Catagenesis (geology)

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See Catagenesis (biology) for usage in the field of biology, where it refers to retrogressive evolution. Contrast with anagenesis.

Contents 1 Theoretical reaction 2 Important parameters 3 Erroneous assumption 4 See also 5 References

Catagenesis is a term used in petroleum geology to describe the cracking process which results in the conversion of organic kerogens into hydrocarbons.

[edit] Theoretical reaction

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.

[edit] Important parameters

Several generally unrecognized but important controlling parameters of metamorphism have been suggested.^[1]

• 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.

[edit] Erroneous assumption

A large body of petroleum-geochemical data suggests these are not first-order reactions.^[1] This means that geologic time has a minimal role.

• Experiments in closed, wet, pressurized systems are higher ordered reactions rather than first-order reactions.
• Ample geologic evidence supports the possibility that the effect of geologic time may be overestimated. The hypothesis of geologic time being a controlling parameter was based upon rocks which are at low present-day burial temperatures, but it was later discovered in all those basins that high to extreme heat flow had previously existed in those basins. The influence of long geologic time on a rock formation increases the probability that high heat flows will occur in that formation.
• Water suppresses thermal destruction of hydrocarbons, and hydrogen from water seems to be incorporated in kerogen.
• Closed chemical systems suppress catagenesis. Regional shearing of fine-grained rocks opens up closed systems and strongly promotes both catagenesis and rock metamorphism at much lower burial temperatures than in unsheared rocks.
• Increasing static fluid pressure strongly retards hydrocarbon generation. This has been found in experiments and helps 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.

[edit] See also

• diagenesis
• fossil fuels

[edit] References

1. ^ ^{a b} Price, Leigh C. (1997). "Minimum Thermal Stability Levels and Controlling Parameters of Methane, As Determined by C₁₅+ Hydrocarbon Thermal Stabilities". Geologic controls of deep natural gas resources in the United States (USGS Bulletin 2146): 139-176. Retrieved on 2006-10-10.