Evidence of Coal Composition and Coalbed Methane (CBM) Source and Reservoir Potential in Coal Cores

Judith Potter¹ and David Marchioni^2
1 JP Petrographics, Calgary, AB
² Petro-Logic Services, Calgary, AB

Coal is both the source rock & reservoir for coalbed methane (CBM). Source rock potential is typically assessed by desorption testing of coal core or cuttings. Reservoir potential is ultimately assessed by production testing.

The cores cut for gas content testing can also provide valuable information on reservoir potential and/or differences in reservoir potential/performance between wells.

Coal is a bedded organic sediment and contains laminae that vary markedly in macroscopic brightness. Coal lithotypes are the small sub-sections of the seam within which the proportions of these bands are relatively uniform, but significantly different from the proportions in bounding beds. Lithotypes reflect the composition of coal – the proportions of macerals (organic micro-components) and minerals in different portions of a seam. The lithotype profile of a seam reflects the variation in depositional conditions in the precursor mire.

Lithotypes influence gas content as different coal macerals have different potential to generate and store gas. In addition, lithotypes influence cleat development, with cleat density decreasing as the brightness of coal decreases.

The fractures in coal (aka cleat) are the principal permeability pathways. Description of cleat spacing, length and mineral infilling offers insight to the permeability potential of the coal. Cleat development is influenced by coal rank (aka maturity) and by coal composition, which is reflected in visible lithotypes. However, lateral permeability at lithoptype boundaries tends to be underestimated. Examples of micropermeameter tests on slabbed and polished coal cores suggest that permeability at lithotypes boundaries is significant Cores will be presented from coals of different rank and from the major coal zones that are targets of CBM exploration, to illustrate features of lithotype and cleat variation. Detailed lithotype/cleat profiles will be displayed alongside the cores.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005