Abstract: Critical Review: Maturation Criteria Utilized in the Evaluation of Source Rocks

By

Paul J. Cernock

The interrelationships between petroleum and source rocks can be understood only if the important geochemical and geological data of both systems are known and correctly interpreted. In evaluating suspected source rocks, the geochemist has primarily focused attention on the organic richness (quality), organic matter type (gas, condensate or oil prone) and state of thermal maturity of the organic matter within these rocks. Various geochemical techniques have been proposed and accepted over the years to define organic richness, matter type and maturity. However, there have been questions and apparent contradictions in the industry primarily with establishing the thermal maturity of various source rocks. The geochemical data have been, in most cases, correct; unfortunately, the available geological data have often not been properly utilized to construct a valid geochemical/geological interpretation.

A critical review will be presented of various maturation criteria which are generally utilized in source rock evaluation studies, and the pitfalls which can trap the unwary if the geological data are not properly incorporated into the final maturity interpretation. Maturity may be defined by the color of dispersed organic matter (kerogen) viewed in transmitted light and/or by the vitrinite reflectance technique, which compares the reflectivity of the maceral vitrinite with a scale based on coal rank. Any change in the level of maturation caused by an increase in geothermal history (time-temperature) results in a corresponding change in color of the dispersed kerogen and by changes in reflectivity (Ro) of polished faces of the vitrinite materials.

This review will show that identical samples of rock can produce "kerogen" and "vitrinite" data which can be (mis) interpreted several ways, because the compounding geological data have not been properly utilized.

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