Origin of Oil and Gas

We know that oil and gas are extracted from deep below the earth's surface. But does anyone knows how did oil and gas gathered there in the first place? The origin of gas and oil has been a topic of debate by chemists and geologists for a long time. The most common theory that is accepted by all is the organic theory. According to scientists, the bodies of prehistoric plants and sea animals were trapped in sediments, which after many many years due to heat and pressure were changed into crude oil and natural gas. They are usually found together in the crust of the earth and to get the gas, it is necessary to drill into the earth's crust.

The Organic Theory
According to this theory the natural gas is formed by a chemical reaction in the earth of the marine organisms that were buried in the sands, which was initially the seashore. The organic theory states that oil and gas have biological origins. When the earth was mainly covered by water, small sea creatures and plants that were dead settled to the bottom of the ocean floor. Over the years layers and layers of sand, silt and clay built up on top of them. This decayed matter from plants and animals is called organic matter. As the years passed, these sands were further overlaid with sediments pushing the organic matter further down. The increased burden, the resulting heat, pressure made the sands and other materials transform into rock formations known as "reservoir rock". Over time, the reservoir rock covered the organic material and trapped it beneath the rock. Through the ever increasing heat and pressure and the process of decay, the decayed sea creatures and plants were converted to oil and gas. Thus almost all oil and gas are derived from tiny decayed plants, algae, and bacteria. Oil forms first, then with the increase in the temperature and pressure at greater depth gas begins to form.

Oil and gas form as a consequence of a environmental conditions occurring in a sequence

• The presence of organic material
• Organic remains are trapped and preserved in sediments
• The material is buried deeply. Then it is heated by increased temperature and pressure.

Characteristics of Reservoir Rock for Oil and Gas Exploration
A piece of sandstone or limestone when seen under a microscope show many tiny openings, which are referred to as pores by geologists. It is to be noted that it is through such rocks having pores that oil and gas can be extracted.

The diagram shown below is a rock with pores and is referred to as porous. This means it has tiny holes through which gas and oil may flow. Reservoir rocks must be porous, because hydrocarbons (oils and gas are also known as hydrocarbons as they consists of hydrogen and carbon) can occur only in pores

A reservoir rock through which gas is extracted is also permeable. That means the pores of the rocks are connected. If gases are in the pores of a rock, they must be able to move out of them. The arrows in the diagram below shows how the pores can be connected.

A suitable reservoir rock is that which is porous, permeable, and contain enough hydrocarbons to make feasible economically for the operating company to drill the oils and gases and produce them.


Diagrammatic Representation of Formation of Oil and Gas

First stage: Sea and plant animals died. They were buried on the ocean floor. They were covered by sand and silt over the years.

Second stage: The remains went deeper and deeper and with increased heat and pressure they were turned into oil and gas.

Third stage: Through layers of silt, sand and rock, drilling is done today to reach the rock formation that contain oil and gas deposits.