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HOW OIL WAS FORMED [A] Nearly all living things draw their energy from the sun through the food they eat. In order to USE this energy, plants and animals must "unlock" it from the food. To do this they need oxygen from the air.
[C] Scientists can tell from fossils how old the rocks are. They know which are the richest oil-bearing rocks. Black shale and bituminous limestone are two important sources of oil.
[B] Because there was little oxygen in the buried layers of plankton, they kept their stores of energy. Instead of rotting, it is thought they were changed by the pressure and by bacteria into the black liquid we call CRUDE OIL.
[D] Even though oil is made deep under the ground, it can gradually seep upwards or sideways through POROUS rock. If it seeps to the surface it will evaporate into the air.
[C] At the same time, wind, waves, rivers and glaciers eroded some of the earth's crust. This formed mud, sand and gravel which was washed down by rivers. It settled on the sea bed. More and more of these SEDIMENTS mixed with, and pressed down on, the layers of plankton. As a result, the sediments were compressed into hard layers of SEDIMENTARY rocks.
[B] Millions of years ago, when the sea was full of plankton, the sea bed became covered with their dead remains. Sometimes the plankton was buried in layers of mud and sand several hundred metres thick.
[D] The whole process of forming this oilfield might have taken many MILLIONS of years.
[A] When creatures die, the energy still locked inside them is usually released by the oxygen in the air. We call this process ROTTING.
[C] Many different types of sedimentary rock were formed. Grains of sand became sandstone; mud became mudstone or shale, and the skeletons of millions of shellfish became limestone.
[A] But rotting cannot begin if there is no oxygen present. The energy cannot be released. Instead it stays locked within the creatures' remains
[D] However if it reaches a barrier of NON-POROUS rock, it will build up and build up behind the barrier and form what we call an OILFIELD.
A
good site to look at on the BBC
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