Upper Carboniferous-Lower Permian Gipsdalen Group karstified reservoir
carbonates of the Loppa High, Barents Sea; reservoir potential and drilling
challenges.
Geir Elvebakk*, Kai Hogstad*, David W. Hunt**, Jan Pajchel**, Bjarne Rafaelsen*** and
Håkon Robak*
*
**
***
Norsk Hydro, N-9480 Harstad, NORWAY
Norsk Hydro Research Centre. P.O. Box 7190, Bergen, Norway
Institute of Geology, University of Tromsø, N-9037 Tromsø, NORWAY
Seismic Area C on the Loppa High, southwestern Barents Sea, is covered by a 1 x 1 km grid of 2D
seismic and with a 3D grid of 970 sqkm covering the most prospective areas. In-house pre-stack depth
migration of the 3D data has resulted in a high-quality seismic data set that reveals the 3D
heterogeneity of the carbonate buildups and karst systems in the Upper Palaeozoic strata with unusual
clarity. Such data is essential for the prediction of reservoir properties and in steering well location
away from cavernous areas where the risk of mud-loss during drilling is considered to be greatest.
The present exploration efforts have focused on a gigantic structural high referred to as the “Obelix
Structure”. The structure is located in the uplifted footwall block of the extensional N-S trending
Polhem Fault. The main prospective Upper Palaeozoic carbonate reservoir section is truncated on the
eastward-dipping eastern flank of the Obelix Structure, where it is overlain with angular unconformity
by Triassic shales. The unconformity between the Upper Palaeozoic reservoir ant the mid-Triassic seal
spans the Upper Permian-Ansian, approximately 25 million years. Drainage systems, sinkholes
(dolinas) and irregular hummocky topography are considered indicative of penetrative karstification of
the carbonates.
Basin
Outer ramp
Inner ramp/
platform
Basin
Middle
ramp
Figure 1: Palaeogeographic map, near Top Gipsdalen Group (main reservoir). The light grey coloured area is
truncated carbonates and Caledonian-deformed basement. The truncated carbonates represent mainly inner
ramp/platform carbonates that represent the main reservoir target for the Obelix Structure, Loppa High. The
darker grey areas represent middle and outer ramp carbonates with large polygonal network buildups (see also
Figure B) and with a significant lower reservoir potential than updip. Notice also the fault control on deposition
with abrupt change in buildup size across faults.
3rd order regression.
Progradation of lagoons and sabkha across inner
and middle shelf areas contemporaneously with
evaporite deposition in outer shelf, slope and
basin. Dolomitisation was pervasive both in inner
and middle shelf carbonates and in reefs as the
results of flushing by evaporite brines and fresh
water. Subaerial exposure of the entire shelf led to
extensive dissolution of aragonite. The resulting
porosity was in the order of 20% and permeability
between 10 and several hundreds of mD.
Stage III
Palaeoaplysina
4th - 5th order regression.
Differential subsidence resulted in progradation of
sabkha and lagoonal deposits in inner shelf areas
followed by suberial exposure of the inner and
middle shelf areas. Vertical growth of reefs in outer
shelf setting continued. Dolomitisation was mainly
restricted to evaporite lagoons and sabhkas and
was caused by evaporite brines, whereas
dissoultion of aragonitic material occurred
throughout the inner and middle shelf
Stage II
Stage I
Sea level
Basin
Limestone
Barrier
reef
Shelf/lagoon
Sabkha and evaporite
lagoonal dolomite
Flooding of the platform and initial growth of
Palaeoaplysina-phylloid algal reefs on flooding
surface.
Mixed-water dolomite
Evaporite
96052006.Geo.GeE.SAOE
Figure 2: Carbonate reservoir model for the Loppa High. Dolomitization was early diagenetic processes and
was controlled by depositional environments (sabkha and hypersaline lagoons) of inner ramp/platform settings.
These areas also have the strongest impact from subaerial exposure and together they produced the carbonates
with the highest reservoir potential. A later diagenetic Dolomitization stage, perhaps from modified marine
waters, is locally pervasive and dolomite cements partially fills some pores and are also associated with
anhydrite cements.
5 . U p p e r P a la e o z o ic & M e s o z o ic B u r ia l:
K e y C o n c e p t s a n d C o n t r o ls o n K a r s t P la y
Figure 3: Schematic model illustrating the karst development. There are at least three main karstification
episodes related to 1) high-frequency subaerial exposures at glacioeustatic sea level cycles, 2) at major 3. and 2.
order sequence boundaries (e.g Top Gipsdalen Gp.), and 3) at an approximately 25 mill. year unconformity
between the Palaeozoic and mid-Triassic. Open palaeocaverns existed far into the Late Triassic when they
collapsed at approximately 500m of burial.
The first well on the Obelix Structure will target the warm-water carbonates and the basal
transgressive sands of the Gipsdalen Group. The Group is characterized by carbonate buildups
deposited on a series of fault-controlled eastward tilted ramps, with a tectonic control on
accommodation space, thickness and depositional setting. Carbonate buildups are not isolated pinnacle
structures, as earlier thought from study from regional 2D data, but instead form a polygonal network
enclosing deeper mini-basins or lagoons. The buildups appear to be preferentially located along the
downdip footwall margin of the basement fault blocks, and their distribution closely mimics the
basement fault trends. The newly recognized buildup patterns are important when considering the
lateral connectivity of the most favourable reservoir strata. The connectivity is considered to be much
greater now than when interpreted from 2D data as the buildups are linked into mosaics. The overlying
Bjarmeland and Tempelfjorden groups (late Early Permian and Late Permian) are dominated by coolwater carbonates and are considered less prospective. They were deposited during a period of renewed
tectonic activity along the Polhem Fault that started in mid-Permian and lasted to Anisian time.
Epikarst: Seismic Evidence 1
Deep sinkhole fairway
Immediately undip
Of base of Bjarmeland Grp.
•
•
•
•
Incised valleys in basement – clastic fairway
Drainage channels in carbonates with sinkholes
Both fault-controlled
Related to unique surfaces (tG/tP)
Date: 2002-09-25 - Page: 15
Figure 4: Seismic examples of large-scale karst features including sinkholes (dolinas) and irregular “chaotic”
(darker grey) (A), excavated palaeocaverns developed along linear faults (B) and seismic lines illustrating
palaeocaverns (C).
Between the mid Permian and mid Triassic the Obelix Structure was uplifted and tilted eastwards. It is
considered to have formed an ocean island with a maximum relief of approximately 500 m during the
Early Triassic. During this time period the Gipsdalen, Bjarmeland and Tempelfjorden groups were
subaerially exposed, truncated and karstified. Seismic-scale dolinas (sinkholes), palaeocaverns and
hummocky irregular topography related to this exposure event are superbly imaged. In the basal part
of the overlying Triassic succession localized deformation is considered to be related to the collapse
karst palaeocaverns in the underlying Palaeozoic. The preferential development of karst features
adjacent to faults indicates that both faults and fracture systems were selectively solution-enlarged by
meteoric waters that drained from the land area in the west. The stratigraphic control on the karst is
also clear in the north of the Obelix Structure. Here stratiform area of approximately 50 sqkm,
characterized by a particular chaotic seismic signature and irregular topography, is interpreted to be
related to preferential dissolution of evaporite beds within the Gipsdalen Group and brecciation of the
overlying strata.
Epikarst: Seismic Evidence 3
• Line chosen through major sinkholes
• Location of major sinkholes has structural control
• Areas of chaotic reflectors coincident with low
velocities
• Contrasting seismic properties of ’lower’ and
’upper’ Gipsdalen Grp.
• Downdip change in character of ’upper’ Gipsdalen
Date: 2002-09-25 - Page: 17
Figure 5: Seismic line across a sinkhole (A). Karst features are associated with chaotic seismic facies (B), low
acoustic impedance (C), and lower velocities (D).
L&U Harstad
Aug 2002
Epikarst: Seismic Evidence 4
Stacked shelf margin build-ups
Low velocity ‘upper’
Gipsdalen
A
High velocity
’lower’ Gipsdalen
Karstified ‘upper’
Gipsdalen
IL 8677
Basement
3D PSDM
1 km
• Platform succession,
paleoageomorphology and
velocity model are all related
• High-velocity ‘lower’ unit
• Lower-velocity ‘upper’ unit
• High velocity outer zone
coincident with stacked buildups
Basement
A
High velocity
’lower’ Gipsdalen
Triassic
A => Outside 3D area
5 km
Interval velocity map, constant depth 1360 m
Date: 2002-09-25 - Page: 18
Figure 6: Seismic line (A) and time slice velocity map (B) with karstified low-velocity inner ramp/platform areas.
The first well is planned for within the low-velocity area.
Gravimetric data and velocity data extracted velocity tomography analysis focused on the carbonate
succession, show that the karstified areas are characterized by low gravimetry and velocity,
suggesting the presence of potential porous reservoir carbonates in this area. Geophysical studies of
the carbonates show that truncated carbonates have significantly lower acoustic impedance than the
non-truncated carbonates. Integrating geological and geophysical knowledge in this area is crucial to
meet the challenge of: 1) mapping the prospective buildups, 2) the carbonate and sulphate karst
systems and 3) understanding the fill of the palaeocavern systems.