RICE, DUDLEY D., U. S. Geological Survey, Denver, CO

ABSTRACT: Composition and Origins of Coalbed Gas

Coalbed gases are quite variable in composition. In addition to methane, they can contain significant amounts of heavier hydrocarbon gases (C<2+>>20%) and carbon dioxide (>99%). Coalbed gases are also variable in their isotopic composition: (delta){13}C<1>:-70.4 to -16.8 ppt, (delta){13}C<2>: -29.2 to -22.8 ppt, (delta)D<1>:-333 to -117 ppt, and (delta)C<CO2> -26.6 to +18.6 ppt. The primary controls of hydrocarbon

gas composition are coal rank and composition and depth/temperature. Biogenic gas is generated by the degradation of organic matter at shallow depths and low temperatures in coals of any rank and is mainly methane. Thermogenic coalbed gas results from devolatilization of coal at ranks of high-volatile bituminous and higher. These gases can be wet at intermediate ranks (high- to medium-volatile bituminous) and are dry at higher ranks. (delta){13}C and (delta)D values become more positive with increasing rank. In addition, at intermediate ranks, hydrogen-rich coals generate wetter gases than do oxygen-rich coals.

Shallow coalbed gas is relatively dry with isotopically light methane as compared to gas from deeper coal, regardless of rank. This trend results from the original gases being altered by relatively recent bacterial activity (aerobic oxidation of heavier hydrocarbons and/or anaerobic generation of biogenic methane). This alteration occurs at depths <3,000 ft and is controlled by the physical characteristics of the coal beds, burial history, and groundwater flow.

Carbon dioxide generated during devolatilization is commonly not preserved in present-day coalbed gases because it is highly reactive and soluble in water. Significant present-day amounts of carbon dioxide can be the result of several processes not related to coalification, such as recent bacterial activity, thermal destruction of carbonates, and migration from magma chambers or the upper mantle.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.