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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.

Tectonic plates

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.

Sedimentation

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

Did you know?Finding 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 quickly.

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.

Limestone caves

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’.


mOre For more information about the Kingdom’s caves and geology:

www.saudicaves.com

www.sgs.org.sa/index_en.htm

Origins of Arabia
Andrew Thompson, ISBN 1-900-988-046

The Desert Caves of Saudi Arabia
John Pint, published by Stacey International 2003

The Children’s Encyclopaedia of Arabia
Mary Beardwood, ISBN 1-900988-33-X


Photographs courtesy of Peter Sanders and John Smith.


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