Porosity and Permeability in Bioturbated Sediments - Trace Fossils as Indicators of Sedimentary Environments - page 834

4.3.4.2 Ula Formation (Jurassic), Offshore Norway
Within the aforementionedUla Formation, the influence of cryptically bioturbated
sandstones on reservoir quality can be demonstrated. The studied reservoir units
comprise upper fine- to lower medium-grained sandstone that contains rare sedi-
mentary and biogenic structures. Themassive appearance of the intervals is attrib-
uted to cryptobioturbation. A petrographic thin-section study showed that the
porosity is enhanced slightly (by 5-10%) within the highly bioturbated sections.
Spot permeametry revealed uniform permeability distributions ( Fig. 14 ), which
explain the correspondence of permeability to the geometric mean ( Fig. 11 ).
4.3.5 Diagenetic Textural Heterogeneities
Diagenetic textural heterogeneities (DTH) occur typically as bioturbated,
dolomite-mottled limestones or as nodular sandstones/siltstones. With carbon-
ates, the dolomitic mottles result from dolomitization of calcareous sediment
adjacent to body fossils, sedimentary lamination, and trace fossils ( Fig. 6 E;
Abdel-Fattah et al., 2011; Corlett and Jones, 2012; Gingras et al., 2002a,
2004a,b; Kendall, 1977; Zenger, 1992, 1996 ). Another important diagenetic
process is fabric-selective carbonate dissolution, which is discussed in detail
by Cunningham et al. (2012) .
The presence of fabric-selective dolomite in association with trace fossils is
attributed to chemical and physical alteration of the substrate by burrowing
organisms. Physical alteration of the substrate includes modification of the
grain size, redistribution of grains, compaction, and sorting. Compositional het-
erogeneity is due to the incorporation of localized, concentrated organic mate-
rial in the form of mucous or fecal material. It is most likely that physical and
chemical factors together determine the resulting fabric in burrow-mottled
limestones ( Gingras et al., 2004b ).
Burrow-associated cement precipitation and nodule formation have four
main modes of occurrence: (1) cementation of a finer-grained burrow wall, such
that siderite, dolomite, calcite, or pyrite occur as very fine-grained crystals in
the burrow-wall material (preferred tube cementation); (2) cement precipitation
or cement dissolution within a coarser-grained burrow fill, in which mosaic cal-
cite is distributed heterogeneously throughout the burrowed medium (preferred
burrow cementation); (3) cement—usually dolomite or calcite—precipitation
or cement dissolution adjacent to a trace fossil, forming a diagenetic halo
around discrete trace fossils (fabric-mimicking hypoburrow cementation);
and (4) concretion formation, usually calcite and rarely dolomite, with a trace
fossil as the nucleation point (nodular hypoburrow cementation). This range of
diagenetic-ichnological associations shows that the impacts of diagenesis are
extremely variable.
In most cases, burrow-induced diagenesis profoundly alters the rock's pet-
rophysical characteristics. The permeabilities and porosities of burrow-affected
and unburrowed zones are commonly different (e.g., Knaust, 2009 ), and