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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) hydrocarbons associated with kerogen-bearing rocks. Any of these modes of occurrence can result in a subsurface hydrocarbon "show." Each type of show has strongly different implications for exploration and must be differentiated as the first step in show interpretation. 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, or subsurface hydrocarbon saturations of greater than 50%, indicate a continuous-phase occurrence.
Continuous-phase shows can be interpreted quantitatively. The static hydrocarbon column downdip from a continuous-phase occurrence can be calculated from one well bore if the subsurface oil or gas saturation, capillary properties, hydrocarbon-water interfacial tension, oil density, and water density of the reservoir are known. Producing wells are by definition continuous-phase oil or gas, and estimates of oil-water or gas-water contacts from this type of analysis can be useful in orderly and profitable field development.
Continuous-phase oil or gas can extend either updip or downdip from a commercial reservoir. Continuous-phase shows can also be interpreted quantitatively to determine how large an oil or gas column is required downdip to explain the show. By this method it can be determined whether an exploratory well penetrated the updip waste zone or downdip transition zone of an oil or gas field. Field studies indicate that quantitative interpretation of a noncommercial show can provide reliable estimates of the downdip hydrocarbon column. This type of data can be used in a systematic manner to explore for subtle stratigraphic and combination traps.
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