ROKOS, S.I. JSC "ARCTIC MARINE ENGINEERING GEOLOGICAL EXPEDITIONS, "Russia

Wide spread occurrence of gas containing sediments is a characteristic of the Barents and Kara shelf The signs of gas presence could be seen on the time sections of shallow seismic reflection survey as "bright spots", correlation ghosts and other irregularities of acoustic record.

Every area where gassy sediments occur has its own features of gas manifestation depending on geology, Pleistocene paleography and sedimentation environment.

hi the deep area of the Barents and Kara shelf free gas occur within relatively small scattered sites. On the seismic records there are amplitude anomalies in the inner reflectors within the Mesozoic strata and correlation ghosts corresponding to pockmarks on the seabed. Obviously the presence of free gas in the deep areas of the Barents and Kara shelf is related to fluid migration out of deep sedimentary horizons and maybe of oil and gas bearing structures.

In the shallow south-eastern part of the Barents Sea (so called Pechora Sea) all the Pleistocene sediments are saturated with free gas. It is thought to be a result of the permafrost degeneration under influence of recent transgression. Sub-zero temperature suppressed decay of buried organic matter, but the effect of comparatively warm transgressing waters cause its drastic decomposition with release of biogenous gas. Moreover, permafrost acted as a seal and after its melting began the gas accumulated underneath was instantly released into sedimentary section.

Distribution of free gas within sediments went in accordance to their reservoir properties. The areas where the upper part of sedimentary section is composed of thick clayey sediments of Karguinsky age are characterized by clear-layered pattern of seismic imaging. The structure of the section can be traced on the seismic records down to the foot of these clays which act as a seal for underlying strata. Local gas vents from underlying gassy sand can be seen in the crests of anticlines and in places of sharp bending of layers. The underlying gas-saturated sands act as a shallow gas trap and are acoustically opaque.

In the estuaries of big rivers, such as the Ob and Taz Bays, active accumulation of recent alluvial-marine sediments comes about. Due to vigorous decomposition of buried organics the sediments here are saturated with shallow gas through the whole section from the very seabed. They’re no reliable reflectors on the seismic records, which can be traced below seabed. Only in some local places where the recent sediments are washed-out or have never been accumulated the section shows acoustic transparency.

There also occur free gas emanations coming out of dislocated basement rocks.

ARCTIC SHELF OIL AND GAS CONFERENCE 2004