Porosity and Permeability in Bioturbated Sediments - Trace Fossils as Indicators of Sedimentary Environments - page 836

massive. Recrystallization associated with dolomite formation generated intra-
crystalline porosity.
4.3.5.3 Palliser Formation (Devonian), Alberta, Canada
Another excellent example of burrow-selective dolomitization concerns the
Devonian Palliser Formation (Wabamum Group in the subsurface) of the West-
ern Canada Sedimentary Basin ( Fig. 6 ). These rocks are composed of a series of
highly bioturbated carbonate fabrics interpreted to represent a series of shallow-
water ramp deposits. Spot permeametry of samples indicated that the burrows
are preferentially higher in permeability than the matrix. In general, the samples
have a low-permeability limestone matrix with negligible effective porosities.
In turn, the limestone encompasses chaotically distributed, fabric-selective
dolomite with an effective porosity as high as 5-6% ( Gingras et al., 2004b ).
Analysis of SEM micrographs has indicated that preferential dissolution and
precipitation of planar dolomite crystals within and adjacent to burrow fabrics
and physical sedimentary structures occurred. This implies that bioturbation
influenced the pore-water geochemistry, and in so doing encouraged dolomiti-
zation.
5. CONCLUSIONS
Biogenic fabrics in sedimentary rocks can strongly influence the permeability
fabric and therefore fluid flow in sedimentary strata. Bioturbation may
impose well-defined, highly contrasting permeability fields, referred to as dual
permeability, or subtly contrasting permeability fields, referred to as dual poros-
ity. Both types of flow media influence the reservoir quality of fluid-bearing
rocks. Dual porosity leads to (1) the entire rock contributing to fluid or gas pro-
duction; (2) in the presence of more than one fluid phase, flow is focused
in higher porosity/permeability zones; and (3) moving of the fluid in the
lower-permeability portions of the rock into higher-permeability media through
diffusion and advection (mechanical movement).
Dual-permeability flow media have poorer resource characteristics, result-
ing in (1) burrows comprising the effective flow medium present, (2) fluid
resources being more slowly produced by diffusion from the tighter rock,
and (3) secondary recovery techniques possibly isolating large parts of the flow
network. Other factors that influence the overall behavior of the flow-medium
class are the burrow density, burrow connectivity, burrow/matrix permeability
contrast, burrow-surface area, and burrow architecture. For the most part, these
parameters can be assessed and their impact estimated.
Dual porosity and permeability fabrics are a component of the burrow-media
classification presented in Pemberton and Gingras (2005) : (1) SCD, (2) NCD,
(3) non-constrained subtle heterogeneities, (4) CBH, and (5) diagenetic hetero-
geneities.