FLUORESCENCE ALTERATION AND THE SUPPRESSION OF VITRINITE REFLECTANCE

Vitrinite reflectance is a highly important thermal maturity indicator in petroleum exploration, but there are a number of problems in its successful application which are basically the consequence of it being a single parameter technique. Fluorescence alteration of macerals, which can be conveniently measured with the laser Raman microprobe, is a multi-parameter technique which can be used to overcome some of these limitations. On a fluorescence alteration diagram most vitrinite plots on, or close to, a line which may be called the "normal" vitrinite line. Fluorescence alteration data for a suite of randomly selected macerals from coal, or dispersed organic matter, plot along a curve which cuts the "normal" vitrinite line at the equivalent vitrinite reflectance of the sample, as determined by calibration against a suite of reference coals. The technique possesses a high degree of objectivity and can be employed to determine the equivalent vitrinite reflectance of samples in which the vitrinite is difficult to identify, or even absent. Perhydrous vitrinites, with suppressed vitrinite reflectance, are readily recognized as they plot in a field laterally displaced with respect to the "normal" vitrinite line. The method is exemplified by a study of coals from the Greta and Pelton seams, Sydney Basin, which provides a classic example of vitrinite reflectance suppression, and is further illustrated by a study of the Dampier #1 well from the North West Shelf of Australia which contains both perhydrous macerals and dispersed organic matter in which vitrinite is difficult to identify.