Application of vitrinite reflectance anisotropy in the evaluation of coal metamorphism

Measurement of the maximum plus either the minimum or mean vitrinite reflectance provides a method for quantification of anisotropy of the reflectance. Previous research has suggested that anisotropy of vitrinite reflectance may be a direct indicator of static pressure (depth of burial) at which coal metamorphism occurred. The combination of thermal information (from vitrinite reflectance) and pressure information should, therefore, provide a means of evaluating the metamorphic history of a single coal seam or a series of coals in a sedimentary basin (given the independent placement of coal-rank parameters on a pressure-temperature grid). Occurrence of coals in strata with diagnostic metamorphic minerals is rare. Therefore, evaluation of coal metamorphic conditions can seldom proceed parallel to mineralogical evaluation of rock metamorphic conditions. Rather, the coal petrologist has generally relied upon time-temperature nomograms to estimate metamorphic temperature from coal rank. In this study, the level of organic metamorphism (LOM)-time-temperature scale of Hood and others (1975) was considered the most versatile nomogram for evaluation of metamorphic conditions. Coals from throughout the Appalachians were studied, but this study concentrates on coals from the Allegheny plateau in western Pennsylvania. Vitrinite maximum reflectance (in oil at 546 nm) and reflectance anisotropy ((R maximum − R mean )/R maximum = anisotropy) were measured on Pennsylvania coals. In order to estimate rank gradients and subsequently paleogeothermal gradients, however, it was necessary to rely upon previously published volatile matter data from the Lower Kittanning, Upper Freeport, and Pittsburgh seams. From equivalent metamorphic temperatures from the LOM-time-temperature scale and from vertical separation of the seams, paleogeothermal gradients were calculated for Somerset County, adjacent to the Allegheny front, (about 40°C/km) and for Westmoreland County to the west (about 33°C/km). Past depths of burial for both localities were calculated to have been 3.5 to 4 km for the Allegheny Group coals. Calculated gradients are each in areas on the plateau where vitrinite reflectance is nearly constant within each area. Two similar areas, for which geothermal gradient could not be calculated directly due to insufficient vertical exposure of coals, lie immediately to the north. The crustal blocks appear to be divided by the southwest-to-northeast Chestnut Ridge anticline, which marks a major tectonic division on the plateau, and by the southeast-to-northwest “Gwinn-type” lineament, which marks a northward reduction in amplitude of the plateau folds. Coal rank is 1.6% R (low volatile bituminous) in the Somerset County block (associated with the 40 °C/km gradient), 1.3% R (medium volatile) in the block northeast of the lineament (centered in Clearfield County), 0.9% R (high volatile A) in the Westmoreland County block (associated with the 33 °C/km gradient) west of Chestnut Ridge, and 0.8% R (high volatile A) in the block northeast of the lineament (Clarion County). The rank decrease west across Chestnut Ridge occurs in a zone about 50 km wide. Width of the zone of northeastward rank that decreases across the lineament is not as well defined but is probably of the same magnitude. As burial depths of the blocks appear to have been similar, the primary cause for rank variation was variation in geothermal gradients between blocks. Anisotropy did not change significantly across the region, suggesting that it can be correlated with past depth of burial.