Earth Science 13.2B Paleozoic Era : Life explodes Paleozoic Era : Life Explodes: Part B.
A Model for Late Paleozoic Carbonate BuildupsIn the Early Permian an abrupt change towards a cooler...
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Reseach news 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. Growth of the buildups In the Barents Sea, the Late Carboniferous to Early Permian sediments are mainly shallow marine, locally evaporitic warm- water carbonates with Palaeoap- lysina-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 depo- sition put a temporary end to car- bonate 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 has changed dramatically. What geologists once interpreted as iso- lated 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-dimensio- nal maps. These maps show that underlying bathymetry and faults, in addition to changes in cli- mate and depositional environ- ment, have affected the carbo- nate 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. More than half of the world’s oil and gas is produced from carbona- te 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-Pecho- ra 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 carbona- tes 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 reser- voirs are all buried several thou- sand 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 Carbonife- rous to Early Permian. The Barents Sea was then a part of the north- ern 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 seis- mic 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 TEXT: Bjarne Rafaelsen, StatoilHydro ASA and Karin Andreassen, University of Tromsø 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). 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). 88 gEo June 2008 ➜
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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.
Growth of the buildupsIn the Barents Sea, the Late Carboniferous to Early Permian sediments are mainly shallow marine, locally evaporitic warm-water carbonates with Palaeoap-lysina-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 depo-sition put a temporary end to car-bonate 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
has changed dramatically. What geologists once interpreted as iso-lated 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-dimensio-nal maps. These maps show that underlying bathymetry and faults, in addition to changes in cli-mate and depositional environ-ment, have affected the carbo-nate 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.
More than half of the world’s oil and gas is produced from carbona-te 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-Pecho-ra 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 carbona-tes 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 reser-voirs are all buried several thou-sand meters below the sea floor.
The value of 3DThe growth of carbonate buildups in the Barents Sea lasted for 35-40 million years, from Late Carbonife-rous to Early Permian. The Barents Sea was then a part of the north-ern 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 seis-mic 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
TEXT: Bjarne rafaelsen, Statoilhydro aSa and Karin andreassen, university of Tromsø
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).
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).
88 gEo June 2008
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90 gEo June 2008
The warm- and cool-water carbonates discussed in this article belong to the Late Carboniferous-Early 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.
references: rafaelsen, B., Elvebakk, g., andreassen, K., stemmerik, l., colpaert, a., samuelsberg, t., 2008: from detached to attached carbonate buildup complexes - 3d seismic data from the upper Palaeozoic, finnmark Platform, southwestern Barents sea. sedimentary geology 206, 17-32. 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.
AcknowledgementStatoilHydro 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.
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 carbo-nate 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).
evaporite succession. After the eva-porites 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.
From warm to coolIn the Early Permian an abrupt change towards a cooler climate occurred. In the study area this is marked by a transition to calcite-dominated deposits (Bjarmeland Group) where large bryozo-Tubip-hytes buildups dominate. Here 0.4-5 km wide, 1.5-27 km long and 60-420 m thick bryozoan-dominated cool water carbonate buildups occur as straight, sinus-shaped and continuous ridges that grew on top of the warm-water carbonates. After the cool-water carbonate buildups were estab-lished 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 surround-ing palaeoseafloor is 150-350 m, while the relief from the top of the carbonate buildups to the palae-oseafloor 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 car-bonate 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 strugg-led 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.
A valid play modelIn 1993, Shell drilled an explora-tory well on the Finnmark Platform close to the centre of the 3D seis-mic 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. Unfortuna-tely 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.
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