Discovering a potential new oil and gas province in the north-western sector of the Russian Arctic.
The complete regional geological assessment of the northern part of the Barents Sea is nearing completion, with the density of seismic observations reaching over 0.2 line-km/km2 (Figure 1). This density has been largely achieved through surveys by Marine Arctic Geological Expedition (MAGE), a joint-stock company commissioned by the Federal Subsoil Resources Management Agency under the Ministry of Natural Resources of the Russian Federation in 2006–2012.
Figure 1: Available seismic information in the northern part of the Barents Sea. (Source: MAGE)
Completed geophysical investigations, including 2D CMP reflection seismic acquisition, shipboard gravity and differential marine magnetometer measurements, have now totaled over 30,000 line-km. The resulting seismic data along with maps of anomalous potential fields deduced from the magnetic and gravity surveys now make it possible to evaluate the oil and gas potential of this underexplored region.
A primary geological objective of these surveys was to establish a subsurface structural and tectonic framework that would include the main reflectors identified in the sedimentary cover and thickness maps, as well as a tectonic classification based on anomalous potential field data. MAGE also examined the formation of geological petroleum plays and their seismic facies classification in order to investigate the probable location and hydrocarbon content of potential reservoirs. An additional aim was to quantify the region’s potential and undertake a cost estimation of its total subsoil resources.
It should be noted that the hydrocarbon potential identified to date in the North Barents Basin is not commensurate with the large scale of the sedimentary basin. In addition, only the gas and condensate content has been included in estimates published so far.
Unique Sedimentary Basin
Due to its thickness and structural evolution, the East Siberian megatrough, and in particular its northern part, is considered a unique sedimentary basin. The trough reaches a thickness of up to 20 km of Paleozoic and Mesozoic sedimentary fill, with almost no Cenozoic interval. Its geological evolution clearly features three phases of tectonic stabilization followed by a persistent subsidence of the basin within its present boundaries. However, the cause of this subsidence remains unclear, since there are no signs of the rifting that is commonly associated with accumulations of post mid-Devonian sedimentary strata.
Major tectonic elements identified beneath the sedimentary cover from west to east are the Aleksandrovskaya high zone, the North Barents syneclise, and the Prednovozemelskaya structural area (Figure 2).
In the North Barents syneclise the basement is not traceable on the 2D CMP seismic sections. However, according to the refraction and integrated modeling data derived from anomalous gravitational and magnetic fields, the depth to basement is estimated to be 16 km.
Figure 2: Cross-s ection through the East Siberian megatrough with gravity and magnetic anomaly profiles. (Source: MAGE)
Geoseismic Sequences
The base of the sedimentary cover is marked by the early mid-Paleozoic geoseismic sequence (GS), which rests with angular unconformity on the surface of the heterogeneous base and is limited by a mid-Devonian erosional surface. The top of this sequence lies between 4,500 and 16,000m and is between 1,000 and 6,000m thick. It is most prominent in trough-shaped depressions in the marginal parts of the sedimentary basin and is less certain in the depocenter.
The Upper Devonian to mid-Permian GS rests with angular unconformity on the sediments of the early mid-Paleozoic GS and is traceable throughout the basin. In the northern part of the Barents Sea, this GS varies in thickness between 7,000 and 500m from east to west. In the Prednovozemelskaya structural area, the Admiralteiskaya-1 well, drilled on the crest of the Admiralteisky megaswell, penetrated 60m of late Carboniferous carbonates overlapped against the erosional truncation by Permian terrigenous deposits.
This GS is sub-divided into the Upper Devonian to Lower Carboniferous and the mid-Carboniferous to mid-Permian geoseismic sub-sequences, which form the lower parts of large alluvial cones filling the Sedov and Western Fobos troughs (see Figure 3). The maximum thickness of sediment found in an alluvial cone of the Fobos trough is 6,000m, while the alluvial cones of the Sedov trough are marked by lesser thicknesses of about 5,000m. The clinoform structure of the Upper Devonian to mid-Permian GS suggests it has promising potential for oil and gas prospects.
Figure 3: Prednovozemelskaya structural area: (a) basement; (b) sedimentary cover. (Source: MAGE)
Figure 3: Prednovozemelskaya structural area: (a) basement; (b) sedimentary cover. (Source: MAGE)
The overlying mid to Upper Permian GS, limited at the top by an unconformity surface with toplap features, is divided into two large geoseismic sub-sequences. The GS varies in thickness between 250 and 4,500m, with the lower sub-sequence in the western margin of the Aleksandrovskaya high zone wedging out. On the whole, the mid to Upper Permian GS demonstrates a time of quieter sedimentation, in contrast to the ‘avalanche’ sedimentation of the Upper Devonian to mid-Permian GS.
The Triassic GS is limited at the top by a strongly pronounced erosional surface which is most prominent in the Prednovozemelskaya structural area. A stratigraphic gap has been observed on well data between the Triassic and Jurassic deposits, marking a pause in sedimentation within the northern part of the East Barents megatrough. As a result about 1,000m of Triassic sediments were exposed and denuded on the margins of the basin. The mid-Triassic formations in the eastern periphery of the sedimentary basin have a distinct clinoform structure which is not typical of the rest of the GS.
The Jurassic GS is limited at the top by a reflector which is associated with Upper Jurassic black clays and is a regional datum in the Mesozoic. The sequence is 2,000m thick in the North Barents syneclise. Within the Aleksandrovskaya high and the Prednovozemelskaya structural area, the Jurassic deposits wedge out against the erosional truncation extending beneath the seabed.
Jurassic deposits are related to the largest gas condensate fields of the Barents Sea, including the Shtokman, Ludlovskoye and Ledovoye fields.
In the Cretaceous GS deposits on the margins of the North Barents syneclise have been eroded but are found at the seafloor in the center of the trough. There has been a long sedimentation gap recorded in the post-Neocomian age. The depth of the Early Cretaceous erosion, which affected the Jurassic and Upper Triassic deposits in the Prednovozemelskaya structural area, is estimated in terms of kilometers. The Cretaceous GS is 1,000m thick.
The Neocomian clinoforms show an apparent dip towards the center of the North Barents syneclise both south-eastward and southward from the rise of Franz Josef Land.
Potential Trapping Mechanisms
From this analysis it could be argued that most sediments in the northern part of the East Barents megatrough accumulated in the Upper Devonian to Permian and Triassic ages, while the surface that divides these strata shows no indication of any significant tectonic patterns such as rifting. Heavy sedimentation occurred in the Mesozoic age, particularly in the Cretaceous period. The Paleozoic and Mesozoic sedimentary cover captures three sedimentation gaps, including the major mid-Devonian, early Jurassic, and post-Neocomian unconformities, which are most prominent in the marginal parts of the sedimentary basin, particularly within the Aleksandrovskaya high in the west and the Prednovozemelskaya structural area in the east.
Notably, during tectonic stabilization the sedimentary cover was more substantially denuded in structures adjacent to the area of the present North Barents syneclise. A similar stability in subsidence occurs in other superdeep depressions, such as the North-Chukchi, Peri-Caspian, and South Caspian troughs, where the thicknesses of accumulated sediments are hard to explain by heavy tension (rifting). For instance, an estimation of the total displacement of the basement in the North Barents trough indicates that the relative stretching of the crust is less than 10%, which would have only resulted in the deposition of sediments in the order of 2 km. The joint analysis of seismic and gravity data therefore suggests that the subsidence is attributable to rock compaction in the lower crust in the transition of gabbro to a denser eclogite.
In addition to the fact that the sedimentary cover mechanism remains ambiguous, the geology of the northern part of the Barents Sea is distinguished by a lack of large anticlinal structures. However, a vast number of zones have been identified in the sedimentary cover which can be related to non-structural traps, arising from the frequent tectonic rearrangement, as described above.
Video created from the official documentary on the activities of MAGE, timed to coincide with the 40th anniversary of MAGE. (External site – in Russian)
Inferred Hydrocarbon Resources
Until this major regional assessment, the hydrocarbon potential of the northern part of the Barents Sea was estimated according to ultimate potential resources (UPR), which meant that no quantitative estimate had been made, while the structural potential was limited to the Orlovskaya anticline structure, which only covers about 1,000 km2. However, as a result of this targeted regional exploration program, 79 local anticline structures have been identified in the northern part of the Barents Sea over a total area exceeding 42,000 km2. Furthermore, extensive areas of potential non-structural traps have been recognized, covering a total area in the order of 3,080 km2.
The resulting estimate of inferred hydrocarbon resources shows that the value of subsoil resources of the region has increased by multiple orders of magnitude, becoming more attractive to potential explorers. At present, the entire northern part of the Barents Sea is divided into license blocks which have either been acquired or are pending license approval.
Based on the completed surveys, inferred resources (D2) of the sedimentary cover of the North Barents shelf exceed 23,000 MMtoe up to a depth of 7,000m. The share of recoverable resources accounts for more than 18,000 MMtoe. At a price of $250 per ton of oil equivalent in 2015, the cost of subsoil resources, including inferred (D2), recoverable (D2), and localized (D2loc), is estimated at $1.19 bn.
Thus, it appears that this region can now be considered to be an independent potential oil and gas province. These remain unproven oil and gas reserves simply as a result of insufficient exploration and a lack of drilling.
