ABSTRACT

A wide range of features observed on a shallow tune slice series from the upper continental slope has been plotted and classified. They are interpreted as sediment dislocation planes, gravity flows and manifestations of shallow gas. Stratigraphic subdivision revealed marked variations in the assemblages of features present down through the sequence, including indications of earlier gas hydrate activity. No evidence was found for modern gas hydrate but the present pattern of gas migration is seen in considerable detail. Tune slices clearly provide a valuable method for locating and identifying potential engineering geohazards throughout an area. This has practical applications both for site selection and investigation.

INTRODUCTION

A detailed desk study was carried out on a shallow tune slice sequence from Elf's UKCS Block 205117a on the upper West Shetland Slope (Fig. 1) to identify potential geohazards. The 3D seismic dataset was Elf Enterprise Caledonia courtesy data from a regional survey acquired by BP, with time slices generated at 4 ms intervals. This survey extended into Block 205/17a as an essentially triangular area of about 45 km2, with water depth increasing steady towards the NNW, from 245 to 395 m; the shelf break at approximately 220 m lies 3 km SSE of the area.

The advantage of investigating a larger area than a site survey is that it can provide asufficiently comprehensive database for features to be classified into specific types and their distributions examined. Significant interrelationships may then be seen; features of little practical significance on their own may m combination constitute a geohazard. Once this background information has been acquired, a target site can be placed in context and evaluated with considerably more confidence.

Computer image reconnaissance revealed large numbers of features in the upper 200-300 ms below seabed, with highly variable and sometimes complex distribution patterns. While many features were clearly geological, others appeared to be manifestations of migrating gas. However, neither the identity of the features nor the reasons for the spatial variation were known.

GEOLOGICAL SETTING

The Ternary and Quaternary deposits form a prograding wedge extending into the Faeroe-Shetland Basin with post-Eocene deposits accounting for the top c.200 m in this area. The British Geological Survey (BGS) informal seismostratigraphy is fully described in the BGS regional offshore report (Ref 1) and on the "Foula" and "Judd" map sheets (Refs. 2–5). Well and borehole information for the upper slope is scarce throughout the region.

A relatively thin "Oligocene-Pliocene" interval of probably mid-to late-Miocene deposits is succeeded by the 140-200 m thick Morrison sequence which spans some 2 million years from late Pliocene through almost the entire Quaternary. It is divided by BGS into two seismically defined packages. The lower Package 1 was considered to predate glaciation of the shelf, but Package 2 is characterised by higher rates of sediment supply attributed primarily to the Anglian and Devensian ice sheets. These existed at about 0.4 ma and 20 ka BP, respectively. Much of the upper sequence was interpreted as slumps

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