Petroleum is classified as a nonrenewable fossil fuel. The reason for this is that the prevailing opinion on how oil is formed is that it is biogenic. According to the biogenic theory, when microbial life at the surface of the ocean, including algae, phytoplankton, zooplankton and the tiny organisms which feed upon them die, their bodies fall to the ocean floor in a continuous rain of organic debris.
As this organic debris accumulates, the lower levels are put under enormous pressure. The pressure causes the organic debris to heat. The continued build-up of pressure and heat break down the organic compounds and turn them into hydrocarbons. These hydrocarbons are a mix of different organic chemicals of liquid and gas. The ratios of the various chemical compounds varies with each field.
The alternative explanation for the origin of petroleum is as abiogenic source. Proponents assert that petroleum is actually formed well below the earth’s surface in areas just above the mantle as part of a chemical process associated with volcanic magma. As the hydrocarbon accumulates, the liquids and gas push themselves up through fissures into sediment chambers.
The mechanism for this abiogenic process is not well understood. But support for it comes from two sources. The first is that astronomers have detected hydrocarbon lakes and rivers, even precipitation, on the moons of Jupiter and Saturn. The second is that oil deposits are known to exist deeper than the known depths of ancient oceans.
In both cases, the hydrocarbon source must be buried deep enough to maintain a temperature of between 50o C and 100o C. If the temperature falls lower, the hydrocarbons form kerogen. The amount of time necessary to form petroleum and natural gas is not known. However, the petroleum pumped today was laid down at different time intervals over a period of between 65 and 300 million years ago.
Whether it is formed biogenically or abiogenically, one can argue in a technical sense that petroleum and natural gas are perpetual resources. They are continuously formed by natural processes. However, no matter how they are formed, it is clear from the present Peak Oil declarations of the major producers, that the rate of consumption is surpassing the rate of production substantially.
For more information see “The Mysterious Origin and Supply of Oil”