Vitrinite reflectance (Ro)
Vitrinite reflectance is the proportion of incident light reflected from a polished vitrinite surface. Vitrinite is a maceral (an organic component) of coal. As vitrinite is heated in the earth during burial, it systematically changes its reflective properties. These changes are caused by increasing condensation of aromatic (carbon-ring) structures in the coal matrix (Stach and others, 1992; Bustin and others, 1985; van Krevelen, 1992; Mukhopadhyay and Hatcher, 1993).
Vitrinite reflection is a standard method for calculating the relative amount of coalification (rank, maturation) that a coal (or other organic substance) has undergone. Measurements of reflectance are used in some international coal classification systems. Although not officially used to define rank in the United States, vitrinite reflectance is commonly used to corroborate rank determinations based on calorific value, volatile matter, and fixed carbon contents.
The methods for vitrinite reflection analysis of coal samples are standardized in ASTM method D2798-11 (American Society for Testing and Materials, 2013, p. 474-478). For reflectance measurements of dispersed vitrinite in other sedimentary rocks ASTM method D7708-11 (American Society for Testing and Materials, 2013, p. 846-853) applies. To determine vitrinite reflectance, a sample of coal is ground to the consistency of fine sand (-20 mesh = particle sizes less than 850 microns) and made into a pellet for examination. The top surface of the pellet is polished and coated with a prescribed (immersion) oil. The pellet is placed under a petrographic microscope. Reflectance measurements are first taken with a glass standard of known reflectance to provide a reference point for subsequent measurements. Following this calibration stage, a number of reflectance measurements (usually 50 to 100) are collected from particles of vitrinite in the coal sample. The amount of light being reflected is measured by a photomultiplier tube, which converts light intensity to a numerical value. The relative reflection (essentially, a gray-scale hue) of vitrinite macerals in the pellet is then measured by observation and comparison to standard reflection values. Values range from 0 (low reflectance) to 4 (high reflectance).
Several different types of vitrinite reflectance (Ro) measurements can be made:
Ro random: Random vitrinite reflectance is a measure of random orientations of vitrinite in nonpolarized white light. The Ro value is generally a mean value of 50 to 100 measurements. Ro random is the value most often used in vitrinite reflectance measurements.
Ro min: The minimum vitrinite reflectance is a measure of the minimum reflectance when the stage is rotated 360°, using polarized white light. Coal begins at low rank as a uniaxial, isotropic material. This means that reflectance values are equal in all directions. Coal becomes progressively more biaxial and anisotropic, however, with increasing rank. This means that reflectance becomes increasingly directionally dependent. As such, the difference between Ro min and Ro max increases with rank. Within the bituminous coal rank (Ro = approximately 0.5 to 1.5), directional anisotropy in vitrinite develops and increases with rank. Because of the anisotropy at higher rank, the minimum and maximum reflectance values are commonly measured in higher rank coals.
Ro max: The maximum vitrinite reflectance is a measure of the maximum reflectance value obtained when rotating the stage 360°, using polarized light. Within the bituminous coal rank (Ro = approximately 0.5 to 1.5), directional anisotropy develops and increases with rank. For higher-rank coals, the minimum and maximum reflectance values are commonly measured.
Vitrinite reflectance in other sedimentary rocks
Vitrinite particles are not only found in coal, they are preserved in other sedimentary rocks as well. Organic-rich shales may contain significant amounts of vitrinite. Because vitrinites essentially record the maximum heating the organic material has undergone, they also record the maximum burial heating a sample has undergone. This is a very important criteria in petroleum geology. Burial heating of organic-rich deposits has distinctive ranges (windows) in which petroleum and gas are produced from source rocks. Vitrinite reflectance measurements provide data for understanding if oil or gas in a reservoir was sourced from the immediate vicinity or had to be sourced from a deeper deposit. The data are also used to determine burial histories of petroleum and natural gas generation, which are important for oil and gas exploration. A brief, yet thorough, review of the use of vitrinite reflectance in petroleum exploration can be found in Bustin and others (1983).
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