A well drilled to measure the size / quality (commercial potential) of a hydrocarbon discovery. Before development, a discovery is likely to need several such wells.
a well drilled as part of an appraisal drilling programmewhich is carried out to determine the physical extent, reserves and likely production rate of a field;
a well drilled to further confirm and evaluate the presence of hydrocarbons in a reservoir that has been found by a wildcat well. 
A well drilled as part of an appraisal drilling programme which is carried out to determine the physical extent, reserves and likely production rate of a field.