ORGANIC THEORY OF ORIGIN OF PETROLEUM

There are two theories regarding the origin of petroleum

1. Inorganic Theory
2. Organic Theory

INORGANIC   THEORY

According to this theory petroleum  products orginated from the carbon available inside the mantle of the earth. Carbon reacted with water and hydrogen inside the earth in the presence of some metals that functioned as catalyst. This happened due to the high temperature existing at that time. Now this theory is more or less discarded and the organic theory is found accepted.

ORGANIC THEORY

Organic matter is the dead remains of plant and animal organisms. As these organisms died they went down to the shallow sea bed. Through millions of years rivers flowed down to these sea beds and were covered with mud and slit. Over a period of time they were covered with mud and slit and got buried deep and deeper. Once the organic matter got buried  below, they did not decay due to lack of contact with atmosphere.

CHEMISTRY
Oil and gas are made of a mixture of different hydrocarbons. As the name suggests these are large molecules made up of hydrogen atoms attached to a backbone of carbon.

PLANKTON
Most oil and gas starts life as microscopic plants and animals that live in the ocean.

BLOOMS
Today, most plankton can be found where deep ocean currents rise to the surface. This up willing water is rich in nutrients and causes the plankton to bloom.

ON THE SEA BED
When the plankton dies it rains down on sea bed to form an organic mush. If there are any animals on the sea bed these will feed on the organic particles.

BLACK SHALE
However, if there is no oxygen in the water then animals cannot survive and the organic mush accumulates. Where sediments contain more than 5% organic matter, it eventually forms a rock known as Black shale.

COOKING
As black shale is buried, it is heated. organic matter is first changed by the increase in temperature into kerosene, which is a solid form of hydrocarbon. around 90°c , it is changed in to a liquid state, which we call oil. around 150°c, it is changed into a gas. a rock that has produced oil and gas in this way is known as a source rock

MIGRATION
Hot oil and gas is less dense than the source rock in which it occurs Oil and gas migrate   upwards up through the rock in much the same way that theair bubbles of an underwater diver rise to the surface. The rising  oil and gas eventually gets trapped in pockets in the rock called reservoirs.