About 62% of the caprock seals for the world's 176 giant gas fields are shales and about 38% are evaporites. These two lithologies make up essentially all of the caprock for the 2,650-Tcf of gas expected to be recovered from these fields. Caprock thickness data is lacking from many fields, but 20 to several hundred meters are typical.

Optimal conditions for seal preservation occur in areas which had a comparatively simple geologic evolution. Complex fold belts and overthrust belts are commonly subject to seal destruction. Of the world's 25 largest gas fields, 21 are in cratonic settings and four in fold belts; those in fold belts have evaporite seals.

Lateral continuity of caprock has a favorable impact on the retention of gas over large areas such as the Arabian platform.

Gas hydrate accumulations illustrate both seal and reservoir. Seal destruction is caused by changes in the phase equilibrium.

Considerable multidisciplinary research still needs to be done to quantify knowledge of seal prediction for giant gas fields.

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