Abstract: Measurement of Vitrinite Reflectance and its Use with Temperatures Based on Apatite Fission Track Analysis (AFTA^TM)

A. C. Cook, I. Duddy, P. Green

Vitrinite reflectance was developed as a technique for assessing coal rank. The maturation concept does not differ essentially from that of rank. Within coals, distinction of vitrinite from the other macerals is primarily based on morphology even though reflectance contract with other macerals may be marked. For dispersed organic matter, distinction of vitrinite should be made primarily on the basis of morphology. Where they are available, coals provide the most robust data on maturation level. However, vitrinite reflectance measurements can be made on samples from sequences lacking coals. Vitrinite is present in most fully marine sequences that are Devonian or later in age, although usually a minor component. The use of graptolite and chitinozoan tests and bitumen reflectance, permit the technique to be used for many sections of Lower Paleozoic or Precambrian age.

Where the organic facies differs from humic coals, reflectance obtained are different from those that would be obtained from humic coals if present in the section. Most differences in reflectances related to facies can be assessed given information on vitrinite type and the associated anomalously low vitrinite reflectances.

For accurate identification of vitrinite and organic facies, whole rock rather than demineralized strew mounts should be used. Measurement of reflectance without the use of a polar is common practice. However, if the stage is not rotated, local flatness is not known. Use of a polar permits measurement of maximum reflectance and gives more precise data for higher ranks. Additionally, bireflectance data provide an indication of contract alteration.

The annealing of fission tracks in apatite represents an independent method of identifying a specific temperature range, the time when section cooled below that range and the style of the subsequent cooling history.

Both vitrinite reflectance and the annealing of apatite fission tracks are primarily driven by maximum temperature. The two techniques are synergistic in that together they provide more information about the thermal history than either can provided by itself. Vitrinite reflectance covers a wide range of temperatures. The fission track data provides fixed time/temperatures points normally lacking in constraining thermal models. Together the two forms of data are especially useful for calculating section loss as they provide more accurate results than can be obtained by, for example, back projection of vitrinite reflectance profiles. At higher levels of maturation, vitrinite to middle part of the oil window the two methods provide independent assessments of temperature history that are ifferent in kind, leading to more robust interpretations.

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