Coals hold a unique position in conventional and unconventional
hydrocarbon systems in that organic matter forms the bulk of the rock. Gases within
coals have a dominant storage mechanism of adsorption within the fundamental
structure of the organic matter rather than the conventional state as a compressed gas in
intergranular spaces or dissolved within liquid hydrocarbons. Gases form an integral
part of the coal substance. Multiple phases of gas generation are the norm and result in
complex relationships between gas content and organic petrology. Thermogenic gases
are produced during normal burial metamorphic processes. Biogenic gases are
produced during earliest coalification as well as post-coalification, after uplift.
Understanding the variability of the organic matter is key to understanding the
variability in gas content and gas composition. Coal type is determined at the peat stage
and strongly influences mechanical properties of the coal, amount and type of gas
generated, timing of gas generation and storage capacity. Coal type needs to be studied
at the macroscopic level through lithotype analysis; at the microscopic level via
maceral analysis and; geochemically. Rank studies by vitrinite reflectance are preferred
over bulk geochemical techniques and provide insights into the timing and generation
of thermogenic gases. Petrographic studies can also be used to detect the presence of
igneous intrusion, which affects both gas composition and storage capacity. The
importance of individual, detailed basin studies cannot be overstated. Attempts to make
global generalizations usually fail due to the complexity of the starting material
coupled with unique geological histories.
Keywords: CBM, Lithotype, Maceral, Coal type, Coal rank, Gas variability, Gas
content, Methane.
