Geoscience Reference
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Updated depositional and stratigraphic model of the Lower
Jurassic Åre Formation, Heidrun Field, Norway
CAMILLA THRANA*, ARVE NÆ SS*, SIMON LEARY
†
, STUART GOWLAND
‡
,
MALI BREKKEN* and ANDREW TAYLOR
‡
*
Statoil ASA, Strandveien 4, N-7501 Stjørdal, Norway
†
Statoil ASA, 2103 CityWest Blvd., Houston, Texas, 77042, USA
‡
Ichron Limited, Century House, Gadbrook Business Centre, Northwith, Cheshire CW9 7TL, UK
ABSTRACT
The Jurassic reservoirs of the Heidrun Field, offshore mid-Norway, have produced in
excess of 135 M Sm
3
of oil. The oil recovery factor of the fluviodeltaic (Rhaetian to Early
Pliensbachian) Åre Formation is the lowest amongst the reservoir formations, which
makes this an important stratigraphic interval when it comes to remaining reserves and
IOR potential. In order to address challenges related to stratigraphic correlation and
detailed reservoir modelling, a major core and log based reservoir characterisation study
of the Åre Formation was performed. The outcome of this study includes a new under-
standing of the depositional setting and a revised reservoir zonation of the Heidrun
Field. The new stratigraphic framework defines seven main zones, which comprise
genetically linked facies bounded by field-wide stratigraphic surfaces. The Åre Formation
is an overall transgressive succession that progresses upward through: 1) non-marine
coastal plain deposits in Åre 1 to 2 reservoir zones, 2) a succession of lower delta plain
and brackish-water interdistributary bay deposits in Åre 3 to 4 reservoir zones, 3) mixed
wave-influenced and tide-influenced estuarine deposits in Åre 5 to 6 zones; and 4) open
marine shoreface deposits in the uppermost Åre 7 reservoir zone. Although the regional
sequence stratigraphic significance of surfaces separating these reservoir intervals could
not be determined in every case, the stratigraphic division presented here has proven
more robust on a field-scale than in previous models. It provides improved stratigraphic
control for drilling operations and more accurate predictions of the spatial distribution
and geometry of hydrocarbon bearing facies. Long-term benefits will include more pre-
cise production forecasts and improved drainage strategies.
Keywords:
Fluviodeltaic, estuarine, sequence stratigraphy, industrial application,
Jurassic, Norwegian Sea.
INTRODUCTION
within the Lower Jurassic Åre Formation. Current
development plans focus on a revised drainage
strategy, utilisation of more complex well solu-
tions and evaluation of small and economically
marginal drilling targets. Continued effort to
develop improved oil recovery (IOR) measures
and enhanced oil recovery (EOR) methods are
necessary as significant business opportunities
still exist.
Historically, geological understanding and
reservoir characterisation of the Åre Formation
has faced various challenges: 1) Reservoirs are
composed of complex fluvial to marginal-marine
deltaic and tide-influenced deposits. Although
conceptual depositional models have been in
The Heidrun Field, located approximately 190 km
offshore mid-Norway (Figs 1 and 2; Koenig, 1986;
Whitley, 1992), has produced in excess of 135 M
Sm
3
of oil (per 2010) since its discovery in 1985.
Hydrocarbons are accumulated in the Lower and
Middle Jurassic deposits of the Båt Group (Åre
and Tilje Formations) and the Fangst Group (Ile,
Not and Garn Formations; Fig. 3). Production rate
of the Heidrun Field is declining and one of the
main challenges today is to target and drain effi-
ciently the remaining oil reserves. A relatively
large fraction of remaining reserves is allocated in
heterogeneous and low-productivity reservoirs
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