Recognition of vitrinite with non-uniaxial negative reflectance characteristics

Abstract

Non-uniaxial negative reflectance characteristics are being reported for coals from around the world with increasing frequency. These data have traditionally been collected from oriented sections cut from a block of coal to determine the magnitudes of the principal reflectance axes. To make data collection less time-consuming, a method of determining the principal reflectance axes from a standard, crushed coal sample has been developed. The method requires measurement of the maximum and minimum apparent reflectances from a series of randomly oriented vitrinite particles. The reflectance crossplot displays these two reflectance values against their difference or bireflectance. The paper describes the method of interpreting the resultant pattern to determine the values of the three principal reflectance axes. Analysis of coals from the British Columbia Rocky Mountains and Foothills revealed biaxial and uniaxial positive as well as uniaxial negative reflectance characteristics. A new measure of maturation, R ev , is proposed which is based purely on the lengths of the three principal reflectance axes. R m approximates this value but is influence by any eccentricity of the reflectance indicating surface (RIS). Vitrinite with a non-uniaxial reflectance characteristics cannot be adequately described by a single measure such as R max or R m but requires reference to both The size and shape of the RIS. A reflectance determination such as R ev or R m provides rank (ASTM) information whereas the new measures R st and R am describe a reflectance indicating surface shape which is the result of maturation and tectonic histories. Vitrinites with non-uniaxial negative reflectance characteristics are abundant in the coalfields of the Rocky Mountains and Foothills of British Columbia. R max does not fully describe the reflectance character of these coals and does not describe the potentially useful RIS shape.