Porosity and Permeability in Bioturbated Sediments - Trace Fossils as Indicators of Sedimentary Environments - page 827

4.3.3 Weakly Defined Textural Heterogeneities
Sedimentary media that exhibit less than two orders of magnitude between
the trace-fossil and matrix permeabilities comprise weakly defined textural
heterogeneities (WDH) (i.e., dual-porosity system). This class of flow media
dominantly comprises sand-filled burrows within a sandstone matrix or very
fine sand in siltstone matrices ( Figs. 6 and 11 ). In examples of WDH, the sorting
of the burrow infill tends to be better than the matrix, and the grain packing is in
some cases likely to be looser—especially with passively infilled burrows. Bio-
genic permeability modification ascribed toWDT most commonly is associated
with the presence of Planolites , Thalassinoides , Skolithos, and some expres-
sions of Macaronichnus . In the light of their branching morphology, deeply
penetrative nature and passive infill, Thalassinoides and Ophiomorpha are
likely the most important permeability-enhancing trace fossils, although
Tonkin et al. (2010) observed permeability reduction with Ophiomorpha as
well. La Croix et al. (2012) and Lemiski et al. (2011) drew attention to another
style of WDH comprising Phycosiphon -dominated intervals in siltstones. In
these examples, the small trace fossils provided additional flow paths for natural
gas to reach both the well bore and bedded reservoir units.
WDH are mostly associated with lower shoreface, sand-rich proximal off-
shore settings, and various bay-margin deposits ( Pemberton and Gingras,
2005 ), with Phycosiphon -bearing (impoverished Cruziana Ichnofacies) examples
largely identified within offshore siltstones. Sand-dominated intertidal-flat
deposits should be included in WDH, but no studies have been conducted to
demonstrate this. Due to the variable sedimentary conditions present in lower-
shoreface and (particularly) tidal-flat deposits, the thicknesses and lateral extents
of associated facies are generally limited. In strata hosting these fabrics, affected
beds and bedsets reach thicknesses of up to 10 m ( Pemberton and Gingras, 2005 ).
The lateral extents of subtly burrowed facies range between hundreds of meters
(tidal-flat settings) and several kilometers (offshore associations).
4.3.3.1 Ula Formation (Jurassic), Offshore Norway
Located in the north-eastern corner of the Norwegian Central Graben, the Late
Jurassic (Kimmeridgian-Oxfordian) Ula Formation hosts significant hydrocar-
bon fields within the oil-rich Ula Trend ( Spencer et al., 1986 ). Interpreted as a
storm-influenced, fault-controlled shoreface deposit, hydrocarbon production
from the Ula Formation occurs primarily from bioturbated units. Spot permea-
metry, Micro-CT, and Visual Modflow modeling of the Ula Formation demon-
strated that the permeability enhancement within Ophiomorpha -dominated
sandstone intervals represents a dual-porosity network ( Fig. 11 ). The distal
Skolithos Ichnofacies, that is gradational with proximal expressions of the
Cruziana Ichnofacies, constitutes a good example of a dual-porosity rock fabric
that can be categorized using the arithmetic mean. An example includes hori-
zontally oriented networks of Ophiomorpha with high bioturbation intensities