Seals are an important and often overlooked component in the evaluation of a potential hydrocarbon accumulation. Effective seals for hydrocarbon accumulations are typically thick, laterally continuous, ductile rocks with high capillary entry pressures. Seals need to be evaluated at two differing scales: a "micro" scale and a "mega" or prospect scale. Quantitative "micro" data measured on seal rock hand specimens are difficult to extrapolate a billion-fold to the scale of the sealing surface for a hydrocarbon accumulation. Fortunately, each class of exploration prospects has recognizable seal problems. Technical review should focus on the characteristic seal problems that result from the structural form and the stratigraphy of classes of prospects. Anticlines have relatively little seal risk since any horizon serving as a top seal will also be a lateral seal. Stratigraphic traps have substantial seal risks. Faulted prospects need to be very carefully analyzed, as they have inherently large seal risks. Hydrocarbons are not distributed randomly or arbitrarily on complexly faulted structures. Hydrocarbon entrapment follows very simple physical principles, and preferential hydrocarbon distribution can be predicted, given adequate data. An improved assessment of seal risk for an exploration prospect directly affects the accuracy of estimation of exploration success.

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