
Geologists try to
unravel the mystery of this pre-history by studying the rocks and answering questions as to how, when and what
happened to form them.
The hard rocky exterior of the earth’s crust is constantly moving. At depths greater than 100 kilometres
below, enormous heat and pressure on minerals creates a liquid mantle, which we see occasionally on the surface
as volcanic lava and magma. The crust is pulled and pushed in different directions, which breaks it into large
segments called tectonic plates. Six large plates and a dozen smaller ones cover the earth’s surface.
The Arabian plate is moving very gradually north-eastwards, separating from the African plate along the line
of the Red Sea and the Gulf of Aden. Where two plates meet together collisions can occur, causing high, folded
mountain ranges. If one of the colliding edges is an ocean crust, being much denser than the other, it will
glide under the continental mass.
About 600 million years ago, an immense diversity of animal forms began to appear in the world’s seas.
The structure of the Arabian Peninsula at that time had essentially been formed. Onto its vast, barren plains
crept a shallow sea, full of animal life. Tiny sea creatures and plants that took their energy from the sun’s
heat and light sank to the sea bed when they died and their bodies became covered by tiny particles of rock
and minerals called sediment. Over millions of years, the layers of mud and sand containing the little creatures
turned into sedimentary rock. As the animals decayed, heat and pressure on them produced fossil fuels, and these
now form the vast oil reserves lying in the sedimentary rock regions of Eastern Arabia.
The search for oil
this treasure within the rocks required many skills of the geologists
who came to Arabia to search for oil. In the 1930s technology was
not as advanced as it is today, and they had to use a mixture of
guesswork and astute observation to survey the possible locations
for oil. It was a frustrating job. Today, satellite images show
rock types and formations. Computers turn magnetic surveys from
the air into pictures of rock structures hundreds, or even thousands,
of metres below the surface. If an area looks promising a seismic
survey is carried out.
In a seismic survey, sound waves generated from a vibrator truck on the earth’s surface travel downwards
and bounce back from the top of certain underlying layers of rock, known as seismic reflectors. If the rocks
are porous, oil may be present and the echoes return to the surface slowly. If the rocks are solid there is
little chance of oil, or other liquids, such as water, being present. The echoes from these rocks bounce back
The search for water — aquifers
In the past the search for underlying reservoirs of water was as crucial to the people of Arabia as the search
for oil. It is a surprising fact that colossal quantities of water lie trapped under the Arabian sands.
The water in these aquifers is very old, collected in a time thousands of years ago when the climate was much
wetter than it is today. Sandstone, a sedimentary rock, is porous and allows water to flow freely through it.
However, other types of rock, such as clay, are impermeable and do not allow the passage of water. Reservoirs
of water become trapped between these different types of rock. Water entered the sandstone rock formations in,
or near, an area where the rock crops out above the ground. Water flows down the strata until released naturally
by a fault, or artificially, by sinking a well.
Oases are formed
where aquifers come naturally to the surface. Where bright green foliage dots the landscape it is certain that
water is not far below. Huge oases extend for many kilometres, supporting vast areas of palm trees such as those
in the old towns of Hofuf and Riyadh.
Arabia has nine principal aquifers, six of which are in sandstone and three in limestone, which is also a sedimentary
rock. Underground water resources are thought to be huge, probably holding hundreds of billions of cubic metres.
Saudi Arabia also has some of the world’s largest systems of limestone caves. These, too, indicate greater
past rainfall. When rain falls, tiny amounts of carbon dioxide from the air dissolve in it to convert it into
a weak solution of carbonic acid. That seeps downward, etching features into the limestone. Saudi Arabia’s
limestone has been eaten away, creating a multitude of caves and intricate passageways. The caves have been
closely studied by the Saudi Geological Survey and reveal information from bones, pollen and spores trapped
in the dust of the caves about past plants and creatures that lived there.
Inside the caves dramatic shapes are created by deposits formed
from drops of water containing calcium carbonate. As the caves are
now air-filled, carbon dioxide escapes from the water leaving ‘speleothems’
as the deposits are technically called. Chemical analysis of the
deposits gives information about their age and the climate when
they were formed. There are two main types of deposits: stalactites,
which hang from the cave roof and stalagmites, which grow up from
the floor. The walls of some caves glisten with beautiful coatings
of minerals where thin sheets of water flow down the cave walls
or along inclines.
The earliest explorers
of Saudi Arabia’s caves were the Bedouin, who sheltered in the caves or dahls (sinkholes) to escape from
the heat or howling sandstorms. The holes held the possibility of water at the bottom, which may have meant
life or death for a desperately thirsty traveller.
The Dahl Sultan
In the early 1980s an survey by the Saudi Geological Society discovered
one of the largest caves in Saudi Arabia lying east of the Dahna
Desert in a small settlement called Ma’aqala. (200-250 km
north-northeast of Riyadh.) Named the Dahl Sultan, it was entered
through a small hole no larger than a dinner plate and had to be
enlarged with a chisel to allow a thin person to squeeze through.
Once inside, the cave explorers encountered incredible sights –
a combination of low crawl-ways, spacious passageways several kilometres
long, and spectacular formations of stalactites, hundreds of thousands
years old. It was a stunning introduction to the underground world
below Arabia. Until Dahl Sultan was discovered there was no evidence
that Saudi caves contained such marvels.
The team went on to discover a complex system of caves in this area. One cave consisted of a vast hole, 100
metres in depth and 100 metres in diameter, located in the desert east of Majma’ah. Local people call
it Dharb al Najem, meaning ‘Place Where the Star Fell to Earth’.
For more information
about the Kingdom’s caves and geology:
Origins of Arabia
Andrew Thompson, ISBN 1-900-988-046
The Desert Caves
of Saudi Arabia
John Pint, published by Stacey International 2003
Encyclopaedia of Arabia
Mary Beardwood, ISBN 1-900988-33-X
Photographs courtesy of Peter Sanders and John Smith.