Hydrocarbons occur in the subsurface in four modes: (1) continuous phase oil or gas, (2) isolated droplets of oil or gas, (3) dissolved hydrocarbons, and (4) associated with kerogenous rocks. All of these modes can result in what is described as a subsurface hydrocarbon "show." Only continuous phase occurrences of oil and gas indicate that a trapped and potentially producible accumulation of hydrocarbons has been discovered. Free oil or gas recovery from the formation on subsurface hydrocarbon saturations of greater than 55% indicates a continuous phase occurrence. Continuous phase oil or gas can extend either updip or downdip from a commercial reservoir, and can also be interpreted quantitatively to determine how large an oil or gas column is required downdip to explain the show. Reliable estimates of the downdip hydrocarbon column; exploration for subtle stratigraphic and combination traps.--Modified journal abstract.