Low-Porosity
Triassic Reservoirs of Barents Sea
Internet Geology News Letter No. 104, July 2, 2001
Murmansk gas field in Triassic clastic rocks
on the southwest
border of the South Barents depression was the first offshore discovery
on the Arctic Shelf of Russia. Delineation drilling, however,
disclosed
several unfavorable geologic features: block structure of the field,
fault
disruption of the strata, their lens character in places, and pinchouts
- all leading to reassessment of the commercial importance of the
field.
These Triassic reservoir rocks are fine- to
medium-grained
sandstones, commonly polymict. Cement is largely clay, rarely
clay-carbonate.
Porosity is not high. Values determined by geophysical logging
range
from 8.5 to 25.1 percent, and averages in the interval of perforation
are
from 12.1 to 19.6 percent. Porosity determined from cores from
the
productive interval ranges from 6.6 to 19.3 percent, and average rarely
exceeds 16.3 percent. The higher values determined by the geophysical
logging
are attributed to high clay content of these Triassic reservoirs and
the
impossibility for correcting for this clay content.
The high degree of cementation by clay and
clay-carbonate
has resulted in low permeability. Yields of gas have ranged from
35 to 738 thousand cubic meters per day. The most common yields have
been
200 thousand (3.5 million cubic meters).
Measurements made on core from the Triassic of Severo-Kil'din gas field
gave values of porosity of 10.6 to 21.2 percent. This parameter
is
rarely less than 12 nor greater than 20 percent. Average is 16
percent.
Permeability is 0.1 to 21 md with a tendency to increase with depth.
The reservoirs in Shtokmanov field are fine-
to medium-grained
Jurassic sandstone, which is weakly cemented. Porosity of Stratum
VI in the 1920-1963 m interval measured on core is 19.5 to 25.6 percent
with average of 23.9 percent. Permeability ranges from 264 to
1097
md with average of 878 md. Gas yields have been around 200
thousand
cubic meters per day.
Although reservoir properties are
substantially different,
yields at Murmansk and Shtokmanov have been about the same. This
is attributed to anomalously high formation pressure in the Triassic
rocks
at Murmansk field.
The Triassic rocks in the west Cis-Novaya
Zemlya area
are expected to be host to oil pools.
Overpressure is predicted for depths greater
than 3000
m in the Triasssic on structures in the north of the South Barents
depression
and northwest flank of the Gusinozemel plateau. This overpressure may
compensate
for the low permeability of these Triassic reservoir rocks.
(Taken from Tumanov and Serebryanskaya, 1991;
digested
in Petroleum Geology, vol. 28, no. 9/10, one map)
Copyright 2001 James Clarke. You are
encouraged
to print out this News Letter and to forward it to others. Earlier News
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