The Garn Formation (Bajocian-Bathonian) in the Kristin Field, Halten Terrace: its origin, facies architecture and primary heterogeneity model - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 539

Smørbukk Field by Corfield et al ., 2001); (3) the
individual grabens and half-grabens of the seaway
would likely differ in their internal pattern of sub-
sidence and hence the parasequence stratigraphy
of the Garn Formation may not be regionally cor-
relative; and (4) the eastward transfer of sand
across these tidal current-dominated depressions
depended on their accommodation space and was
probably highly non-uniform on regional scale.
The revised model for the primary heterogene-
ity and permeability compartmentalisation of the
Garn Formation in the Kristin Field (Fig. 12) may
explain why the reservoir's initial pressure has so
rapidly declined (Quin et al ., 2010) and the recov-
ery factor has been considerably lower than in
other gas-bearing and condensate-bearing fields.
The reservoir's overall permeability is low. The
release of fluids from the more permeable reser-
voir compartments (tidal sandstone ridges) is
probably hindered by their high degree of inter-
nal heterogeneity and baffled by the surrounding
less permeable facies, which delays the spatial
transfer of fluids and equalisation of reservoir
pressure. Considerable amounts of condensate
liquid may be entrapped by capillary forces in
these compartments at the early stage of produc-
tion, as the background pressure decreases and
pore-water saturation rises. This aspect of the
recovery efficiency should thus be simulated and
carefully assessed with the aid of the quantitative
model provided by the present study. In addition,
a possible role of faults (Fig. 4) in secondary com-
partmentalisation of reservoir should also be
taken into account.

sand ridges, accompanied by episodic storm action
and concurrent sand accumulation in inter-ridge
swales. Fairweather wave action predominated as
the accommodation was gradually filled up by sea
floor aggradation, until a new increase in accom-
modation occurred due to tectonic subsidence.
Littoral wave-dominated sedimentation eventu-
ally prevailed, but soon retreated onto the graben
flank as the regional rise in relative sea-level
caused stepwise encroachment of muddy neritic
environment and a general marine flooding.
The depositional setting and facies anatomy of
the Garn Formation revealed by the study explain
the spatial distribution of sandstone facies and have
important implications for reservoir heterogeneity.
Tidal sandstone ridges are a major architectural ele-
ment of the Garn reservoir in the Kristin Field.
These semi-isolated, more permeable sandstone
bodies constitute locally up to ~ 70% of the bulk
formation thickness and are estimated to be up to
~ 20 km long, 1.7 km wide and 10 m to 15 m thick,
with a lateral spacing of 2.9 km to 6.5 km.
The tidal sandstone ridges are encased in less
permeable sandstone facies and consist of dune
cross-strata sets that act as highly anisotropic
permeability mini-containers. The combination
of the two scales of reservoir heterogeneity is
considered to be responsible for the rapid decline
of reservoir pressure and low recovery factor.
Large amounts of liquid condensate may poten-
tially be withheld in these sandstone bodies at
the early stages of production, to be released only
later when eventually turning into gas.
A statistical method has been used to estimate
the cross-set volume frequency distribution from
the frequency distribution of the cross-set thick-
nesses measured in cores. Such quantitative esti-
mates are valuable for the characterisation and
modelling of petroleum reservoirs, where only
bed thicknesses can be measured from the cores.
On this basis, the sandstone ridge bodies in a res-
ervoir model for the Garn Formation in the
Kristin Field can readily be populated with real-
istic cross-set volumes and their anisotropy be
accounted for to simulate the flow of fluids in the
reservoir and assess better the recovery.
The study indicates that a high net/gross and
seemingly homogeneous sandstone reservoir with
low overall permeability may in reality be highly
heterogeneous and show a production perfor-
mance suggesting strong compartmentalisation,
even though the reservoir contains only high-
pressure gas and condensate.

CONCLUSIONS

Analysis of the Garn Formation in the Kristin Field
indicates that sand deposition occurred in a sub-
siding, shallow incipient graben within a narrow
Jurassic seaway linking the Boreal and Tethys
seas. Deposition involved repetitive cycles of
tide-dominated to wave-dominated sedimentation,
attributed to tectonically-driven changes in relative
sea-level. The transgressive-regressive cycles formed
a transgressive parasequence set ~ 100 m thick, cul-
minating in a major marine flooding and the
deposition of the muddy Melke Formation as a
result of the climax of Jurassic rifting, which caused
a structural collapse of the shelf.
Facies analysis indicates that the sedimentation
involved a vertical stacking of tidal dunes into