Catagenesis (geology)

Catagenesis (geology)

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Catagenesis is a term used in petroleum geology
Petroleum geology
Petroleum geology refers to the specific set of geological disciplines that are applied to the search for hydrocarbons .-Sedimentary basin analysis:...

 to describe the cracking
Cracking (chemistry)
In petroleum geology and chemistry, cracking is the process whereby complex organic molecules such as kerogens or heavy hydrocarbons are broken down into simpler molecules such as light hydrocarbons, by the breaking of carbon-carbon bonds in the precursors. The rate of cracking and the end products...

 process which results in the conversion of organic kerogen
Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. It is insoluble in normal organic solvents because of the huge molecular weight of its component compounds. The soluble portion is known as bitumen. When heated to the right...

s into hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....


Theoretical reaction

This chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

 is believed to be a time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

, temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 and pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 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
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume 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
Differential equation
A differential equation is a mathematical equation for an unknown function of one or several variables that relates the values of the function itself and its derivatives of various orders...


where X is the reactant (kerogen) and κ is the reaction rate constant which introduces the temperature-dependence via the Arrhenius equation
Arrhenius equation
The Arrhenius equation is a simple, but remarkably accurate, formula for the temperature dependence of the reaction rate constant, and therefore, rate of a chemical reaction. The equation was first proposed by the Dutch chemist J. H. van 't Hoff in 1884; five years later in 1889, the Swedish...


Important parameters

Several generally unrecognized but important controlling parameters of metamorphism have been suggested.
  • The absence or presence of water in the system, because hydrocarbon-thermal destruction is significantly suppressed in the presence of water.
  • Increasing fluid pressure strongly suppresses all organic-matter metamorphism.
  • Product escape from reaction sites, as lack of product escape retards metamorphism.
  • Increasing temperature as the principal driver of reactions.

Future Work

A great deal of future research is required to isolate the parameters are most significant for inducing the Catagenetic process. Future work in the field will involve the following:
  • Establishing the precise relationship between burial time and hydrocarbon cracking.
  • Determining how hydrogen from water is ultimately incorporated in kerogen.
  • Establishing the effect of regional shearing.
  • Determining how static fluid pressure affects hydrocarbon generation. Some experiments have demonstrated that static fluid pressure may explain the presence of hydrocarbon concentrations at depths where their composition would not otherwise be expected.
  • Many measurements of hydrocarbon content in sample rocks have been done at atmospheric pressure. This ignores the loss of large amounts of hydrocarbons during depressurization. Rock samples at atmospheric pressure have been measured at 0.11–2.13 percent of samples at formation pressure. Observations at well sites include fizzing of rock chips and oil films covering drilling mud pits.
  • Types of organic matter can not be ignored. Different types of organic matter have different chemical bonds, bond strength patterns, and thus different activation energies.
  • C15+ hydrocarbons are stable at much higher temperatures than predicted by first-order reaction kinetics.*

For example, while it was once assumed that catagenetic processes were first-order reactions, some research has shown that this may not be the case.