Abstract

Vitrinite maximum reflectance trends for the Middle Pennsylvanian Springfield (No. 9) coal bed in the Western Kentucky coalfield indicate that coal rank is high volatile A bituminous (0.70-0.85% R_max) in portions of the Webster syncline in Webster and Union counties (south of the Rough Creek fault zone) and is generally high volatile C bituminous (<0.60% R_max) in the remainder of the coalfield. Within the Webster syncline, the reflectance of the Springfield coal does not vary significantly despite a difference in elevation of nearly 300 m. Reflectance gradients in the Webster syncline are not well defined; reflectances are generally highest in the Baker (No. 13) coal bed and are lower in the underlying Herrin (No. 11) and Briar Hill (No. 10) coals. Rank trends from the Springfield data alone suggest pre-tectonic coalification, but the lack of well-defined reflectance gradients complicates this interpretation. Hydrothermal alteration may have been a significant cause of the coalification in this region. Previous investigations of coals in fault blocks have considered the role of hydrothermal mineralization in coalification. Southern Union and Webster counties are adjacent to the Fluorspar District, a region of extensive hydrothermal metamorphism. It is possible that convective heat transfer related to Fluorspar District mineralization led to vertical heterogeneity in the temperature profile and consequent metamorphic history. In Henderson County, north of the Rough Creek fault zone, coal rank is significantly lower, as low as 0.42% R_max for the Springfield coal. Reflectance gradients are, again, poorly defined.

Extensive oil and gas development has occurred in the high volatile C bituminous region to the north of the Rough Creek fault zone, whereas fewer pools, with lower production, are known within the Webster syncline to the south of the fault zone. The rank of the Middle Pennsylvanian coals can be used to estimate the level of maturation of the Devonian New Albany shale, a likely source rock for much of the oil and gas in the coalfield. Previous studies on the maturation of the New Albany shale, which lies about 1 km below the Springfield coal, indicate an equivalent medium volatile bituminous (1.0-1.2% R_m) rank in the Fluorspar District. New Albany rank decreases to an equivalent high volatile B/C (0.60% R_m) north of the Rough Creek fault zone. The significance of the New Albany reflectances is dependent on the level of suppression of vitrinite reflectance in organic-rich shales. The existence of reflectance suppression would imply that the shales could be more mature than studies to date have indicated. It is possible that the episodes of basin dewatering which influenced the coal rank south of the Rough Creek fault zone also flushed the potential oil reservoirs in the same region.