Abstract: Reasons Why Vitrinite Reflectance Values May Be Precisely Inaccurate

Brian J. Cardott

Maximum vitrinite reflectance is measured on coal samples to determine coal rank. In general, large vitrinite particles in coal are abundant, easily identifiable, and usually indigenous (first cycle). Application of the vitrinite reflectance analysis to dispersed vitrinite in shale samples has numerous sources of error.

Caving contamination from well cuttings is a major source of error. Examination of whole-rock particles allows the identification of vitrinite in the host lithology. However, vitrinite dispersed in shale is often in low abundance and of small size, requiring kerogen isolation and measurement of random reflectance (stationary stage). When isolated, vitrinite from caving contamination and drilling mud must be properly interpreted; vitrinite-like bitumen may be mistaken for vitrinite. Random reflectance may be low at high thermal maturity (>1.0% R_o) owing to the anisotropy of vitrinite.

Recycled vitrinite can be identified on a reflectance histogram. The reflectance spread of first-cycle vitrinite is usually about 0.3-0.4% at low maturity (<1.0% R_o) and increases to >1.0% at higher maturity (>3% R_o). Caving contamination will have a lower reflectance than the first-cycle vitrinite, while recycled vitrinite will have a higher reflectance. Poor quality vitrinite particles (small size, scratched, pitted, etched) should be rejected.

The vitrinite reflectance analysis is a quantitative thermal maturity indicator. The reported vitrinite reflectance value is the mean of numerous (>20) measurements determined precisely to 0.01%. However, misidentification of vitrinite-like organic matter, contamination, and high standard deviation from vitrinite anisotropy at high thermal maturity and too few measurements can result in inaccurate determination of thermal maturity.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994