In all producing basins, most of the oil and/or gas reserves are concentrated in a minority of fields. This distribution of oil and gas reserves is the result of ＇＇focusing＇＇ during lateral and vertical basin-wide migration. Although the physico-chemical and geological processes involved in hydrocarbons migration are not yet fully clarified, lateral migration directions can be established by integrating geological, geophysical and geochemical data. The Petroleum Systems concept is of use here. When combining Petroleum Systems data with well-controlled regional structure maps, areas of hydrocarbons concentration (i.e. migration focusing) can be defined as areas of the highest exploration potential. Some basic petroleum-geological data are reviewed from the Eastern Venezuela Basin and the Llanos Basin region in Colombia. By using published Bouguer gravity maps as regional structural form-line maps, areas of preferred lateral migration are shown to be areas of maximum hydrocarbon accumulations. This leads on to an exploration approach in which Petroleum Systems data and regional (gravity or other) structure maps are used to evaluate acreage for its exploration value with a minimum expenditure of time, manpower and related funds.