Abstract

The coal resources of the southern (Grand County) part of the Sego coalfield are found in seven Neslen Formation coal beds, which are named (in ascending stratigraphic order) the lower, green, Ballard, Chesterfield, lower carbon, upper carbon, and top. About 71% of the available coal resource occurs in the lower carbon bed, 12% is in the upper carbon, and the remaining 17% is distributed among the other five beds. The coal occurs at depths ranging from surface exposures to over 3000 feet in the northwestern part of the study area; the deepest coal occurs below minable depths, but may be a suitable reservoir for production of coal-bed methane. The coal beds are best developed in a belt that parallels the current outcrop, which approximates the paleoshoreline during Neslen deposition in the Late Cretaceous.

The sulfur content of coal in the Sego coalfield is low, and typically varies between 0.6 and 0.7%. With an average heating value of about 11,850 Btu per pound, these coals just meet the compliance level of having less than 1.2 pounds sulfur per million Btu (lbs S/106 Btu). Available data indicate that coal in the Neslen Formation contains more inertinite than the average Blackhawk Formation coal. This may be the result of the more frequent periods of swamp drying due to the more frequent shoreline fluctuations. The rank of the near-surface coal is high volatile C or B bituminous based on chemical analyses and vitrinite reflectance data (Ro = 0.54 to 0.70%). Vitrinite reflectance data from oil and gas as well as coal test-hole samples, show that the coal rank generally increases with burial depth toward the center of the Uinta Basin, reaching high volatile A bituminous rank (Ro = 0.71 to 0.84%) at depths of about 5000 feet. While lower than the reflectance level needed for maximum gas generation (Ro = 1.2%), these coals are sufficiently mature to be in the early stage of thermogenic gas generation.

Over 6.8 billion tons of Neslen Formation coal has been identified in the 16-quadrangle study area in beds at least one foot thick, of which about 1.1 billion tons is in beds more than four feet thick and under less than 3000 feet of overburden. Almost half of this 4-foot-thick coal (555 million tons) would be off-limits for development because of Wilderness Study Area land-use conflicts, and another 1% is encumbered by current natural gas developments. The remaining 578 million tons is available for mining. Considering coal bed thickness, distribution, and current mining practices, we estimate that about 125 million tons of the 193-million-ton available lower carbon coal resource might be recovered from the southern Sego coalfield in Grand County.