A natural oil or gas reservoir that exists below a layer of sedimentary rock. Wildcat wells are the first wells to be drilled in a particular geographic location. Appraisal wells are drilled after hydrocarbon has been identified. Development wells are drilled according to a predetermined pattern to maximize the amount of oil hydrocarbon that can be recovered. This is based on available seismic surveys and other geological data. Injection wells are used for the injection of fluids into the reservoir for enhanced oil hydrocarbon recovery. The wellhead is the top of the oil or gas well.