Thermal Maturation and Petroleum Generation
When sedimentary organic matter is buried in basins it is exposed to increasingly higher subsurface temperatures. At temperatures of approximately 60°C and higher, the thermal degradation of kerogen yields hydrocarbons under reducing conditions (Hunt, 1996). Type I and II kerogens generate most of the world's oil when subjected to burial temperatures between 60°C and 160°C. Type III kerogen generates natural gas, condensate, and waxy oil. Type IV kerogen generates small quantities of methane (CH 4 ) and carbon dioxide (CO 2 ). Most oil expulsion occurs within this burial temperature range too, when the organic matter is in the “oil window”. Recall that petroleum expulsion from the source rocks is very inefficient; roughly 85% of the hydrocarbons generated in a mature source rock are retained within the micropores of the fine-grained sediments. These hydrocarbons are eventually cracked to natural gas as temperatures increase beyond the oil widow. Three levels of maturity are recognized by petroleum geochemists, early , peak , and late mature . Hunt (1996) “Postmature for oil is mature for the gas window. Between one-half to two-thirds of thermogenic gas forms during the thermal cracking of previously generated oil in both source rocks and in reservoir rocks and in coal” (p. 140).
Stage of Thermal Maturity
<60 o C
Bacterial and plant organic matter converted to kerogens and bitumen
Methane generated by microbial activity
60 o C -160 o C
Rock generates and expels most of it's oil
>160 o C
Postmature for oil/mature for gas
The geochemical parameters used to determine level of thermal maturation in the source rock are vitrinite reflectance, thermal alteration and, a Rock-Eval pyrolysis parameter called T max . The production index can be derived from Rock-Eval pyrolysis data and is also indicative of the rock's thermal maturity.
T max is the temperature at which the maximum amount of S2 hydrocarbons are generated during Rock-Eval pyrolysis. T max is dependent on the type of kerogen present in the source rock. Plotting T max versus the Hydrogen Index (HI) shows the different maturation paths of different kerogen types. T max values correlate directly with vitrinite reflectance values.
Vitrinite reflectance is a measure of the amount of light reflected by vitrinite present in the rock's organic component. The value is measured in oil immersion and is expressed as percent reflectance in oil, R o . Vitrinite is a direct microscopic measure made on the macerals extracted from the kerogen in the source rock samples. It is widely used as a maturity indicator, but is dependent on kerogen type and cannot be measured in rocks that lack vitrinite.
The production index (PI) is the ratio of already generated hydrocarbon to potential hydrocarbon [S1/ (S1 + S2)] derived from Rock-Eval pyrolysis (Peters, 1994). Low ratios indicate immaturity or extreme postmature organic matter. High ratios indicate either the mature stage or contamination by migrated hydrocarbons or drilling additives. PI values less than 0.01 indicate the immature stage. Early and peak mature stages correspond to PI values of 0.10 to 0.15 and 0.25 to 0.40, respectively. PI values greater than 0.4 indicate the late mature stage.