The geographic extent of a petroleum system is defined by the observed occurrences of genetically related hydrocarbons that emanated from a given pod of mature source rock. These related hydrocarbon occurrences and associated migration routes, in turn, are confined and limited by the presence of sealing surfaces. Where confining seals are lacking, hydrocarbons escape to the surface. Therefore, the seal rock is an essential element of the petroleum system.

In the petroleum system, there are two important classes of seals: regional seals that roof migrating hydrocarbons and local seals that confine accumulations. Major roofing seals act to confine migrating hydrocarbons to particular stratigraphic units. Any lithology can serve as a seal for a hydrocarbon accumulation. The only requirement is that minimum displacement pressure of the lithologic unit comprising the sealing surface be greater than the buoyancy pressure of the hydrocarbon column in the accumulation. In practice, however, the overwhelming majority of effective seal rocks are evaporites, fine-grained clastics, and organic-rich rocks. These lithologies are commonly evaluated as seals because they have high entry pressures, are laterally continuous, maintain uniformity of litho ogy over large areas, and are relatively ductile.

Regional evaluation of the exploration potential of an area should start with (1) determination of stratigraphic position and areal distribution of thermally mature source rocks, (2) identification of the regional seals for migrating hydrocarbons, (3) an analysis of trapping conditions focused on the areas under the regional seals and updip from the thermally mature source rocks, and (4) an examination of the distribution of hydrocarbon shows and production.