Porosity and Permeability in Bioturbated Sediments - Trace Fossils as Indicators of Sedimentary Environments - page 817

3.3 Burrow-Surface Area and Trace Architecture
The presence of permeable burrows within a comparatively tight matrix
increases the surface area of flow conduits within reservoirs. For instance,
the surface area represented by 100 Thalassinoides /m 2 , with 2 cm diameter
tubes that descend 50 cm into the substrate, is a little over 3 m 2 . This is three
times more than the horizontal section and six times the vertical section. A sim-
ple fracture would possess a surface area roughly equivalent to twice the cross-
sectional area of the block. It is easy to imagine extensive burrow-to-matrix
interactions with higher burrow densities.
Storativity within burrow fabrics is simply related to the volumetric fraction
the biogenic fabric occupies. Crustacean-generated traces, such as Thalassi-
noides and Ophiomorpha , can occupy more than 50% of the local sample vol-
ume. Macaronichnus may easily exceed 70% of the rock volume. Diagenetic
fabrics around burrows (e.g., dolomitized burrows) range up to 80% of the vol-
ume before the matrix is no longer discernible. These examples illustrate that
burrowed horizons not only change flow characteristics within a reservoir, but
also have an impact on resource production and reserve calculations as well.
In addition to its size, a burrow's overall architecture can influence the degree
of connectivity and tortuosity present in biogenically modified flow media
( La Croix et al., 2012 ). Branching burrows with vertical elements provide the
highest likelihood of developing an effective, isotropic network. A common
example of this is Thalassinoides . Many trace fossils do not exhibit branches,
and thus rely on chance interpenetration to connect the biogenic flow paths
(e.g., Zoophycos ). Cryptic bioturbation is so highly interconnected that the flow
medium is essentially isotropic and permeability-dead zones likely are rare.
4. ANALYTICAL METHODS
The assessment of ichnological permeability revolves around measuring the
permeability of matrix and trace fossils, and resolving the shape and distribution
of trace fossils for upscaling. In both areas, analytical techniques have a range of
limitations.
4.1 Permeability Assessment: Spot Permeametry
Permeability assessments most commonly are derived from spot permeametry,
which provides small-scale measurements that are not particularly accurate.
Plug, core, and bulk permeability measurements, which normally are some
modifications of a Darcy experiment, are time-consuming and technically prob-
lematic as summarized in Gingras et al. (1999) . Darcy experiments are not fur-
ther discussed here, because they are well outlined in other works (e.g., Freeze
and Cherry, 1979 ). The strategy for most ichnological spot-permeametry studies
is outlined below.