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P032 PALAEOKARST RECOGNITION & 3D
DISTRIBUTION – NEW INSIGHTS FROM THE
UPPER PALAEOZOIC, LOPPA HIGH, BARENTS
SEA
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D. HUNT , G. ELVEBAKK , B. RAFAELSEN , J. PAJCHEL , K. HOGSTAD , H. ROBAK and T. RANDEN
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Norsk Hydro ASA, Bergen, Norway
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Norsk Hydro ASA
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Institute of Geology, University of Tromsø
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Schlumberger Research
Upper Carboniferous-Lower Permian carbonates are truncated with erosional angular
unconformity below Triassic sediments in the uplifted footwall block of the Polhem fault (Fig.
1), Loppa High, Barents Sea, N. Norway. The Palaeozoic carbonates are prospective
hydrocarbon reservoirs, and were extensively karstified during a c. 25 Ma period of subaerial
exposure prior to Triassic transgression. In-house pre-stack depth migration (PSDM) of a
240 km2 3D seismic grid, collected from the most prospective part of the high, has revealed
the surficial geomorphologic features of the upper Palaeozoic karst surface with exceptional
clarity. Autotracking on the zero crossing is considered to make for vertical resolution down
to a few metres (Figs. 2, 3). However, using a traditional approach to the interpretation of the
seismic, an enormous gulf is apparent between the fine picture of surficial paleokarst visible
on dip maps (Figs. 2, 3) and the ability to recognize and map karst in 3D between the
reflectors as is required for reservoir prognosis. Thus, on the Loppa High, a challenge has
been to map the internal form of the karst system in 3D. Here we report on I) the
stratigraphic and structural controls on karst distribution apparent from the PSDM data and II)
on the methods used to deliniate the karst system in 3D within the upper Palaeozoic
succession.
Figure 1. Simplified east-west (left to right) seismic-based geologic cross section though the eastern
flank of the Loppa High Barents Sea, Northern Norway. Here, prospective upper Palaeozoic warm
and cool-water carbonates are truncated with angular unconformity (tP surface) below Triassic strata
(seal). A separate karst surface separates the warm and cool water carbonates (tG surface). While
superficial geomorphical features of both paleokarst surfaces are readily recognisable from dip maps,
a major challenge is to map the karst within the Palaeozoic succession in 3D.
EAGE 65th Conference & Exhibition — Stavanger, Norway, 2 - 5 June 2003
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Controls On Karst
The distribution of karst features related to the top Palaeozoic (tP) surface is primarily
controlled by 1) the bedrock lithologies and 2) faulting and fracturing (Figs. 2, 3). The upper
Palaeozoic bedrock, in which the karst system developed, is quite heterogeneous. The
lower part of the succession, the Gipsdalen Group (Fig. 1), was deposited in warm arid
climate under the icehouse conditions. This highly cyclic succession is characterized by
limestones and dolomites mixed with sulphates, and localised siliciclastics towards its base.
In contrast, the overlying Bjarmeland and Templefjorden groups were deposited in cool water
conditions. They are comprised of limestones with abundant marine cement, cherts and
minor shales.
Dip maps and seismic sections reveal the lithologic control on the geomorphology of
the tP karst surface (Figs. 2, 3). Several distinctive geomorphological zones are apparent
running parallel to structural dip (Fig. 2A, B). The oldest part of the Gipsdalen Group has a
characteristic scarp and dip morphology (zone B, Fig. 2). It is characterized by the most
continuous reflectors (Fig. 4A, B), and high velocities (Fig. 4C). To the east are two fairways
of the most-penetrative karst (zones C and E, Fig. 2), associated with the lowest velocities
and discontinuous-chaotic reflectors (Fig. 4). Zone C is thought to be controlled by the
distribution of sulphates within the succession and is often chaotic in seismic section (Fig.
2B). In contrast, zone E is characterised by the most intense karst, and its location is
apparently controlled by carbonate buildup distribution and the base of the overlying
Bjarmeland Group (Fig. 2).
Figure 2. Dip map (A) and seismic line (B) from PSDM cube of the ‘top Gipsdalen’ surface (tG, Fig. 1).
Several distinctive geomorphical zones are distinguished in the basement (zone A) with incised
palaeovalleys, the karstified Gipsdalen (B-E) and Bjarmeland-Templefjorden groups (zone F) that are
cut by faults of upper Palaeozoic and Mesozoic age. Major dolines occur at the intersections between
faults (circled in B, above and imaged in Fig. 4B, C).
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The stratigraphic control on karst development is locally overprinted where the
carbonates are cut by faults and fractures. The fault/fracture controlled karst is localised by
syndepositional faulting and differential compaction within vertically stacked carbonate
buildups. Thus, while in general the most intense karst occurs just off the flanks of the
vertically stacked buildups (Figs. 2, 3) linear fault and fracture-controlled karst channels and
palaeocaverns can cut straight through them (Fig. 3).
Figure 3. A) Dip map from the PSDM data showing the polygonal arrangement of fault-controlled
carbonate buildups within the Gipsdalen Group (at right). Fault/fracture controlled karst channels
locally cut straight through the Gipsdalen carbonates. B. Seismic line through the fracture/fault
controlled karst channels. They normally have a chaotic appearance in seismic section. As shown
above (B) and in Fig. 2, karst is typically best developed in the lows flanking the buildups.
The fault and fracture control on karst distribution is also demonstrated by
comparison of velocity tomography, variance and seismic reflection data (Fig. 4). Sinkholes
apparent from dip maps often appear at the intersection between faults. They are coincident
with zones of poor reflector continuity and anomalously low velocity, and have a vertical
extent of up to 300 m (Fig. 4).
Figure 4. Sinkholes apparent on dip maps often occur at the intersections of faults (Fig. 2). The karstmodified faults are zones of chaotic reflectivity (A) and poor reflector continuity (B) and localize areas
of lower velocity (C). Note how the geomorphically distinctive lower part of the Gipsdalen Group, with a
characteristic scarp and dip slope geomorphology (i.e. Fig. 2, zone B), has better reflector continuity
(A, B) and higher velocities (C).
EAGE 65th Conference & Exhibition — Stavanger, Norway, 2 - 5 June 2003
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The 3D Mapping of Karst
Figures 2 and 3 reveal spectacular surficial detail of the karst, and aid in advancing
understanding of the geologic controls on its distribution. The velocity tomography and
variance data also give an indication of the potential penetratrion of paleokarst within the
upper Palaeozoic succession. However, using conventional analysis it is not possible to use
the different cubes together in a quantitative manner to map geobodies, such a paleokarst, in
3D. In order to try and map the karst network within the upper Palaeozoic succession in 3D
we have used Schlumbeger Seisclass mapping technology. This software is able to
simultaneously compare data from multiple subcubes of the seismic data (i.e. reflection
strength, variance, flatness, fault edge, velocity tomography etc.). Once data or seed points
are selected the program isolates parts of the data with common features. It has proven the
most useful approach to mapping the karst system on the Loppa High (e.g. Fig. 5).
Figure 5. Geoviz image showing a 3D representation of the ‘karst’ subvolume from a seisclass
classification. This classification was considered successful since it mapped out karst features visible
on dip maps of the top Palaeozoic surface from a few carefully placed training data. Note the linear
fairway of karst features that map-out in the Gipsdalen Group just up dip of the base of the Bjarmeland
Group (right of the cube), where a distinctive fairway of sinkholes is apparent from dip maps (i.e. zone
E, Fig. 2). North is to the top of the diagram and the volume cube is c. 100 m high and 1km2. The
following subcubes were utilized in this classification: flatness, gradient length, fault edge, variance (24
ms window) and VRS_12.
The results shown in Fig. 5 were produced after a relatively few runs (<10) using a
small number of carefully selected seed or training points. The ‘karts’ subvolume volume
identified honours the karst features observed on dip maps of through-going reflectors.
Having obtained this result, the challenge that now presents itself is to further subdivide the
‘karst’ subvolume identified in order predict the lithology of the cavern fills.
Conclusions
The 3D PSDM seismic from the Loppa High reveals a picture of subsurface
paleokarst with exceptional clarity. The lithologic and structural controls on paleokarst
development are made clear from these data. However, while traditional 3D mapping
techniques provide superb surface-based images of the karst and an idea of its extent, little
is revealed of the exact 3D form the karst. Seisclass mapping has proven to be the only
effective way of mapping the karst system in 3D and is also a relatively rapid process.
Acknowledgement:
The partners of Seismic Area C; Norsk Agip, Fortum and Statoil are gratefully acknowledged
for permitting this publication.