Anomalous Volume-Strain Relationships in Bituminous Coals

CODY, GEORGE D., and ALAN DAVIS, Pennsylvania State University, University Park, PA

Most bituminous coals are optically anistropic, a phenomenon that results from reversible strain of coal's macromolecular structure. Although this strain is reversible, it is known to be "locked" in place via intermolecular hydrogen-bonding interactions between phenolic and carboxylic acid functional groups. Thus, appropriate agents are necessary to relax the strained macromolecular structure. Such agents typically are basic organic solvents capable of neutralizing the acidic oxygen containing functional groups. Within coal-forming sedimentary basins, the nature of the external stress field responsible for the strain of coal is simple. Assuming that tectonic stresses are minimal, the stress regime is uniaxially compressive. Under such a stress field, a reversibly strained elastic body naturally is expected to lose volume. Therefore, upon relaxation, uniaxially strained coals would be expected to regain their original volume. When lower rank bituminous coals are relaxed in appropriate solvents, however, the net volume change is negative and they lose volume. This seemingly anomalous larger (strained) molar volume was presumably sustained in the presence of the lithostatic stress field by the presence of volatile (light hydrocarbons, CO2, and/or water vapor) trapped within the macromolecular network. Data on the volume-strain relationships of two coal suites from different sedimentary basins and across regional coalification gradients aids in illustrating potential mechanisms of volatiles storage as well as possible causes leading to the release of trapped volatiles.

AAPG Search and Discovery Article #91005 © 1991 Eastern Section Meeting, Pittsburgh, Pennsylvania, September 8-10, 1991 (2009)