just because there is coal doesn't mean its suitable for gas extraction...The potential of a particular coalbed as a CBM source depends on the following criteria.
Burial depth- of coal seams influences coal rank, capacity of gas generation, gas content and preservation. Seams buried to a great depth generally have low permeability leading to higher development cost whereas seams with too small a burial depth could be too low in rank and gas generation capacity.
Thick coal seams generate large amount of coalbed methane and also have a large volume for coalbed methane preservation. Thin coal seams generally do not allow for horizontal drilling and can be unproductive...
Cleat density/intensity: cleats are joints confined within coal sheets. They impart permeability to the coal seam. A high cleat density is required for profitable exploitation of CBM. Also important is the maceral composition, a high vitrinite composition is ideal for CBM extraction, while inertinite hampers the same.
The rank of coal has also been linked to CBM content: a vitrinite reflectance of 0.8-1.5% has been found to imply higher productivity of the coalbed.
Last but not the least, the gas composition must be considered: the methane composition of the extracted gas should not be less than 92%.
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