Abstract: Implications of the Suppression and Enhancement of Vitrinite Reflectance Related to Geological Factors
Wolfgang D. Kalkreuth, David
L. Marchioni
Vitrinite reflectance values may be enhanced or suppressed relative to regional levels. While "anomalous" reflectance values can arise from a variety of secondary sources, the focus here is on geological causes related to compositional and/or depositional aspects of the organic matter.
In coal samples, reflectance varies with bed thickness; thicker units yielding the highest reflectances. Coaly inclusions in clastics and carbonates typically yield lower reflectance than coals. Reflectance of vitrinite picked from coaly lenses in clastic cores will most likely be suppressed relative to cuttings derived from relatively thick seams.
Reflectance is strongly suppressed in proportion to the abundance of alginite and to a lesser degree, proportional to other liptinite components. Consequently, it should be expected that reflectance in potential source rocks will be suppressed.
Hydrogen-rich and hydrogen-poor vitrinites in coal and dispersed organics will yield suppressed and enhanced reflectance values respectively. Further, hydrogen-rich vitrinites are commonly associated with marine influenced deposition. Due both to this effect and to the typical restriction of reflectance measurement to thin coaly lenses or dispersed organics, reflectance in marine sequences can be expected to be lowered.
The effects of reflectance anomalies can be very significant in the interpretation of the depth to specific hydrocarbon generation or destruction thresholds and in the interpretation of thermal histories. It is critical to interpret reflectance data in terms of maceral composition, host lithology and depositional environment of the sequence sampled. Corrections should be made to standardize reflectance data sets with regional values.
AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994