Editing Petroleum (section)

==== Transformation of kerogen into fossil fuels ====
Kerogen formation continued to a depth of about 1 [[km]] from the Earth's surface where temperatures may reach around 50 [[°C]]. Kerogen formation represents a halfway point between organic matter and [[fossil fuels]]: kerogen can be exposed to oxygen, oxidize and thus be lost, or it could be buried deeper inside the [[Crust (geology)|Earth's crust]] and be subjected to conditions which allow it to slowly transform into fossil fuels like petroleum. The latter happened through '''catagenesis''' in which the reactions were mostly [[Radical (chemistry)|radical]] [[Rearrangement reaction|rearrangements]] of kerogen. These reactions took thousands to millions of years and no external reactants were involved. Due to the radical nature of these reactions, kerogen reacted towards two classes of products: those with low H/C ratio ([[anthracene]] or products similar to it) and those with high H/C ratio ([[methane]] or products similar to it); i.e., carbon-rich or hydrogen-rich products. Because catagenesis was closed off from external reactants, the resulting composition of the fuel mixture was dependent on the composition of the kerogen via reaction [[stoichiometry]]. Three types of kerogen exist: type I (algal), II (liptinic) and III (humic), which were formed mainly from [[algae]], [[plankton]] and [[woody plant]]s (this term includes [[tree]]s, [[shrub]]s and [[liana]]s) respectively.<ref name="Schobert-2013" />

Catagenesis was [[pyrolytic]] despite the fact that it happened at relatively low temperatures (when compared to commercial pyrolysis plants) of 60 to several hundred °C. Pyrolysis was possible because of the long reaction times involved. Heat for catagenesis came from the decomposition of [[radioactive]] materials of the crust, especially [[Potassium-40|<sup>40</sup>K]], [[Thorium-232|<sup>232</sup>Th]], [[Uranium-235|<sup>235</sup>U]] and [[Uranium-238|<sup>238</sup>U]]. The heat varied with [[geothermal gradient]] and was typically 10–30&nbsp;°C per km of depth from the Earth's surface. Unusual [[magma]] intrusions, however, could have created greater localized heating.<ref name="Schobert-2013" />