Environmental Engineering Reference
In-Depth Information
the pore-throat distribution and by acting as
loci
of cementation and dissolution
processes during early and late stages of diagenesis. These processes commonly
act together, and the range of permeability fabrics generated by biogenic modi-
fication is broad.
Factors such as the morphology of the original burrow(s), the impact of
bioturbation on grain-size distributions, the distribution of organics within incipient
trace fossils, the mineralogy of fossils and grains, and a host of post-depositional
chemical processes determine the distributions of porosity and permeability (sum-
marized in
Lemiski et al., 2011; Pemberton and Gingras, 2005; Tonkin et al., 2010
).
Burrowed fabrics are commonly three dimensional in their geometry and burrow
shapes and sizes are variable. Thus, bioturbated sediment is more spatially complex
than bedded sediment, and flow through it is difficult to model (see
Knaust, 2012
).
Even very simple fabrics contain a range of burrow forms, and it is difficult to sta-
tistically characterize that range (cf.
McIlroy, 2007
). An important outcome of
burrow-associated permeability is that burrows tend to improve the vertical perme-
ability (
k
v
) over the horizontal permeability (
k
h
). In unfractured laminatedmedia,
k
h
is almost always larger than
k
v
.
Many oil and gas reservoirs contain zones that are bioturbated, or are
entirely composed of burrowed strata. In such cases, ichnological permeability
and porosity likely contribute to storativity and deliverability of resource,
and important relationships, such as
k
v
versus k
h
are altered. The analysis of
bioturbated strata in hydrocarbon reservoirs should constitute part of reservoir
analyses. However, due to the aforementioned complexities, this seldom occurs.
This chapter provides a summary of the concepts associated with the
applications of ichnology to reservoir fluid flow. We focus on the nature of
permeability and porosity and scrutinize the ways in which burrows might influ-
ence those parameters. Physical and chemical modifications resulting from bio-
turbation are discussed and their relationships to a classification system for
burrow-related permeability are considered.
2. THE NATURE OF ICHNOLOGICAL PERMEABILITY
Trace-fossil permeability is broadly comparable to fracture permeability in that
trace fossils are discrete entities displaying predictable permeabilities that, due
to their large surface areas, have significant flow interactions with the surround-
ing matrix. There are differences: trace fossils are tubular, trace-fossil perme-
abilities are lower than (open) fracture permeabilities (
Gingras et al., 1999
), and
the volume comprising the biogenic fabric is commonly much higher than
the volumes of fracture systems (10-100%
versus
1-3%). Flow interactions
between burrow fabrics and matrix are therefore more extensive than those
between fractures and their hosting matrix. The burrow distribution can also
be much extensive, whereas fractures present localized permeability streaks.
The permeability/porosity modification associated with burrow fabrics
ranges between severe reductions of the bulk permeability and notable
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