A trap is of no value unless it has oil or gas in it. Prospecting, therefore, should include efforts to determine if petroleum was generated by the enclosing rocks and if it was likely to have collected behind the barriers that constitute the trap.

Observations can be made to see if the rocks and fluids contain traces of hydrocarbon which would suggest that they are source rocks. Oil seeps from breached traps around the margin of a basin commonly suggest that similar traps may contain oil downdip.

The key to stratigraphically trapped oil is the presence of barriers to fluid flow. Such barriers can be located by discontinuities in the patterns of fluid pressures. In mountainous areas, meteoric water commonly has gained access to strata which have regional continuity of permeability. Abrupt changes in water composition in these areas indicate barriers where stratigraphic factors may have preserved the petroleum.