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Some of the world's best scientists and most advanced technology are
to be found in the oil industry, engaged in the task of discovering
new reserves of oil that will satisfy the needs of modern society.
Where to look?
There are many ways that oil and gas are trapped under the ground and
the experts seek evidence for the presence of sedimentary rock formations.
For example, oil is likely to be held within reservoirs such as limestone
or sandstone and it will need to have an impermeable cap rock, such as
shale. Also, the Earth's crust must have moved in such a way that the oil
or gas reserves have been unable to escape.
This is the job of the geologists who study rocks and their formation,
and the geophysicists who are concerned with the physical characteristics
of rocks. Together they use their knowledge and experience to locate sites
where oil might have been trapped.
One example of entrapment (the anticline) is given in the previous section. Another occurs when movements of the earth have cracked and lifted the rock layers (known as a fault), fracturing the oil-bearing stratum. The fault can result in a seal of impermeable rock (see diagram).
Nowadays, geologists use evidence from satellite photographs, together with aerial photographs taken by flying over carefully chosen strips of land. These are used to build up a map of the topography (surface shape) of the land below. This gives clues to the history of its formation and might show up an anticline for example. Alternatively, a sudden change in terrain or foliage might indicate a fault where oil could be trapped. All kinds of evidence can help to locate sites where further investigation might pay off.
Further exploration
Once a possible area has been decided on, the next stage (if on land)
is to do an intensive ground study of it. If there are any exposed layers
of rocks the geologists might find clues to millions of years of history.
They will even examine the exposed strata for fossils. These will help
date a rock layer because each era has its characteristic set of fossil
remains.
Most important, however, is the seismic survey (see the diagram above), where geophysicists map the rocks beneath the surface. It works because shock waves travel at different speeds depending on rock types and patterns. They are reflected at different angles and refracted (bent) differently as they meet new strata. The signals are collected with geophones and processed by computer to give a three-dimensional picture of the rock structures below.
| In most cases, the reservoirs hold gas, oil and water, which is present as separate layers - gas, the lightest, at the top, oil in the middle and water at the bottom. |