Preliminary report on methodology for calculating coal resources of the Wyodak-Anderson coal zone, Powder River basin, Wyoming and Montana

The National Coal Resource Assessment of the Wyodak-Anderson coal zone includes reports on
the geology, stratigraphy, quality, and quantity of coal. The calculation of resources is only one aspect
of the assessment. Without thorough documentation of the coal resource study and the methods used, the
results of our study could be misinterpreted. The task of calculating coal resources included many steps,
the use of several commercial software programs, and the incorporation of custom programs. The
methods used for calculating coal resources for the Wyodak-Anderson coal zone vary slightly from the
methods used in other study areas, and by other workers in the National Coal Resource Assessment.
The Wyodak-Anderson coal zone includes up to 10 coal beds in any given location. The net coal
thickness of the zone at each data point location was calculated by summing the thickness of all of the
coal beds that were greater than 2.5 ft thick. The amount of interburden is not addressed or reported in
this coal resource assessment. The amount of overburden reported is the amount of rock above the
stratigraphically highest coal bed in the zone. The resource numbers reported do not include coal within
mine or lease areas, in areas containing mapped Wyodak-Anderson clinker, or in areas where the coal is
extrapolated to be less than 2.5 ft thick.
The resources of the Wyodak-Anderson coal zone are reported in Ellis and others (1998). A
general description of how the resources were calculated is included in that report. The purpose of this
report is to document in more detail some of the parameters and methods used, define our spatial data,
compare resources calculated using different grid options and calculation methods, and explain the
application of confidence limits to the resource calculation.