Evaluation of a prospect is not limited to the prospect proper, i.e. the site preparation, wells, installations for the field, flowlines to a gathering point, but also the larger infrastructure. But such infrastructure, e.g, a pipeline to an onshore terminal, is often shared by more fields. In that case the question is "What part of the infratructure should be assigned to a particular prospect". This is of particular importance in the exploration phase, where an economic cutoff has to be estimated.
Therefore group of prospects requires a two-stage evaluation. Several prospects will use the same infrastructure for the evacuation of the oil. A prospect in the group should not be penalized by the cost of the infrastructure, i.e. a long pipeline. So the evaluation goes in the following steps: