Characterization of the Light Oil and Heavy Hydrocarbon Content and Composition of Coal Seam Gas Reservoirs

Wenbao Li, Iulia M. Lazar, Abdul Malik, Milton L. Lee, Charles R. Nelson

Coal seam gas reservoirs contain complex mixtures of hydrocarbon constituents. Although only gaseous (C_1-5) hydrocarbons are commercially producible, other hydrocarbons are known to be present since gas chromatograms of solvent extracts of reservoir coals generally exhibit prominent molecular signatures of C₁₅₊ n-alkanes, pristane, phytane, and alkyl substituted aromatics. By contrast, relatively little is known about the content and composition of light oil (C_6-14) hydrocarbons in these reservoirs due to their preferential susceptibility to volatility fractionation and loss during the solvent extraction and evaporation procedure.

We report the development and use of supercritical CO₂ extraction and chromatography techniques to quantitate and characterize the C_6-14 hydrocarbon components in 15 U.S. coals of varied geologic ages and a wide thermal maturity range (lignite through low-volatile bituminous). The C_6-14 hydrocarbon analysis data are compared with Soxhlet solvent extract analysis data for the C₁₅₊ components.

In the C_6-14 hydrocarbon fractions, the aliphatic components predominate over the aromatics while the opposite is the case in the C₁₅₊ hydrocarbon fractions. The C_6-14 aliphatic hydrocarbon content data ranged from 0.02 to 0.20 weight % of the bulk organic matter in the rock. In higher rank coals, the C_6-14 aliphatic components were generally more abundant than the heavier C₁₅₊ aliphatic components. The identified C_6-14 components include homologous series of n-alkanes, branched and cyclic alkanes, alkyl benzenes, and alkyl naphthalenes.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995