Introduction
The Barents Sea region represents the one of the major undeveloped petroleum province of
the world. The area is partly less studied because of difficult nature-climatic conditions.
Large-scaled geological and geophysical investigations have been carried out in the Russian
and Norwegian onshore and offshore regions.
The present work is devoted to characterization and distribution of the Jurassic reservoirs of
the Barents sea region. Compilation of literature combined with field-trips materials, seismic
interpretation and well-logs analysis have been done in order to describe the focused intervals.
Geological settings
During the Early Jurassic the Barents sea Basin became submerged. The mainland and
continental shelf assumed much of their present form. At the same time the marked change
occurred from an arid to a humid climatic regime, resulting in increased run-off and the
transport of gravel, sand and mud from the mainland out onto the shelf. Many of the most
important oil and gas reservoirs of the Barents sea basin were deposited during the Early and
Middle Jurassic.
The climate change resulted in increased erosion and run-off from mainland. Erosion leveled
out the topography and great rivers transported gravel, sand and mud to the shelf via variety
of deltas distributed along an elongated coastline.
The coastline migrated back and forth repeatedly during the Early and Middle Jurassic.
Limited basin subsidence, combined with the supply of enormous volumes of sediment,
caused the coastline to advance seawards. When basin subsidence increased the sea level rose
and the coastline retreated. Meandering and braided river drainage patterns characterized the
coastal plains and coastal sediments were reworked by wave action and the tides.
The Lower and Middle Jurassic deposits along the coast represent a greit stratified sandpit,
with thick units of sandstones interbedded with mudstones. The sandstone units cover vast
areas across much of the Barents sea shelf. Jurassic sand units are very porous. And the
normally water-filled pores between the sand grains are in many places occupied by oil and
gas. Jurassic sandstones represent the most important oil and gas reservoirs of the Barents sea
basin.
Factual data and methods
The description of the Jurassic reservoirs has been identified according to the description of
the geological sections of Svalbard and the Frantz Joseph Land and in the wells in the Eastern
and Western parts of the Barents Sea
Cyclicity of the Jurassic section
Comparison of Lower and Upper Jurassic sections of the different localities of the Barents sea
allows to mark out five cycles of sediments accumulation, caused by lateral zoning and eustatic
periodicity
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Jurassic succession of the Barents sea basin present by quite thick sandstone layers
overlapping by clay beds. The last ones become thicker to the uppermost part of the Jurassic
section.
The Jurassic sediments encountered in Shtokmanovskoye gas field in the Barents Sea
represent more or less the entire Jurassic stratigraphy by all series. Maximum thickness of the
Jurassic succession is around 900 m.
Also in the Shtokmanovskoye field the Jurassic stratigraphy could be subdivided into five
cycles caused by eustatic sea level fluctuations and sediment characteristics. The upper cycles
are quite well correlated, the lower ones have a lot of internal unconformities and they are
being changed by age (Fig. 1).
Fig.1. Litho-stratigraphic chart of Barents-Kara region.
Cycle One, sediments of the lower series (Pliensbachian – Toarcian) is represented by
interbedded sandstones, siltstones and clays. Sandstones in layers 4–50 m are separated by
relatively thin (4-10 m, rarely up to 25 m) mudstones and siltstone units. At the various levels
of stratums middle part there are usually found relatively thin (up 10 m) lens-like interbeds of
conglomerates, built by gravel and pebbles of siliceous rocks, and thin beds of coal with
vegetation detritus.
Conglomerates occur rarely with fragments of dark-gray mudstones, cemented by clay mass.
In general, sharp contrasts between sandstones with traces of underlying mudstones, indicates
several periods of erosion. The sediment thickness of 40 – 120 m is represented in 4-5 cycles.
Middle Jurassic sediments (Aalen to Callovian) form cyclic-built stratum. The thickness
varies from 25 to 520 meters. Thickness of “clay” and “sandy” stacks in the cycles content are
comparable. Increasing clay content upwards in the section indicates a general deeping of the
sediment basin.
Cycle two (Aalenian) includes mudstone-sandstone stratum. The rocks are characterized by
significant sandstone content. The silt and clay content increases upwards in the section in
Shtokmanovskaya-1 well. The depositional environment varies from marine to continental.
The whole stratum is divided into three – four transgressive sub-cycles.
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Cycle three (Bajocian) includes mudstone-siltstone-sandstone stratum.
Sandstone portion in the stratum content drops to 29-54% and gradually decreases from South
to North. If in the Southern part of observed territory beds of sandstone are distributed in the
stratum either evenly or attracted to its upper part, in the Northern part of it sandstones are
gradually “drifting” to the lower cross-sections half.
Cycle four (Bathonian - Early Callovian) includes siltstone-mudstone stratum.
Stratum with thickness up to 200 m contains mainly clay with lower siltstone and sandstone
content. The latter are confined to the stratum’s middle part and are found mainly in the
middle part of Shtokmanovskiy-Ludlovskiy saddle. The Bathoninan mudstones can be
described as dark-gray, often of brownish color, fine-sorted, and silty to various degrees.
Sometimes landslide textures and turbidite textures are observed in mudstones (usually in thin
interbeds and sandstone stacks), as well as calcite grains, pyritized thread-like residues of
water-plants (alga), coal material.
Cycle five is mapped as sandstone stratum.
Sediments of Lower and Middle Callovian sub-stages are represented by sandstone stratum.
Sandstones are predominantly fine-grained, rarely fine-medium-grained, quartz-like,
sometimes micaceous, massive and with intermittent wavy interbedding with clay and coal
material. Interbedding is often poorly exposed, and irregular. Sandstones are sorted to various
degrees; transition from underlying sediments is gradual.
Upper Jurassic sediments are distinguished by clay composition and by presence in their
profile of specific “black shales”, enriched by organic substance. Specific appearance of these
clays and corresponding specific gamma-logging curve made these sediments almost major
marking horizon, which permits orienting in Jurassic – Cretaceous profile. They are
represented by “black shales” and dark-gray mudstones. “Black shales” are black and darkbrown weakly silty clays, concentrated with coal material.
Middle Jurassic reservoirs
In the Jurassic section of the Barents sea three reservoirs J2, J1 and J0 are mapped. These
collectors are represented by terrigenic section of sand stratum. In the Barents sea basin
reservoir J0 has regional extension, reservoirs J2, J1 are not regional and can be mapped only
on local structures.
Sediments of producing reservoir J2, which was penetrated by several wells in the Eastern part
of the Barents sea, were studied in most details. The profile of J2 reservoir consists of three
units, different in structure, sediments composition and genetic types. Profile is characterized
by transgressive-regressive structure, where sedimentary conditions are changing bottom-up
from fluvial-deltaic, directly deltaic to delta fronts. Composition of sandy fractions also
changes in this direction from quartz greywacke to medium-grained rocks.
Several levels of ancient oil-water contact are mapped within gas-saturated part of the
reservoir.
This reservoir have passed through the complex generating process. Apparently, primarily
small oil pools were formed in the Middle Jurassic sediments due to hydrocarbons generation
by Triassic source rocks of predominantly humus composition. Then reservoir was abruptly
filled by gas, which has destroyed original oil pool. Evidence of this is given by residual
bitumen saturation of the rocks. Currently J 2 reservoir is saturated by gas condensate which
was formed by underlying and lateral flows of Upper Jurassic hydrocarbons.
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Conclusions
Jurassic sediments of Barents Sea haven’t been studied in a full volume yet and a lot of
aspects of complex structure still stay unknown.
Researching based on genesis knowledge and on the drilling available data of
Shtokmanovskoye and Snohvit fields gives an opportunity to compare complexes structures
and find some regularity. Based on this study it might be possible to correlate the megasequences of the Barents Sea. A gradual thinning of sequences from east towards west
indicates a general increase in accommodation space during deposition towards east. In
general the Jurassic sandstones have rather good reservoir properties. The regional extent of
the Jurassic reservoirs indicate that the reservoirs will be present in the entire area accept for
places where the formations have been subject for erosion.
Acknowledgements
Lomonosov Moscow State University, Arctic Research Cooperation, StatoilHydro ASA,
VSEGEI (All-Russia Scientific and Research Geological Institute Named after Karpinsky) is
acknowledged for help and information.
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71st EAGE Conference & Exhibition — Amsterdam, The Netherlands, 8 - 11 June 2009