Formation

Structure of vanadium porphyrin compound extracted from petroleum by Alfred E. Treibs, father of organic geochemistry. Treibs noted the close structural similarity of this molecule and chlorophyll a.

Petroleum is a fossil fuel derived from ancient fossilized organic materials, such as zooplankton and algae. Vast quantities of these remains settled to a sea or lake bottoms, mixing with sediments and being buried under anoxic conditions. As further layers settled to the sea or lake bed, intense heat and pressure built up in the lower regions. This process caused the organic matter to change, first into a waxy material known askerogen, which is found in various oil shales around the world, and then with more heat into liquid and gaseous hydrocarbons via a process known as catagenesis. Formation of petroleum occurs from hydrocarbon pyrolysis in a variety of mostly endothermic reactions at high temperature and/or pressure.

There were certain warm nutrient-rich environments such as the Gulf of Mexico and the ancient Tethys Sea where the large amounts of organic material falling to the ocean floor exceeded the rate at which it could decompose. This resulted in large masses of organic material being buried under subsequent deposits such as shale formed from mud. This massive organic deposit later became heated and transformed under pressure into oil.

Geologists often refer to the temperature range in which oil forms as an "oil window"—below the minimum temperature oil remains trapped in the form of kerogen, and above the maximum temperature the oil is converted to natural gas through the process of thermal cracking. Sometimes, oil formed at extreme depths may migrate and become trapped at a much shallower level. The Athabasca Oil Sands is one example of this.

Abiogenic origin hypothesis

A small number of geologists adhere to the abiogenic petroleum origin hypothesis, maintaining that high molecular weight hydrocarbons of purely inorganic origin exist within Earth's interior and are the source for major petroleum deposits. The hypothesis was originally proposed by Nikolai Kudryavtsev and Vladimir Porfiriev in the 1950s, and more recently Thomas Gold proposed a similar deep hot biosphere idea. The thermodynamic synthesis routes necessary to carry abiogenic source material into subsurface oil are not established, observation of organic markers in kerogen and oil is not explained, and no oil deposits have been located by this hypothesis.