How oil and gas form | About BP | BP Global

How oil and gas form

Why fossils became a fuel source

If you’ve ridden in a car or bus, fossils got you where you needed to go. If you’ve used a gas stove, fossils cooked your food. The petroleum oil that becomes gasoline and many other useful products wouldn’t exist without tiny plants, algae and bacteria, which settled to the bottom of the sea as they died millions of years ago.

There’s no oxygen under the earth’s crust, so the organic matter in the sediment changed into a substance scientists call kerogen. And when the temperatures rose to 110° Celsius or higher the kerogen gradually changed into oil. Under hotter conditions it changed into natural gas. The process takes at least a million years.

Crude oil is a complex mixture of hydrocarbons. In other words, it is made up of hydrogen, carbon and traces of other substances. Its texture varies, but it is generally liquid. Natural gas is mainly made up of the chemical compound methane. It is gaseous, or lighter than air.

Move along, please

If the story ended there, oil and gas might never have become the global energy sources they are today. The deposits would be so scattered that we would have almost no chance of extracting them in usable amounts.

Even after oil has formed in the rock, pressure continues to rise, squeezing the oil out or upwards through rocks that have more pores, or spaces, within them.

All oil moves like this. Some of it eventually reaches the surface and seeps out naturally into land or water, but most of it eventually comes up against a layer of rock that it can’t move through. This impermeable rock forms a seal or trap, and slowly, very slowly, the oil builds up. As it does, it forms a reservoir.

Reservoirs are rock formations that hold oil, natural gas or both within their pores, like a fossilised sponge. Reservoirs can be massive. Some may be as large as London, Hong Kong or New York.

If only finding them was as easy as mapping a city. Rocks also move over millions of years, as tectonic plates shift. Their formations can be extremely complex. This makes oil and gas reservoirs extremely difficult to find.