Abstract
M.Sc.
The potential target areas for coalbed methane in South Africa
are reviewed and a case study based on borehole data from the
north-eastern Karoo basin was undertaken. The Early Permian
coal seams of the Karoo Supergroup occur in several discrete
sedimentary basins in South Africa, of which the Karoo basin is
the largest. Using screening criteria based on geological,
petrographical and analytical data some of the coal deposits can
be excluded as potential coalbed methane producers. These
include the Molteno Coalfield, large parts of the Karoo basin
coal deposits and some of the Northern Province's coal deposits
which are structurally disturbed. The traditional mining areas
in the Free State, Witbank and Highveld coalfields are excluded
from the coalbed methane study because the target seams occur
at less than 200 metres below surface, too shallow for gas
retention. Some of the coal seams in the Waterberg Coalfield
occur at depths of several hundred metres below surface and
these are unlikely to be mined by conventional means. These
deep coals may be ideal coalbed methane producers. This
regional overview was based on available, published data and
two important parameters, permeability of coal and coalbed
hydrology are unknown but important factors that will need to
be taken into account in any future evaluations.
The case study focused on an area close to Amersfoort that has a
predicted potential for coalbed methane production. The study
utilized 465 borehole descriptions from which isopach maps
and geological cross-sections were constructed. Limited samples
of borehole core provided lithological information from which
a facies analyses was undertaken so as to establish the
hydrodynamic origin of each facies types. The main lithofacies
associated with the coal seams are mudstones, carbonaceous
shales and fine- to coarse-grained sandstone. These data were
combined with analyses from limited permeability data,
petrographical data and proximate analyses for the Gus and
Alfred seams. In addition to the sedimentary rocks, the role of
dolerite intrusions was found to be significant as these occur as
thick sills and dykes that occur below, within and above the
coal seams. These may compartmentalize the seams into
secondary targets within the study area. Thick sills overlying
the coal zone also increase static loading and may be
advantageous with respect to reducing the minimum depthbelow-
surface requirements. Potential coalbed methane target
areas are identified, although the entire study area is not
suitable due to structural displacement of the coal seams,
thinning of coal in places and devolatization caused by the
dolerites.