Six wells were drilled in the extensional

Six wells were drilled in the North Falkland Basin in 1998. Five of these wells recorded oil shows, and up to 32% gas was also recorded in mud returns to the rig floor. However, none of the wells encountered commercially viable petroleum accumulations. The syn‐rift and early post‐rift intervals contain thick, lacustrine claystones with oil source potential as indicated by TOC values up to 7.5% and Rock‐Eva1 S2 values of up to 102 kg HC per tonne of rock. These source rocks were immature or only marginally mature in five of the wells but had attained maturity in one of them. Modelling suggests that the main source interval may well be within the peak oil generation window in deeper, undrilled parts of the basin. Calculations of the amount of oil expelled range up to 60 billion barrels. Most of the wells tested a closely‐related set of plays in large structures asSociated with a sandstone interval near the top of the late syn‐rifi to early post‐rij? source‐rock succession. Post‐drilling geological modelling of the basin suggests that oil is unlikely to have migrated into this sandstone play at the localities tested, and that the wells consequently failed largely due to a lack of charge. Howevel; the play maintains exploration potential elsewhere. Other plays, particularly those stratigraphically asSociated with the base rather than the top of the source rock, may have a higher chance of exploration success.

Recently acquired marine reflection data provide evidence for a major geological boundary at shallow depth under the southern Bristol Channel, in the hangingwall block of the Bristol Channel thrust. The boundary is recognized to equate with the top surface of a high velocity layer previously identified along refraction/wide-angle reflection seismic lines in the north Devon area. Depth estimates from the reflection data and from a new interpretation of the earlier refraction data are combined to produce a contour map of the shallow boundary in the south-central Bristol Channel and north Devon areas. The boundary has a WSW-ENE regional trend and lies at a depth of less than 2 km immediately north of the Devon coast. It is truncated to the north by the Bristol Channel thrust and deepens southwards across north Devon and west Somerset. The boundary is interpreted as the top surface of a major litho-stratigraphic unit older than the outcropping Devonian sequence and in normal stratigraphic contact with it. However, in the absence of regional geological control the nature and age of the underlying high-velocity layer remain uncertain.

The deep structure of the Vale of Glamorgan is investigated using recently acquired seismic data. Reflection sections from a commercial seismic survey in the Vale enable the Carboniferous Limestone, Old Red Sandstone and Silurian sequences to be mapped in the subsurface. The Old Red Sandstone sequence thins westwards under the southern part of the Vale, and in the southwest corner of the Vale the base of the Old Red Sandstone is interpreted to overstep across the underlying Silurian sequence in an analogous manner to its regional overstep in west Wales. The Precambrian basement surface is not clearly represented on the sections but may coincide with the base of a reflective sequence observed in places along one of the reflection lines. Basement depth estimates from the reflection sections are compared with estimates derived from a new time term analysis of data from long seismic refraction lines. Basement depths remain uncertain to 1–2 km but appear to be greater under the central part of the Vale than under the western part and along the south coastal zone. A linked system of Variscan forethrusts is traced to outcrop in the core of the Cowbridge anticline but none involves major displacement. The Ty'n-y-Nant-Moel Gilau fault system of the South Wales coalfield is shown to be a component of the linked fault system and its current net extension is attributed to Mesozoic reactivation of a Variscan thrust involving only limited displacement. The individual thrusts appear to connect to a basal thrust that may extend northwards into or under the South Wales coalfield. The amount of displacement along the basal thrust cannot be determined reliably but it may exceed 10 km and involve large scale repetition of Palaeozoic sequences under the Vale.

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