Multiple Hypothesis Reservoir Description Utilising Advanced 3D Seismic West of Shetland
Authors: Hanna Robert W. , Currie Michael T. , Connolly Pat A. , Cowper David , Smith Robert L. , Linskaill Chris J. 
Published: October 2008

The 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.

Introduction

The discovery of the Foinaven field in November 1992, presented BP and partner Shell with a significant opportunity, to rapidly determine if this new play in a frontier basin could be economically developed, and lead the industry in that knowledge. What was unknown at the end of 1992 was: How big was Foinaven? How profitably could it be developed? Are there any others? How fast could all of these questions be answered?

This paper describes the sub-surface uncertainty facing the Atlantic Frontier team afler the discovery of Foinaven and the methods developed and applied to rapidly understand the value of the play.

History

The Faroe-Shetland Basin (Fig. 1) had been explored for nearly 20 years without a commercial development until the discovery of Foinaven. This history (Ref. 1) had followed a similar general pattern to the North Sea: preand syn-rift exploration followed by Tertiary (post-rift) exploration. Pre-rift discoveries occurred early, such as the 5 billion STOIIP Clair field in 1977, but due to poor flow test results this discovery remains undeveloped. By the early to mid 80's the focus moved to the Tertiary where the promise of better reservoir quality was anticipated. Exploration of this fairway during the 80's found sub-economic gas. The use of 2D seismic amplitude anomalies to guide exploration also suffered as not all anomalies were found to be hydrocarbon related.

In the Faroe-Shetland Basin (Quad 204 area), a 9th round commitment well was drilled in Block 204/24 during 1990. The well was to test a pre-rift structure near its crest. An amplitude anomaly was identified in the Tertiary above the ridge and the well location was modified to encounter it. Amplitude and AVO (Amplitude Variation with Offset) analyses were performed and predicted a 30 meter thick gas sand in the Tertiary target. The well never reached the primary pre - rift target due to closure of the weather window, but did penetrate the entire Tertiary sequence. 8 meters of oil bearing and 7 meters of gas bearing, thin bedded sands were encountered at the Tertiary amplitude anomaly.

What would become the Foinaven field had been discovered, but not recognised!