The Relationship between Vitrinite Reflectance and Transformation Ratio in Basin Modeling

Roger W. Marzi and Douglas W. Waples

It is well known that if two reactions have different activation energies and frequency factors, there is only one temperature profile at which both reactions proceed at the same rate. Therefore, since the kinetic parameters for vitrinite reflectance (Ro) and hydrocarbon generation are different, there can be no universal correlation between Ro and transformation ration (Tr).

However, our opening statement is not valid if at least one of the reactions is described by a distribution of overlaping activation energies. In such cases the reaction with the lowest activation energy will begin first, and will proceed alone until the lowest activation energy for the second reaction is reached. From that point on, until the last activation energy for one of the reactions is reached, the two reactions proceed in parallel, and there is an excellent correlation between the rates of the two reactions.

Since Ro and hydrocarbon generation are both described by activation-energy distributions that overlap to a great degree, there should an excellent correlation between Ro and transformation ratio, commencing at the beginning of oil generation. We have confirmed through modeling experiments that there is such a correlation for any given kerogen that will apply at all geologic heating. Therefore, we can use Ro as a direct proxy for hydrocarbon generation in discussing modeling results or in carrying out modeling studies.

However, the specific correlation between Ro and Tr is kerogen-specific and must be determined empirically for each kerogen, since each kerogen has a different activation-energy distribution. Thus the current perception that there is no universal correlation between Ro and transformation is still correct.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California