Microspectrofluorescence measurements of coals and petroleum source rocks

Abstract

Good, reproducible microspectrofluorescence data, similar to published fluorescence trends, are obtainable from samples of coals and petroleum source rocks (shales). These data are reproducible when measured by different operators, different instruments, or at different times (for example, the remeasurement of samples several months apart). These fluorescence data are used to predict maturity or rank to within about 0.10% R 0 (mean vitrinite reflectance, oil immersion). Commercially available computerized fluorescence equipment, with customized software and specific hardware mod modifications, makes the acquisition of fluorescence data feasible. Fluorescence can be used for measuring low-maturity samples, in which the measurement of vitrinite reflectance is difficult. Source rocks from oil-rich basins frequently do not contain enough particles of vitrinite for a reflectance determination, but they may contain fluorescing particles which can be measured. Although the results from only two different kinds of fluorescence equipment (Zeiss Zonax and Vickers Gamma-Scientific) are discussed, any instrument with equivalent stability and sensitivity can be used to attain reproducible and accurate fluorescence data. We propose the use of the term “ R F” to report fluorescence data in equivalent vitrinite reflectance values. Although the principles and techniques of measuring fluorescence were introduced several years ago, much work remains to establish standardized methods of maesuring as well as data evaluation. The current method of reporting fluorescence data in terms of spectral characteristics (such as the wavelength of maximum intensity or wavelength ratios) is awkward to use and difficult to compare results from various laboratories. R F values convey rank or maturation meaning and can be easily compared among laboratories. Data from fluorescing grains in both coal and petroleum source rock samples are given in this study. Polished samples of coal, this sections, and kerogen strewn slides of source rocks are measured. Spectral data are evaluated by four parameters: the wavelength of maximum intensity (Peak), the red/green ratio ( Q), the coordinates of a metric color system (CIE), and the relative intensity at 490 nm. The measurement of intensity, reported in units of radiance, is possible with the Vickers Gamma-Scientific instrument. Measuring both fluorescent colors ( R F) and intensities may help explain some of the relationships among organic particles, maturation, and hydrocarbon generation.