3D seismic is a powerful and cost-effective geophysical technique to acquire data from which detailed structural and stratigraphical information can be extracted. It generally allows a realistic evaluation of reserves and uncertainty ranges required to optimise field development planning. Such a realistic quantification of uncertainties is a pre-requisite in the application of probabilistic techniques for gross hydrocarbon rock volume estimation. Consequently, the acquisition of 3D seismic at an early stage, i.e., directly following the drilling of a discovery well, is strongly recommended.

Although the cost of 3D seismic is, in some cases relatively high, the incremental cost per barrel is generally small when compared to the costs of "dry" or misplaced wells and of sub-optimal development of a field. Techniques have been developed to reduce acquisition and processing costs as well as to speed up interpretation procedures. Horizon-oriented interpretation, using automatic volume tracking programmes and attribute extraction techniques, has proven to be a fast and efficient way to outline subtle geological detail.

3D seismic interpretation greatly improves the delineation of fault blocks and the understanding of fluid and pressure communication. It has generated new prospects below and adjacent to developed fields, generally resulting in an increase in reserves, especially in structurally complex settings with stacked reservoirs.

3D seismic has contributed significantly in improving the validity of fault seal investigations by juxtaposing slices of 3D seismic data parallel to and on either side of a fault plane. These fault slices have been used to display juxtaposition across faults of seismic amplitudes related to lithology and/or hydrocarbons.

The information obtained from 3D seismic, when using these advanced interpretation techniques, is even more essential in the planning of expensive horizontal wells, which play an increasingly important role in field development.

The importance of early acquisition of 3D seismic will be illustrated by examples from several major hydrocarbon provinces in which the Shell Group is active.

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