Petroleum exploration has extended throughout the world, and current knowledge of world oil occurrence is extensive as well as intensive. World-class giant gas fields represent less than 1 percent of the world's gas fields but contain about 80 percent of the world's proved and produced gas reserves. As modern petroleum exploration is an efficient process, it is probable that most oil in a region will be found by the first 25 to 200 new field exploratory wells, if giant fields exist. When the generation and migration of gas is considered, the extensive vertical gas zone includes shallow biogenic gas, intermediate dissolved gas of the oil window, and deep thermal gas. Low recovery efficiencies and expensive production methods will inhibit heavy oil utilization and prevent it from providing a volume of liquid petroleum similar to that of the lighter oils. A prudent projection of gas production from unconventional geologic sources is that it will supplement rather than replace conventional gas production.