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
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