Contains the full lesson along with a supporting toolkit, including teachers’ notes.
Every day millions of people all over the world depend on oil and natural gas to carry out daily activities such as cooking, heating their homes and transport. Many industries also rely on oil and natural gas as sources of energy for production and as raw material for products, e.g. plastics and fertilisers. In this lesson we will look at some of the techniques employed in finding deposits of oil and natural gas.
How do oil and natural gas form and accumulate?
In ancient seas, plants, animals and microorganisms abounded. When they died they sank to the bottom of the sea where they usually became a source of food for scavengers and decomposers. In certain circumstances, such as highly acidic conditions or lack of oxygen, the remains of the dead organisms did not fully decay and the accumulated material became mixed with silt and clay, to form a sedimentary deposit.
Over time this sediment was overlain by successive layers of deposits – sometimes to a depth of several kilometres. Here it was subjected to high temperatures from geothermal sources and high pressure from the weight of overlying rock.
Under the right conditions of pressure and temperature the organic material in the rock, kerogen, was converted to petroleum – a word that means ‘rock oil’. Higher temperatures favour the formation of gas rather than oil. Hydrocarbons tend to migrate upwards through the rock unless prevented by an impermeable layer of rock (cap rock). As rock layers are often not uniformly horizontal, this migration of fluids is to the highest contained part of a geological structure, known as a trap. The rock in which the oil or gas lies is called the reservoir, while the rock in which it originated is called the source rock (Fig. 1). Crude oil contains a mixture of hydrocarbons with relatively long carbon chains, containing between ten and twenty carbon atoms per molecule. Natural gas consists of mixtures of hydrocarbons with fewer carbon atoms per molecule – only one in the case of methane.