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A Model for Late Paleozoic Carbonate Buildups
In the Late Palaeozoic, warm- and cool-water carbonates in the south-western Barents Sea formed long ridges, atoll-shaped rings,
­polygonal networks and isolated mounds. The warm-water carbonates are potential reservoir rocks and may constitute an interesting
and spectacular play model in this sparsely explored geological province.
Text: Bjarne Rafaelsen, StatoilHydro ASA and Karin Andreassen, University of Tromsø
More than half of the world’s oil
and gas is produced from carbonate reservoirs. The Middle East has
about 60% of the worlds proven
conventional oil reserves, and more
than 70% of these reservoirs are in
carbonate rocks. The Timan-Pechora Basin in Arctic Russia represents
another example of a geological
province with huge reserves found
in carbonate reservoirs.
In the Norwegian part of the
Barents Sea, Palaeozoic carbonates could possibly contain large
amounts of oil and gas and may
provide an important play model
in the future. The most prospective
reservoir rocks are warm water
shelf carbonates and buildups of
Late Carboniferous-Early Permian
age (Gipsdalen Group; Stemmerik
and Worsley, 2005). These reservoirs are all buried several thousand meters below the sea floor.
The value of 3D
The growth of carbonate buildups
in the Barents Sea lasted for 35-40
million years, from Late Carboniferous to Early Permian. The Barents
Sea was then a part of the northern Pangean continental shelf
(present day Canada, Greenland,
Svalbard, Barents Sea and Arctic
Russia), which drifted northwards
around 2-3 mm per year from
a subtropical position at around
30°N, to a temperate position at
approximately 45°N.
Isolated carbonate buildups
form characteristic shapes in many
Late Palaeozoic rocks around the
world. Studies based on 2D seismic data in the Barents Sea show
mounds that have been interpreted
as isolated carbonate buildups.
Following the introduction of
3D, however, our understanding
of the geometry of the buildups
Structural elements of the south-western Barents Sea with the study
area and the location of the 3D-survey shown in red. The red square
on the inset map also shows extent of the main map. FB: Fugløy­
banken; TF: Tromsøflaket; BT: Bear Island Trough; I: Ingøydjupet and
NB: Nordkappbanken. Modified from Rafaelsen et al. (2008).
88 GEO June 2008
has changed dramatically. What
geologists once interpreted as isolated buildups now turn out to be
polygonal networks with laterally
amalgamating and bifurcating
ridges or long straight ridges.
Based on 3D seismic data
acquired offshore Finnmark, the
location, geomorphology, extent,
vertical and lateral growth can be
illustrated using three-dimensional maps. These maps show that
underlying bathymetry and faults,
in addition to changes in climate and depositional environment, have affected the carbonate buildup growth and extent.
The carbonate buildups have also
affected their own depositional
environment by forming lagoons,
atoll-like shapes and possibly
areas with restricted sea water
circulation.
Growth of the buildups
In the Barents Sea, the Late
­Carboniferous to Early Permian
sediments are mainly shallow
marine, locally evaporitic warmwater carbonates with Palaeoaplysina-phylloid algae-dominated
buildups. Based on the seismic
data we learn that the buildups
are several tens of kilometres long,
up to 2,5 km wide and 300 m
thick (Rafaelsen et al., 2008). They
are interpreted to consist of many
vertically stacked complexes each
below seismic resolution.
After a period of favourable
growth conditions, evaporite deposition put a temporary end to carbonate growth. It is, however, still
possible that carbonate buildups
continued to form during ­ pauses
in the evaporite deposition and
therefore can be found within the
➜
Seismic profile showing carbonate buildups under, within, next
to and above the evaporite sequence. It also shows the buildups
­location with regards to palaeohighs and faults. Below is a ­geological
sketch of the Carboniferous-Permian succession, based on the ­seismic
profile. Modified from Rafaelsen et al. (2008).
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evaporite succession. After the evaporites were deposited exposure to
air may have caused karstification
(i.e. dissolution) in the uppermost
evaporite succession, before a
transgression led to new growth of
warm water carbonate buildups.
The warm- and cool-water
­carbonates discussed in this ­article
belong to the Late ­CarboniferousEarly Permian Gipsdalen and
Bjarmeland Group. The Gipsdalen
and ­ Bjarmeland Group has been
penetrated by several wells in the
southern Barents Sea, ­ including
7229/11-1 on the ­ Finnmark
­Platform, but hydrocarbons in
commercial ­ quantities have not
been proven yet.
From warm to cool
In the Early Permian an abrupt
change towards a cooler climate
occurred. In the study area this is
marked by a transition to calcitedominated deposits (Bjarmeland
Group) where large bryozo-Tubiphytes buildups dominate. Here
0.4-5 km wide, 1.5-27 km long
and 60-420 m thick bryozoandominated cool water carbonate
buildups occur as straight, sinusshaped and continuous ridges that
grew on top of the warm-water
carbonates. After the cool-water
carbonate buildups were established on top of the warm water
carbonates, they appear to expand
laterally into deeper water.
Without considering differential
compaction, the relief from the
top of the ridge to the surrounding palaeoseafloor is 150-350 m,
while the relief from the top of the
carbonate buildups to the palaeoseafloor within the atoll-shaped
“rings” is 90-180 m. The largest
cool-water buildups are located
along the edge of the palaeohigh
and are smaller towards the centre
of the high, probably due to less
sea water circulation and/or less
access to nutrients.
The top of the cool-water carbonate ridges are relatively flat.
This may indicate that they were
able to keep up with the sea level
rise, whereas ridges that narrowed
towards the top may have struggled to keep up with the sea level.
An alternative explanation for the
ridge shape is that the access to
nutrients changed with time.
The carbonate buildups were
later draped by sediments of Late
Permian age (Tempelfjord Group)
and are today buried 3900-4500
meters below the present sea floor.
Times-structure maps of three different stratigraphic levels from the
studied 3D seismic ­ survey. Vertical exaggeration is 8x. White line
represents the seismic profile. The maps show the interpreted carbonate buildup growth-pattern through time. Note that the carbonate
buildups from the Bjarmeland Group grew on top of the Gipsdalen
Group buildups. Modified from Rafaelsen et al. (2008).
A valid play model
In 1993, Shell drilled an exploratory well on the Finnmark Platform
close to the centre of the 3D seismic survey we have been studying,
targeting Permian-­Carboniferous
carbonate buildups. The well was
drilled to a total depth of 4630 m
encountering the predicted lime­
stone and buildups. Unfortunately they had insufficient reservoir
­properties and no hydrocarbon
shows were encountered. However,
other wells in the ­Norwegian part
of the Barents Sea have proven
shows in warm-water carbonates,
indicating that the play model may
still be valid.
References:
Rafaelsen, B., Elvebakk, G.,
Andreassen, K., Stemmerik, L.,
Colpaert, A., Samuelsberg, T.,
2008:Fromdetachedtoattached
carbonate buildup complexes 3D seismic data from the Upper
Palaeozoic, Finnmark Platform,
southwestern Barents Sea.
Sedimentary Geology 206, 1732. Copyright Elsevier.
Stemmerik, L. and Worsley,
D., 2005. 30 years on – Arctic
Upper Palaeozoic stratigraphy,
depositional evolution and
hydrocarbon
prospectivity.
Norwegian Journal of Geology
85, 151-168.
Acknowledgement
StatoilHydro ASA and the European Communities project TriTex (IST-1999-20500) are acknowledged for funding the research project. StatoilHydro ASA is acknowledged for
providing the seismic data. The University of Tromsø acknowledges Schlumberger for computer software (GeoQuest and Charisma) and guidance on technical issues. We thank
the Department of Geology at the University of Tromsø and the co-authors of Rafaelsen et al. (2008). Elsevier is acknowledged for allowing this to be published in GEO. Many
thanks to Stephanie Guidard for assistance with the palaeomap and to Rebecca Eve Crompton and Richard Martin for English proof reading.
90 GEO June 2008