The phase of petroleum operations that immediately follows successful exploratory drilling. At this stage, appraisal wells are drilled to further define the resources of the structure. During appraisal, delineation wells might be drilled to determine the size of the oil or gas field and how to develop it most efficiently. When an appraisal well is carried out its location is usually sited to test another part of the structure where the operator expects there to be similar hydrocarbon bearing sands, at a location several kilometers away from the first exploration well, and probably closer to the predicted gas-water contact, in order to see if they can pin down where the edges of the reservoir are in order to best estimate the size thereof. In addition, if the appraisal well shows thicker sands, then this is a further positive indication of the quantity of hydrocarbons, if they come in thinner then potentially there could be less gas present then estimated after the exploration well and after the initial logging and flow rate tests. The greater the number of wells placed the higher the accuracy