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of Fabric and Composition on the
Stress Sensitivity of Permeability
in Some Coals, Northern Sydney Basin, Australia: Relevance to
Coalbed Methane Exploitation1
R. M. Bustin2
Fabric and composition of a series of Upper
Permian high-volatile to low-volatile bituminous coals of the Sydney basin
have a marked effect on stress sensitivity of permeability, and thus on
the reservoir characteristics of the coal. The coals vary in composition
from end members of predominantly bright-banded coal comprised mainly of
the microlithotype vitrite and the maceral vitrinite, to dull coal composed
of significant amounts of ash, inertinite group macerals, and the microlithotype
inertite. The brighter coals are more extensively fractured with one or,
more commonly, two or three regularly spaced fracture sets (cleats) spaced
at 5-20 µm. Fusinite and semifusinite, common macerals in the dull
coals, are characterized by phyteral porosity (mainly cell lumens) and
fabric- selective intergranular porosity.
The permeability of tested samples varies
significantly with composition and effective stress. The fabric of the
samples is the most important factor in determining permeability and stress
sensitivity of permeability. Coals with the highest permeability are those
with at least one well-developed, throughgoing fracture set; these samples
generally include abundant vitrite bands. The lowest permeability samples
are nonbanded, with an attrital fabric and significant authigenic mineralization.
1997. The American Association of Petroleum Geologists. All rights reserved.
received April 1, 1996; revised manuscript received December 4, 1996; final
acceptance June 16, 1997.
of Earth and Ocean Sciences, University of British Columbia, Vancouver,
BC V6T 1Z4, Canada.
study was financially supported by a grant from CSIRO (Syndal) and by The
Earth Resources Foundation, University of Sydney. Samples and some data
were provided by Pacific Power Ltd., Sydney. I thank Paul Gamson, formerly
of CSIRO, for overseeing the permeability analyses and facilitating some
of the study. I thank K. Wright of the Earth Resources Foundation, University
of Sydney; J. Enever of CSIRO; J. C. Close of Meridian Oil; and Chris Clarkson
of University of British Columbia for their comments on an earlier draft
of this paper. Reviewers Andrew Scott, Roger Taylor, and W. Ayers are thanked
for their thoughtful comments.
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