This paper reviews the means by which geological uncertainty has been addressed in a complex marginal field development. The Gannet C field development carries a significant element of risk resulting from a challenging drilling environment and large geological uncertainty due to poor structural definition on seismic. By creating extreme high- and low-case structure maps during a re-mapping exercise following completion of the first drilling round, a better grasp of the possible up- and down-sides to the development has been obtained. This case study shows that targeting horizontal wells using high- and low-case scenarios enables the geologist to optimise the well location within the confines of available data and to anticipate and react confidently to the unexpected drilling result.

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