A trap for hydrocarbons requires the simultaneous existence of (a) a reservoir, (b) on isolated region of low potential in the reservoir, and (c] a barrier seal) with high enough entry pressure to retain a commercially producible volume hydrocarbons. Three kinds of traps exists structural, stratigraphic, and hydrodynamic. All three kinds have a reservoir bounded by 1:1 barrier but differ in what causes the isolated area of low potential. In classification of hydrocarbon accumulations, the conditions that determined the present location of the accumulation should be used where they can be ascertained.