Four main issues control the occurrence and distribution of oil and gas: source, reservoir, seal, and trap.

A source is a fine-grained rock unit containing sufficient organic matter so that when it is heated and/or placed under pressure (maturation), hydrocarbons are generated. If the organic matter is of marine algal origin, then the source rock is most likely to generate oil under optimum maturation conditions, whereas rocks dominated by land plant matter will tend to create gaseous hydrocarbons. The hydrocarbons are of lower gravity than the surrounding groundwater and, therefore, move away from and generally upwards (migration) from the source rock until they are trapped in a reservoir.

A reservoir is a rock unit that acts as a storage device for the hydrocarbons that migrate from the source rock. Hydrocarbons are retained within the reservoir because these rocks contain numerous pores (essentially microscopic-sized holes) between...