The intra-plate earthquake on the Lleyn Peninsula, North Wales afforded a rare opportunity to obtain accurate focal parameters for the main shock and the aftershock sequence. The focal mechanism does not correlate with movement on any of the large faults showing surface expressions in the area but does indicate a NW–SE oriented maximum compressional stress direction, in agreement with measurements made in other parts of Britain. Aftershock activity was concentrated around the focus of the main shock at depths of between 21 and 24 km. The depth indicates an unusually deep transition between brittle and ductile zones that is possibly a consequence of low heat flow in the region.

The Brent Field consists of a major, westerly dipping fault block, with a crestal area broken up into a series of slumped fault blocks. Development of these complex slumped blocks started in 1993 and to date a series of 9 roughly parallel horizontal east-west oil producers have been drilled, some of which have subsequently been converted into water injectors. Oil production to date, at a total rate of up to 100,000 b/d, exceeds the 1993 Development Plan forecast by 11%, despite poorer than expected east-west and north-south communication in some parts of the field. Key conclusions from the first phase of the development are:-–The Development Plan concept of multi-purpose horizontal wells (appraisal, production, injection) has been successful mainly because of the validity of the geological model. This has allowed optimal positioning of wells, with sand-sand juxtaposition initially providing sufficient pressure support via the West Flank.–Controlled reservoir depletion can be used as a method to establish fault block connectivity and to steer the flow of injection water. Uncontrolled depletion will lead to unwanted crossflow, ultimate recovery loss due to oil shrinkage and hazardous drilling due to large pressure differences.–The introduction of 7" monobore completions, combined with consecutive perforation and isolation of 150–250m reservoir sections, has increased ultimate recovery and reduced cost. Multi-straddle assemblies have proved to be ineffective, because communication patterns cannot be predicted accurately enough. An extensive data gathering programme has now been largely completed, consisting of new 3D seismic, borehole imaging logs, radioactive tracer injection, interference testing, advanced PLT logging and permanent downhole gauge output. This has so far resulted in an adjustment to the Development Plan in the compartmentalised Brent Horst and Graben areas, successfully targeting one water injector to support two producers 1500m apart, and in one infill well to appraise the quantity and quality of reservoir rock between the top Etive and the X-unconformity erosional surface. In combination with advanced drilling techniques a recovery factor of 50% is now considered achievable. Introduction The Brent Field consists of a major, westerly dipping fault block, with a crestal area broken up into a series of slumped fault blocks. Development of the Brent slumps started in 1993 and to date a series of 9 roughly parallel, horizontal east-west oil producers have been drilled, two of which have subsequently been converted into water injectors (see Figure 1). The wells in question are listed (in chronological order) in Table 1. They are all part of the first stage of the 1993 Development Plan (ref. 1), eventually comprising of 6 horizontal producers, interspersed with 6 horizontal water injectors. P. 135^

SynopsisAt 10h 47m 59s GMT on April 13, 1980 a seismic event occurred at about 56·5° North, 10·5° West near the Hebrides Terrace Seamount. The seamount is believed to be an early Cenozoic volcanic feature (Buckley and Bailey 1975) and lies in deep water in the eastern part of the Rockall Trough. The event may have been either an earthquake or a large underwater explosion. The seismic evidence, based on recordings from stations in Scotland and Ireland, is that it was an earthquake. It is the first earthquake to be recorded from the contiguous continental margin to the West of Britain and Ireland.

No abstract

As the Brent Field moved into the latter stages of field life it had become increasingly apparent that the resolution of the 1986 Brent 3D survey was insufficient to meet the demands for increased geological detail. Hence in 1995 a 320 sq.km 3D seismic survey was acquired over the field with a denser subsurface sampling than the previous survey. Compared to the previous survey the seismic resolution of this dataset has clearly improved. Fault definition is superior, especially in the so-called "slumped" areas, where a substantial portion of the field's remaining reserves are located. Detailed field wide structural maps will form the basis for the year end review of reserves and have already been incorporated at an early stage into detailed dynamic reservoir models of complex areas. The improved seismic data quality of the new survey justifies the application of "state-of-the-art" geophysical techniques to obtain additional information which could help the determination of reservoir quality characteristics and the prediction of produced fluid contacts. These techniques include Amplitude Vs Offset, Seismic Inversion, Amplitude studies, and 4D Time Lapse studies. Parallel to these initiatives continuous data processing refinements are being tested (amongst others pre-stack depth migration) in an effort to further enhance seismic resolution. The challenge is to manage these large numbers of continuously improving seismic datasets, whilst at the same time maintaining their impact on the maximisation of the remaining value of the Brent Field. Introduction The Brent field, located 160 km NE of the Shetland Islands in the UK sector of the North Sea, was discovered in 1971 and production started in 1976. Hydrocarbons are produced from the Jurassic Brent and Statfjord reservoirs trapped in a westerly dipping monocline with slump (crestal collapse) features. Remaining reserves are currently estimated at around 400 MMbbl oil and 2.6 TCF gas. A new phase of development has started for the Brent field where reservoir depressurisation will gradually transform it from an oil to a gas producing field. Major efforts are now concentrated on locating and developing the field's thin remaining oil rims. It had become increasingly apparent over the last few years that the previous 3D seismic survey, shot in 1986, was insufficient to meet these demands. Re-processing of this survey, carried out in 1989 and again in 1993, had failed to significantly improve the quality of the data which considerably downgrades its usefulness. Hence a new high resolution 3D survey was acquired in 1995. In addition a high quality seismic dataset prior to field depressurisation may provide a suitable base line survey for possible future time lapse seismic surveys. Acquisition and Processing The survey was acquired over a three month period in the summer of 1995 by a purpose built seismic survey vessel, with an eight streamer/one source acquisition configuration. The total survey area covered some 320 sq.km, acquired with subsurface sampling four times denser than the 1986 survey (Fig. 1). A 3D VSP shot in 1993 had demonstrated that enhanced resolution of the subsurface was achievable given recent advances in acquisition and processing techniques. The seismic resolution of the new dataset has clearly improved with respect to the 1986 dataset. P. 83^