Catagenesis (geology)

Catagenesis is a term used in petroleum geology to describe the cracking process which results in the conversion of organic kerogens into hydrocarbons. Catagenesis is the second stage of maturation of organic carbon on the path to becoming graphitic. This geologic process accounts for very significant changes in the biogenic materials that make up the carbonaceous sediment. During catagenesis, the temperature increases, the pressure increases, and both organic and inorganic constituents “adjust” their phase or form to compensate. The process of “lithification” begins during this stage. Generally speaking, a rise in temperature results in the volatization of unstable species or elements that are weakly attached to carbon atoms. Increased temperature and pressure also result in the cessation of biogenic processes. One way to express these changes is to look at the ratio of oxygen to carbon, or hydrogen to carbon as the sediment matures. In almost all cases, as biogenic material matures in a geologic environment, the volatile elements such as oxygen and hydrogen are significantly reduced, resulting in a reduction in the O/C and H/C ratios. A typical O/C ratio value for a fully matured, catagenesis stage carbon might be less than 0.1. This means that for every 100 carbon atoms there are less than 10 oxygen atoms. Similar reductions in the level of hydrogen are also apparent. This chemical reaction is believed to be a time, temperature and pressure dependent process which creates liquid and/or gaseous hydrocarbon Hc from primary kerogen X and can be summarised using the formula: where X0 is the initial kerogen concentration and X(t) is the kerogen concentration at time t. It is generally held that the dependence on pressure is negligible, such that the process of catagenesis can be given as a first-order differential equation: where X is the reactant (kerogen) and κ is the reaction rate constant which introduces the temperature-dependence via the Arrhenius equation. Several generally unrecognized but important controlling parameters of metamorphism have been suggested.