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The Relationship Between Coal Rank and Clear Density--A Preliminary Report

LAW, B. E., U.S. Geological Survey, Denver, CO

The economic production of gas from coal is highly dependent on the development of permeability in the cleat system, yet the characteristics of cleats, from a petroleum geology perspective, are poorly known. Measurements of coal cleat density in outcrops and cores from Upper Cretaceous and Tertiary coal beds in Rocky Mountain basins indicate clear density increases with increasing coal rank. The density of face cleats ranges from 0.5 cleats/cm in lignites [mean random vitrinite reflectance (Rm) 0.28%] to 13.0 cleats/cm in medium volatile bituminous (1.54% Rm) coals. Butt cleats exhibit a similar relationship over this range of coal rank. Crossplots of Rm vs. cleat spacing and Rm vs. bed moisture are nearly coincident, suggesting that shrinkage of the coal due to moisture loss is the m st likely cause of changes in cleat spacing. Within any one coal outcrop or core, cleat density is highly variable and is, in part, dependent on coal quality; the more dense cleating is associated with bright-banded coal, the less dense with dull-banded coals and shaly coals. Although cleat density is not the only factor affecting permeability (cleat width and connectivity are other factors), the preliminary results of this study indicate that cleat density can be quantified, thus providing a partial solution to predicting cleat permeability.

 

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)