Abstracts for FORCE seminar

Abstracts for FORCE seminar
” Jurassic and Lower Cretaceous Sedimentary Systems of the Utsira High”
19.-20.06.12
Integrated geological studies as a key to understanding the geology
of the Johan Sverdrup discovery – with examples from the PL 501
area
Ronald Sørlie 1 , Erik Hammer 1 , Michael Charnock 1 , Hans Amundsen 2 and Lars
Riber 3
1 Lundin Norway AS, Strandveien 50D, NO-1366 Lysaker, Norway
2 Vestfonna Geophysical
3 Department of Geosciences, University of Oslo
Contact email: Ronald.sorlie@lundin-norway.no
Abstract
The Johan Sverdrup discovery was made in August 2010 in well 16/2-6, PL 501. The discovery is
defined as a fault-bounded trap against the Southern Utsira Basement High and overlying Jurassic
shales and/or Cretaceous Åsgard marls. The main reservoirs consist of a sequence of Middle and
Upper Jurassic sandstones of excellent quality. Appraisal wells drilled during 2011 and 2012 in PL501
(16/3-4, 16/3-4A, 16/2-7, 16/2-7A, 16/2-11 and 16/2-11A) and PL265 (16/2-8 and 16/2-10) have
confirmed the extension of the Johan Sverdrup discovery.
Several studies and analyses have been performed, including petrography, sedimentary studies,
isotopic analyses, biostratigraphy and field analogue studies among others.
The Draupne sandstone of Volgian age consists of unconsolidated very coarse to medium grained
sands, with very few sedimentary structures. The sandstones appear massive and are thought to
represent an amalgamated marine, high density gravity flow/bedload deposits. The Middle Jurassic
Vestland Group sequence is dominated by channel deposits in a non-marine setting.
In addition to the biostratigraphic analyses, significant variations in the mineralogical content are
observed and used when defining the different reservoir zones.
The petrophysical properties of the Jurassic age reservoirs are excellent with porosities in the upper 20
to 30%-range and multi-Darcy permeability. Outstanding flow characteristics are confirmed from well
tests (DST) in wells 16/2-6, 16/2-11 and 16/3-4.
Quantitative biostratigraphy has been undertaken on the extensively cored reservoir section. The
primary tools for stratigraphic calibration are dinocysts (marine microplankton), spores and pollen.
Other microfossils (Foraminifera) and macrofossils (Belemnites) have been used to determine the
palaeoenvironmental settings of the reservoir sediments, particularly the presence of marine conditions
is indicated in both the Draupne shale and Draupne sandstone reservoir unit.

Johan Sverdrup sandstone reservoir sedimentological pattern and
evolution: an integrated depositional model
Mario Vigorito, STATOIL ASA
Following the discovery of the Johan Sverdrup giant oil field, a total of 11 exploration and
appraisal wells have been drilled to delineate the discovery. Ten wells have acquired
extensive core coverage of the reservoir section which mostly consists of medium to very
coarse Jurassic sandstones. All the wells show similar lithological and sedimentological
patterns which allow for good correlation across the field. On the basis of detailed
sedimentological, petrographic, palaeontological, and wireline logs analyses, the reservoir is
divided into four units whose relative distribution and volume vary across the field: A) fluvial
coarse sandstone and overbank silty\muddy deposits; B) fluvial and locally fluvio\deltaic fine
to coarse sandstone and silty-muddy overbank deposits, locally pedogenized; C) Tidally
influenced estuarine to marine-tidal medium sandstone, and D) shelfal coarse to very coarse
sandstone, mainly gravitative fan delta deposits locally reworked by bottom currents. Units C
and D generally represent the bulk of the reservoir interval and display exceptionally good
reservoir properties.

Henrik Olsen, Maersk Oil
Reservoir heterogeneities in the Johan Sverdrup field – a sedimentological perspective
The Jurassic reservoirs in the Johan Sverdrup field show variations both vertically/stratigraphically and
laterally across the field. In this presentation Maersk Oil will demonstrate the variabilities of the
reservoirs and the impact of different sedimentological interpretations on the possible heterogeneity
distribution in the reservoirs.

Geological setting in the greater Draupne area with a focus on the Middle and Upper Jurassic
succession
By Evy Glørstad-Clark, Ingeborg Waldal Verstad, Lars Arnesen, Kai Hegglund, Anne Marit Næss, Ifor Roberts
The Draupne Field and Hanz discoveries are located on the Gudrun terrace on the eastern flank of
the South Viking Graben in blocks 16/1 and 25/8. The reservoirs range in age from Middle Jurassic in
the Draupne field to Late Jurassic reservoirs in Hanz (25/8). Rift related faulting and associated
localized tectonic subsidence occurred in the area, influencing sediment routing, gross thickness,
facies distributions and facies belt orientation. Significant reworking of the sediments during this
time period, and accommodation space was fault controlled from Callovian times. Significant paleotopography
developed during Late Jurassic, resulting in localized sand distribution.

Remarkable variations in the Jurassic - Lower Cretaceous
Sequence Stratigraphy of the Utsira High Complex with emphasis
on the western margin of the Haugaland High and Gudrun Terrace.
Michael Charnock, Erik Hammer and Ronald Sørlie
Lundin Norway AS, Strandveien 50D, NO-1366 Lysaker, Norway
Contact email: macharnock@lundin-norway.no
Abstract
The Utsira High has had a long history of exploration in the Norwegian Sector of the North Sea.
Despite this history, recently drilled wells with excellent core coverage and integration with seismic
have provided new insights into the complexity of depositional styles on both western and eastern
sides of the Haugaland High (southern part of the Utsira High complex) during the Middle Jurassic to
Lower Cretaceous interval. It is hoped that highlighting these variations will invigorate new
interpretations and exploration activity of this ‘well-known’ petroleum area.
A comparison is made between the western margin of the Haugaland High - Gudrun Terrace and the
eastern side in the Haugaland High - Karmsund Graben - Avaldsnes High areas (Johan Sverdrup
Field). It can be demonstrated that in addition to some commonality, there is also considerable
variation between the timing and depositional fill of the two areas during this time period.
Depositional styles (along strike) on the western margin of the Utsira High also show remarkable
variation in both large-scale sequence stratigraphy and depositional style. Shoreline and shelf
sequences defined by unconformity bounded packages (SB/TS) are variably preserved in mini-basins
developed along this western margin. Examples are described from three dip sections showing that
sedimentary packages are represented by transgression, progradation, aggradation, passive infill,
sediment by-pass, hanging wall wedge deposition, and/or slumping-sliding depending on palaeolocation
and in response to local variations in tectonic style and accommodation potential. These
examples are linked to the viewing of the Apollo 16/1-14T2 and Tellus 16/1-15 cores in the following
core workshop.

Outcrop analogues for the Jurassic discoveries of the Southern Utsira High. Case histories from
Sardinia Oligo-Miocene rift system and Tertiary basins of Southern Italy
Sergio G. Longhitano
Department of Geological Sciences, University of Basilicata - V.le Ateneo lucano, 10 85100 Potenza, Italy. E-mail:
sergio.longhitano@unibas.it
The Oligo-Miocene Sardinia Rift Basin and other Tertiary basins of south Italy represent excellent
outcrop analogues to the Jurassic deposits surrounding the southern Utsira High. In all study areas,
sediments are dominated by conglomerate and sandstone, and sedimentary body geometries are
also similar. These deposits are representative of continental to marine depositional systems which
developed in tectonically-controlled confined (narrow-linear) basins. The outcrop analogues provide
insight into reservoir architecture and heterogeneity in the Utsira High deposits at a subseismic scale.
Through the use of classical facies analysis techniques on outcrop data, a better understanding of
sediment distribution in extensional basins is achieved. The knowledge gained is demonstrated to be
applicable to future exploration and development of reservoirs in the region.

EROSION OF PERSITENT BASEMENT “HIGHS” AND ASSOCIATED SEDIMENTATION
FROM THE RONA RIDGE AREA, WEST OF SHETLAND
Several present-day large structural features occur in the West Shetland Basin that formed
persistent topographic (above sea-level) features for long periods during the Jurassic. The
structural features comprise complex but poorly constrained assemblages of metamorphic
Precambrian basement. Oil occurs in fractured basement and in Jurassic sedimentary strata
that were derived from erosion of the basement. Precise stratigraphic data are sparse but it
is believed that the coarse-clastic Rona Sandstone, which is the main siliciclastic reservoir
target in the area, records intermittent sedimentation along the margins of degrading islands
from (at least) the Middle Jurassic until the early Upper Jurassic after which the islands
became completely submerged; eventually deep-water conditions prevailed. The
combination of fractured and weathered basement, and the Rona Sandstone form reservoirs
in sub- and supra-unconformity stratigraphic traps that are overlain predominantly by finegrained
Cretaceous strata and locally by Upper Jurassic (Kimmeridge Clay/Draupner
Formation) mudstones intercalated with turbiditic sandstones.
Andrew Hurst
University Aberdeen