From: "James Clarke" Subject: Letter No. 29, January 24, 2000 MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 X-MSMail-Priority: Normal X-Unsent: 1 X-MimeOLE: Produced By Microsoft MimeOLE V4.72.3110.3 Jurassic of Barents Sea

Petroleum Potential of Jurassic of Barents Sea
Internet Geology News Letter No. 29, January 24, 2000

Two large depressions of more or less equal size
comprise most of the Russian sector of the Barents Sea.
These are the South Barents depression on the south,
and the North Barents depression on the north.
Between the two is the Ludlov saddle.

Jurassic sands were deposited on the Russian Shelf of the
Barents Sea under deltaic and near-shore marine conditions,
at a time when the climate was warm and humid and organisms
flourished. Regional clay seals were also deposited along with
the sands. Study of this region is still in an early stage, and
many aspects of its geology are debatable.

The oil and gas fields in the Jurassic section of both the
Russian and Norwegian sectors of the Barents Sea are at
depths of more than 1500 m and occur on saddles and highs
separated by deep depressions and downwarps.

Deposition of the Jurassic sediments and formation of
the tectonic features took place under conditions of uneven
subsidence on a background of regional slope from north to
south. Inheritance of structural plan is clearly apparent in
the North Barents depression, where highs and intervening
saddles have developed. Subsidence was more intensive in
the South Barents depression, particularly in the southern
part where the Goose downwarp formed.

The Lower-Middle Jurassic section consists of alternating
sandstones, siltstones, and clays deposited under continental,
lagoonal-continental, and near-shore marine conditions.
Shallow-water sand-silt facies collected at the crests of
highs, and clayey fractions in lower (starved) areas.
Land areas were located in the regions of the Svalbard
antiklize, Franz Josef Land, central part of a Central
Barents zone of highs, the Baltic Shield, Timan-Pechora
antiklize, and Novaya Zemlya. These areas were the main
sources of detritus. Other sources were large highs out
in the basin, where the Jurassic is partly or fully eroded.

A broad transgression under relatively calm tectonic
conditions began in the Late Jurassic. Bituminous clayey
sediment predominates in the section. Clays high in
organic matter are associated with sectors of uncompensated
downwarping and are Kimmeridgian and Volgian (Tithonian)
in age.

Seismic surveys in the Barents Sea have diclosed
a series of highs including large syn-sedimentary types.
These are host to gas and gas-condensate fields. The main
pay zones of the super-giant Shtokmanov field are Middle
Jurassic sandstones: Callovian, Bajociam, and Aalenian,
with net pays of 51-68, 49-76, and 21-71 m, respectively.
Porosity ranges from 15 to 25 percent, and permeability
is in the 200-800 md range.

A Callovian-Volgian seal is regional and has maximum
thickness of 438 m. A Bathonian-Callovian seal is sub-regional
and has maximun thickness of about 300 m. A lower
Bajocian-upper Aalenian seal consists of two members,
is zonal, and has maximum thickness of 85 m.

Search for new petroleum deposits should be directed
not only toward the many unexplored structures but also
extended to deeper targets below the producing zones of
the known fields (taken from Petroleum Geology, vol, 29,
no. 1/2, p.32-36, 1995, one tectonic map).

Copyright 2000 James Clarke. You are encouraged to
download this News Letter and to forward it to others.
Earlies News Letters are available at:
http://geocities.com/internetgeology/ 1