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 must be evaluated at two differing scales: a "micro" scale, and a "mega," or prospect, scale. Quantitative micro data measured on hand specimens are difficult to extrapolate a billionfold to the scale of the sealing surface for a hydrocarbon accumulation. Fortunately, each class of exploration prospects has a different set of seal problems. Geologic work can be focused on the characteristic seal problems that plague classes of prospects. Anticlines have relatively little seal risk, since any zone serving as

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a top seal will also be a lateral seal. Stratigraphic traps and faulted prospects have substantial seal risks. Hydrocarbons are not distributed randomly or arbitrarily on complexly faulted structures. Their distribution follows simple physical principles, and preferential hydrocarbon distribution can be predicted, given adequate data. Improvements in assessing seal risk for an exploration prospect directly affect the estimation of exploration success.

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