Characteristics of pockmarks in the Norwegian Trench

Abstract

A 3 km wide corridor across the Norwegian Trench was surveyed in detail with high-resolution, side-scan sonar and subbottom profiler (boomer) in order to study pockmarks. The pockmarks in the Norwegian Trench are dish-shaped depressions found in the soft, silty clays. They range from 2 m to 300 m in diameter and appear with a maximum density of 40–50 per km 2. Lineated depressions and strings of small pockmarks were discovered. The strings are generally composed of individual pockmarks 10–15 m in diameter. Each string can be up to 400 m long. Some medium-sized pockmarks (40–50 m in diameter) were found in groups, often associated with a large parental pockmark. The deep-towed, subbottom profiler records indicated columnar disturbances (“fountains”) in the sediments below some of the pockmarks. These fountains are interpreted as vents or fissures through which gas or liquids migrate. Buried pockmarks and anomalies interpreted as boulders, possibly gas bubbles trapped in the substrata, were also found. The recordings bear evidence of gas or liquid migration originating from layers lying beneath the soft-clay sequence. A large hydrocarbon gas reservoir has recently been found during exploration drilling in the immediate vicinity of the studied pockmark area. Pockmarks may be indicators of deep-lying gas or oil reservoirs and therefore prove interesting features in relation to hydrocarbon prospecting.