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"Hydrocarbons" are found in geological formations consisting of strata or layers of rocks. Specifically, they are formed from decomposed organic material (that contains carbon), that bonds with hydrogen, which is a component of water. (Somehow, the resulting oxygen is released).

Such hydrocarbons can take the form of either liquids (e.g. petroleum) or gas (natural gas). There are also "intermediate" products such as propane or butane that can easily take either liquid or gas form (at differing temperatures, but at temperature ranges that are normal for human existence).

What further factors cause hydrocarbons to take the form of petroleum, natural gas, or one of the others?

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Oil and gas both form when organic matter is buried under anoxic conditions, such that it isn't oxidised and subsequently mineralised to the elemental components of the hydrocarbons.

What hydrocarbons are formed depends on both burial depth and temperature. Above approximately 1km below the surface no significant formation of oil takes place, and hydrocarbons are either mineralised or broken down to volatile products such as methane, propane, and butane.

The often so called 'oil window' is located below 1km, but above ~4km, and at temperatures between 40-50 °C and 150 °C. Under these conditions long hydrocarbon chains can form without being broken down to shorter, more volatile chains.

At larger depths or higher temperatures no (significant amount of) oil is formed, and all organic matter is transformed to natural gas.

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An approximate graph after Nichols (2009).

Formation of these fossil fuels often takes place in shales (lithified clay) that are rich in organic matter, but as all sedimentary rocks this is a porous medium and thus liquids will tend to flow upwards to areas of lower pressure. To be preserved two other components are therefore necessary: a reservoir rock (often sandstone) and a sealing rock (numerous types of rock are suitable, including rock salt and certain igneous rocks), often in dome or fault structures.

Source: Nichols, Gary (2009) Sedimentology and Stratigraphy, p 293-295

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  • $\begingroup$ Sorry, but aren't methane, propane and butane considered "natural gas"? Thanks. $\endgroup$
    – André Chalella
    May 14, 2014 at 4:37
  • $\begingroup$ yes they are, I'm not sure what your point is, could you clarify? $\endgroup$
    – hugovdberg
    May 14, 2014 at 15:01
  • $\begingroup$ This answer doesn't mention kerogen type, which has a substantial influence on the phase of the hydrocarbons produced. It's also worth mentioning thermal and biologic cracking. $\endgroup$
    – Matt Hall
    Nov 22, 2015 at 23:42
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petroleum is "OIL" and formed by oils and fats undergo geologic processes. natural gas can occur (methane) under anoxic conditions of organic decomposition of carbohydrates and proteins.

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    $\begingroup$ Looks like you are also a lazy writer. $\endgroup$
    – Rodrigo de Azevedo
    Jan 21, 2022 at 19:29

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