Classification of Torbanite and Cannel Coal I. Insights from Petrographic Analysis of Density Fractions
Torbanite and cannel coal are considered to be coals because of their low mineral content and overall physical morphology. However, the texture and composition of the organic matter in torbanite and cannel coal are similar to the kerogen occurring in oil shales and lacustrine source rocks. Therefore, understanding the nature and origin of organic components in torbanite and cannel coal is of significance in the study of kerogen and petroleum formation. In this research, a set of torbanites and cannel coals from different locations throughout the world were petrographically characterized and processed using a density gradient centrifugation (DGC) technique. Microscopically, the torbanite and cannel coal are composed of coarser maceral particles set in a fine-grained to amorphous groundmass. The groundmass is a mixture of more than one type of substance and accounts for 10 to 80% (by volume) of the torbanites and cannel coals. Botryococcus-related alginite is the most characteristic component of the torbanite. While sporinite typically is the main phytoclast in the cannel coals, in most cases the groundmass is volumetrically the dominant component, determining the overall character of the sample. This observation calls into question the traditional practice of classifying such coals using the alginite to sporinite ratio. Variations in composition, texture and fluorescence permits the recognition of three different types of groundmass: lamalginitic, bituminitic and vitrinitic. High purity alginite, sporinite, vitrinite and varieties of groundmass were separated using the DGC technique. The distribution of density fractions closely reflects the petrographic composition of the various torbanites and cannel coals. Distinct peaks on the density profiles represent the major organic components and peak magnitudes are functions of the percentage of the components, demonstrating that the density gradient profiles can be used to distinguish the different types of torbanite and cannel coal. The separation data also indicate a gradual shift towards higher density from lamalginitic to bituminitic to vitrinitic groundmass.