Signs of gas manifestation in recent sediments of the Barents-Kara shelf

Abstract

It is known that permanent or periodic influx of fluids, hydrocarbon gases included, from the Earth’s interior to its surface indicates the high petroleum potential of a region. Distinct signs of gas emission have been noted by gasometric, geothermal, and seismoacoustic investigations in many areas of the Pechora‐Kara shelf [1‐4]. In this connection, gas manifestations in recent sediments of the shelf are of great scientific and practical interest. Core sampling of recent sediments on the shelf of the southern part of the Barents and Kara seas revealed structural features indicating gas influx that had not been noted by previous researchers. Traces of gas migration are seen clearly in fresh sedimentary cores, particularly along their longitudinal sections. The traces occur as tear-shaped structures or differently shaped hollows related to the decomposition of organic matter in sediments with the release of CO 2 , H 2 S, and hydrocarbon gases. They frequently represent derivatives of hydrotroillite, which is formed in a reducing environment during the interaction of products of the metabolism of sulfate-reducing bacteria (iron hydroxides and free H 2 S). In the shallow southeastern part of the Barents Sea (Pechora Sea), surface sediments are almost continuously saturated with gas. In the study area, Pleistocene sediments are saturated with free gas. Direct signs of gas saturation in sediments were also recorded in cores of some offshore wells. In surface sediments (up to 10 mbsf), this is evident from the poignant hydrosulfuric smell. Below this depth, the gas occurrence is reflected in the development of swells and the emission of bubbles at the core surface. The laboratory treatment of cores in three-axis compression chambers was also accompanied by the release of gas bubbles from the