Porosity and Permeability in Bioturbated Sediments - Trace Fossils as Indicators of Sedimentary Environments - page 825

Sand-filled
Thalassinoides

Zoophycos Silty shale matrix

FIGURE 9 Foram-filled Zoophycos (Zo), encased in a lower permeability matrix, form an impor-
tant part of the Sirasun Field in Indonesia. The burrows are further connected by Thalassinoides
and Chondrites ; thus, the biogenic reservoir is largely isotropic and is a prodigious natural gas
producer.

continuous. Stratal units characterized by non-SCD can exceed several square
kilometers in area ( Pemberton and Gingras, 2005 ).

4.3.2.1 Lysing Formation (Cretaceous), Norwegian Sea
Within the Late Cretaceous Lysing Formation, biogenic fabrics appear to
enhance the reservoir permeability significantly. The Lysing Formation is com-
posed of interbedded very fine- to fine-grained sandstone, siltstone, and shale
that contain a diverse trace-fossil suite. Sediment accumulation is the result of
alternating fine- and coarse-grained sedimentation within a mid- to outer-shelf
setting. The trace-fossil assemblage includes Thalassinoides , Planolites , Scoli-
cia , Chondrites , Palaeophycus, and Zoophycos (i.e., distal Cruziana Ichnofa-
cies). The trace fossils commonly contain coarser sediment (fine-grained
sandstone) than the shale matrix.
Spot-permeability data taken from core datasets indicate that the burrow
permeability can be up to two orders of magnitude higher than the matrix, thus
proffering a dual-permeability flow medium. Importantly, bulk assessments
of permeability based on core-plug data indicate that the harmonic mean of
matrix versus burrow permeabilities provides the most accurate estimate in
the Lysing example ( Fig. 10 ). The harmonic relationship suggests that the
flow is largely directed across the lower permeability matrix from burrow
to burrow. This is, in fact, characteristic of NCD dominated by sparse