Extruded basalt flows overlying sedimentary sequences present a challenge to hydrocarbon exploration using reflection seismic techniques. The Lopra-1/1A re-entry well on the Faroese island of Suðuroy allowed us to study the seismic characteristics of a thick sequence of basalt flows from well logs and borehole seismic recordings. Data acquired during the deepening operation in 1996 are presented here. The re-entry well found that the seismic event at 2340 m, prognosed from the pre-drill Vertical Seismic Profile (VSP) as a decrease in impedance, was not base basalt and the deepened well remained within the lower series basalts. Nonetheless, compressional and shear sonic logs and a density log were recorded over the full open hole interval. These allowed a firm tie to be made with the reflected wavefield from a new VSP. The sonic logs show a compressional to shear wavespeed ratio of 1.84 which is almost constant with depth. Sonic compressional wavespeeds are 3% higher than seismic velocities, suggesting dispersion in the basalt flows. Azimuthal anisotropy was weakly indicated by the shear sonic log but its orientation is consistent with the directions of mapped master joints in the vicinity of the well. The VSP downgoing compressional wavelet shows good persistence, retaining a dominant period of 28 ms at 3510 m depth. Average vertical velocity is 5248 m/s, higher than previously reported. Attenuation can largely be modelled by geometrical spreading and scattering loss, consistent with other studies. Within the piled flows, the effective Q from scattering is about 35. Elastic layered medium modelling shows some hope that a mode-converted shear wave may be observed at moderate offsets. Like its predecessor, the 1996 VSP indicates a decrease in impedance below the final depth of the well. However, it is unlikely to be basement or sediment and is probably an event within the volcanic sequence.

The paper was selecied for presentatmn by Ihe OTC Prcgramme Committee folbwicg rtiw of reformation contained withii an abstracl subnmted by tie authors. Ccmt6mtsof the aPeI as presented have not been reviewed by the Offshore Technology /' Con erence and are SULYOCI to correclti W tie authors. The materiel as presented dc%s not necessarily reflecl any posilmn of the Offshore Tedmolcgy Conference w iis offiiefs. Permiasicn 10 copy is resfrkted 10 an abslract of not more than 303 words. Illustrations may not be copiedThe abstract should contain conspicuous a&nOwMgment of where and by W+VXIIthe paper was wesented. AbstractThe use of advanced 3D seismic techniques to explore, appraise, develop and monitor the West of Shetland Tertiary play has evolved very rapidly. This has been driven by the business desire for rapid pace in a "new" basin. Sub-surface uncertainty has been managed through holding open multiple working hypotheses of the distribution of the reservoirs and their fluid fill.This talk covers the range of possible rock, fluid and seismic response models that the Atlzntic Frontier Programme team has addressed and what has been learned along the way, Also discussed are the techniques developed to rapidly utilise much of the information contained in the 3D seismic data set.Clear understanding of the strategic business value of rapid basin development has driven the sub-surface team to find, appraise and begin development of two major projects encompassing three fields in less than 3 years. This has been achieved with little prior knowledge of reservoir and fluid distribution or productivity in a notoriously difficult seismic quality area.

This paper was prepared for presentation at the 1999 SPE Offshore Europe Conference held in Aberdeen, Scotland, 7–9 September 1999.

This paper describes how fast track reservoir appraisal has been achieved in prospects to the West of Shetlands through an integrated and innovative programme of data acquisition. The approach addressed the key uncertainties in evaluating the prospect by using a combination of the logging, seismic and core techniques. These methods have resulted in significant cost and time savings by reducing core acquisition and minimising the need for Drill Stem Tests. Introduction Background. The West of Shetland province lies approximately 120km west of the Shetland Islands (Figure 1). Following discovery in 1992, a fast track development programme was undertaken in order for the province to sustain BP's maturing North Sea portfolio. The prospects are set in deep water (200–1000 metres) and are exposed to difficult met ocean conditions. Drilling costs typically constitute over 30% of the project budget and any reduction in well numbers and rig time resulting from an efficient reservoir appraisal would heavily impact the project economics. Consequently, the requirement for early oil delivery combined with the expensive drilling environment drove the development of a rapid reservoir appraisal programme. Reservoir Appraisal Traditional reservoir appraisal has taken a sequential approach to addressing the key uncertainties in reservoir prospectivity. As a result the appraisal of North Sea prospects has taken on average nine years from discovery to delivery of first oil. By using an integrated approach which has required the simultaneous planning of field development and infrastructure, it will have been possible for BP and partner Shell to reduce this time to 4 years and dramatically increase the profitability of the project. To assist the integrated approach, reservoir appraisal strategy had to be geared to constraining the most influential reservoir uncertainties as early as possible. The aim of this paper is to illustrate how this was achieved by a holistic and innovative programme of data acquisition. Key Uncertainties The Parametric Method provides a means of illustrating the influence of key uncertainties on reservoir prospectivity. Uncertain parameters are identified and their base, upside and downside cases estimated. P. 63