the Niger Delta. At temperatures above 150 degrees, any oil previously formed is "overcooked" (Cracked) to become Gas.
As the muddy, silty ooze (which geologists call the Source Rock) in which the oil is formed is pressed by successive strata of sedimentation, the oil is squeezed out to accumulate in sandstone strata which are formed from sand deposited by rivers into the sea. The sand is itself turned into rock by the pressure of succeeding sedimentation.
Sandstone is ideal for this purpose because each grain of sand has been rounded by the erosion processes that have carried it from the land mass of which it was once part to be deposited in rivers, lakes or seas. Thus however tightly the grains are crammed together, like marbles in a box, there are still spaces between them. These spaces are called pores and thus porous sandstone is able to hold oil (and gases and other liquids) like a bath-sponge holds water. Where the pores are connected, liquids are able to migrate through the rocks which are Permeable (they can be penetrated by liquids).
The oil in the sandstone sponge is trapped by a stratum of shale which is impervious (cannot be penetrated by liquids and gases), because, unlike sandstones, shales are dominated by clays, the particles of which, being plate shaped, can been forced tightly together without any spaces between them (see figure 6.). The sandstone sponge collects not only oil but also seawater, gas and a variety of associated marine substances such as sulphur and nitrogen compounds.
Spread across horizontal strata of sandstone the oil does not accumulate (we are told) in commercial quantities. However where there are folds or faults, the oil may gather and be trapped in large quantities. This is because oil, being a hydrocarbon, is less dense than water, and thus tends to rise above the water in the sandstone sponge, until it is blocked by a "cap" of impervious shale, as shown in figure 5. Any associated gas, being lighter than both oil and water, sits between the oil and the cap.
To summarise, five geological conditions are required for the formation of an oil field:
- a source rock (formed by marine deposits);
- a reservoir rock (such as a sandstone sponge);
- a cap rock, to prevent the upward movement of the oil such shale);
- a fault or fold so that the oil is trapped in commercial quantities; and
- time – it takes millions of years for commercially exploitable volumes of oil to accumulate.
The Niger Delta is ideal!
Petroleum Hydrocarbons: Hydrocarbons are molecules made up of carbon and hydrogen atoms. The most important Petroleum Hydrocarbons, from the most light to the heaviest, are as follows.
- Natural Gas - methane
- Liquefied Gas - butane
- Petrol - iso-octane
- Kerosene (Paraffin) - decane
- Diesel - tetradecane
- Lubricants - a range of compounds
