Controls on Sodium Distribution in Fort Union Lignites

Robert B. Finkelman, Jesse D. Yeakel, Susan J. Tewalt

Lignite channel samples from the Fort Union Formation were collected from six mines in North Dakota and Montana. Chemical analyses indicate that the sodium (Na) content (on a coal basis) of the lignite beds at four of the mines increases with depth and ranges from about 900 to almost 8,000 ppm. The Na content of the immediate overburden is higher than the maximum Na content of the lignite. However, leaching experiments indicate that water in equilibrium with the high-Na overburden has the capacity to leach Na from the underlying lignites. Thus, leaching of Na from the lignites by downward percolating water may explain the Na profiles displayed by the lignites. All the Na in the lignite is ion exchangeable. However, the bond between Na and organic matter is weaker than the bond between Na and its inorganic host (primarily smectitic clays). Geologic factors that can modify Na concentration in the Fort Union lignites include (1) overburden thickness--for overburdens less than 40 ft thick, Na in the underlying lignite decreases as thickness decreases, (2) topography--Na is lower in lignites under topographic lows, (3) overburden type--Na concentrations may be lower in lignites below sandstones than in lignites below less permeable shales, (4) regional rainfall--Na is lower in lignites in areas of higher rainfall, (5) maceral composition--Na content of huminite macerals is substantiall higher than that of inertinite macerals, as determined by microprobe analysis, and (6) mineral content of the lignites--concentrations of Na-bearing clays (smectites) and feldspars (sanidine) vary between mines.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988