By the year 2020 the world’s known oil reserves are due to run out. By then, new oil fields will need to be found, probably in more and more inaccessible places.
Three things are needed for an oil field to form in nature: the right kind of sedimentary rock to create the oil; a layer of porous rock to store it; and a ‘cap’ of impermeable rock to trap it. Sedimentary rock is formed over millions of years from sediment that contains fish, shellfish, plankton and plants. As these organic materials break down with heat and time, they produce oil and gas. If there is a layer of porous rock, it will soak up the oil like a sponge. An impermeable bed of rock above the oil will contain it, so long as the ‘cap’ is the right shape – ideally a dome.
Prospectors searching for oil look for all three types of rock. To find sedimentary basins which could be oil-bearing, magnetic surveys and gravity surveys are often used. All rocks are magnetic, but the magnetism varies slightly from one rock to another, giving geologists clues to the structure and type of rocks that lie underground. Other clues lie in the varying densities of rocks.
In magnetic survey, magnetometers – instruments which measure magnetism – are usually towed over the region by an aircraft. From readings of the variations in magnetism, a picture of the structure of the ground below can be built up.
Gravitational surveys rely upon the measurement and comparison of the densities of sub-surface rocks. A device called a gravimeter can measure changes in gravity of one part in 100 million of the Earth’s gravitational field. There is even a gyroscopically established version of the instrument to take readings at sea.
The information gathered is processed by a computer and interpreted by geologists. If the results are promising, more detailed information is obtained by seismic surveying.
This starts by sending shock waves into the ground by causing explosions vibrations on the surface. The shock waves travel at different speeds depending on the type of rock they are travelling through. When the waves hit an interface between two different types of rock they are reflected and return to the surface. Microphones and recording instruments pick up the returning shock waves. Computers can then be used to calculate the positions of the rock layers from the time the waves take to return to the surface. A detailed cross-section of the area can then be produced.
This method has proved very successful, but geologists can never be certain that oil will be found even if all the indications are there. Sometimes even the best prospects from seismic surveys have no oil at all, while less promising sites turn up trumps. One of the main reasons for this is the difficulty in telling whether oil has been trapped in its original position or not.
Oil pools can seep upwards or sideways through porous rocks.
Picture Credit : Google