Porosity and Permeability in Bioturbated Sediments - Trace Fossils as Indicators of Sedimentary Environments - page 814

ichnofacies, but dominantly Zoophycos , Thalassinoides, and Planolites (dis-
cussed in Section 4.3.2.). Sedimentary environments associated with biogenic
dual porosity are offshore to lower shoreface, heterolithic tidal bars, and at the
base of transgressive successions ( Fig. 3 ). In the case of carbonate successions,
burrow-associated diagenesis has been associated with carbonate accumulation
in restricted platform and low-energy ramp settings ( Beales, 1953; Corlett and
Jones, 2012; Gingras et al., 2004b; Knaust, 2009 ).
Burrow-induced heterogeneity normally results in the presence of complex,
two- or three-dimensional flownetworks.Where the burrowpermeability is mark-
edlyhigher than thematrix, complex ichnofabrics are prone to flow-conduit cut-off
and a volume of the transmissive rock is rendered ineffective. In these cases, the
assessment of the ineffective flow volume is difficult to determine. Other major
problems include fabric upscaling and determination of the three-dimensional fab-
ric. The latter issue is solved inpart through imaging techniques, such asComputed
Tomography (CT) and Magnetic Resonance Imaging (MRI) (discussed below).
Regarding the former, pervasive fabrics, includingcrypticbioturbationandgeneral
biogenic mixing, are amenable to upscaling. However, the upscaling of biogenic
fabrics composed of discrete trace fossils still remains a leap of faith.
Increased complexity of flow paths results in higher tortuosity, which leads to
higher dispersivity, which is the rock property related to hydrodynamic spreading
of a solute or phase. Low dispersivity means that a fluid can pass through the
porous mediumwithminimal (primarilymechanical) mixing of fluids and gasses,
and is a characteristic of homogeneous flow media. High dispersivity indicates
more flow mixing, which is related to increased rock heterogeneity.
The impact of bioturbation on dispersivity was demonstrated by Gingras
et al. (2004a) . Using laboratory experiments and a burrowed (dual porosity)
limestone medium, they showed that gas was transported rapidly through the
higher-permeability burrows, whereas the matrix contributed minimally to bulk
flux. The dispersivity of the burrowed limestone was higher than other flow
media tested (sandstone and fractured limestone), an outcome of the high
heterogeneity of the burrowed medium. This is important because dispersivity
partly determines secondary recovery efficiency ( Gingras et al., 1999, 2004a;
Pemberton and Gingras, 2005 ; Fig. 4 ).
Simple ichnofabrics have been numerically modeled using flow-simulation
software (e.g., Gingras et al., 1999 ), but complex ichnofabrics (i.e., more than
three trace-fossil types and variably permeable burrow fills) have proven diffi-
cult to model.
3. FRAMEWORK FOR ASSESSING BURROW-ASSOCIATED
PERMEABILITY
A number of factors strongly influence bulk-flow parameters. These are perme-
ability contrast, bioturbation intensity, burrow connectivity, burrow-surface area,
and burrow architecture.