Porosity and Permeability in Bioturbated Sediments - Trace Fossils as Indicators of Sedimentary Environments - page 833

hydrocarbon production ( Gordon et al., 2010; Pemberton and Gingras, 2005;
Tonkin et al., 2010 ). It is therefore important to interpret massive sandy beds
in deep-water successions accurately, in order to create reliable depositional or
reservoir models and to better anticipate a more complex porosity and permeabil-
ity network in seemingly homogenous media.
Cryptic bioturbation (in sandstone) is common in clastic estuarine and dis-
tributary channels, as well as in upper shoreface deposits. More recently,
Greene et al. (2012) identified cryptic bioturbation as a common feature of
Neogene turbidite channels. The aforementioned depositional environments are
dynamic, thus their thicknesses and lateral extents are limited. In general, this
facies is 10 cm and 10 m thick. Amalgamated parasequences may generate
stacked successions that are almost 100 m thick.
4.3.4.1 Bluesky Formation (Cretaceous), Alberta, Canada
Gordon et al. (2010) studied in detail the petrophysics of Macaronichnus and
cryptically bioturbated sandstone. Their study focused on shoreface deposits
of the Early Cretaceous Bluesky Formation in western Alberta, Canada. The
studied reservoir comprises lithic sandstone and several levels that were heavily
bioturbated with Macaronichnus segregatus . Petrographic techniques were
used to assess the effects of M. segregatus on reservoir quality. Thin-section
analysis and SEM (Scanning Electron Microscopy) have shown that the
tracemaker of M. segregatus avoided iron-rich detrital fragments while exploit-
ing the sediment for food ( Fig. 14 ). Grain avoidance led to a re-sorting of the
sand such that the dark-colored grains outline the burrows, and light-colored
competent grains were dominant within the burrow fill. Notably, the light-
colored burrow fills contain a higher chert/quartz ratio. Owing to the fact that
pore-occluding quartz overgrowths do not form on chert fragments, the primary
porosity and permeability were preferentially preserved within the burrow fills
( Fig. 15 ). Gordon et al.'s (2010) study further showed that spot permeability in
the Macaronichnus -burrowed zones was as much as four times higher than in
laminated sandstone of similar grain size ( Fig. 15 ).
Spot permeability (mD)
Fine-to medium-grained,
intensely bioturbated sandstone
Fine-to medium-grained,
cross-stratified sandstone
FIGURE 15 Gordon et al. (2010) showed that permeability was slightly enhanced in Macaronich-
nus -burrowed sand compared to laminated sand, and that the burrowed sediment had less variance.
The above data are derived from a single-cored interval of the Bluesky Formation, Cretaceous,
Alberta (Canada).