Cap-rock seals can be divided genetically into those that fail by capillary leakage (membrane seals) and those that fail due to fracturing or wedging open of faults (hydraulic seals).

A given membrane seal can trap a larger column of oil than gas at shallow depths, but at greater depths, gas is more easily sealed than oil. Where a gas cap overlies on oil rim, however, the maximum-allowable two-phase column is always greater than if only oil or gas occurs below the seal.

This trap contrasts with the hydraulic seal, where the seal capacity to oil always exceeds that for gas. Moreover, a trapped two-phase column, at hydraulic seal capacity, will be less than the maximum-allowable oil-only column, but more than the maximum gas-only column.

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These conclusions, derived for membrane cap rocks, also apply to sealing faults sensu stricto and juxtaposition faults--hydrocarbons trapped laterally against a juxtaposed sealing unit. The two-phase fault seal capacity is greater than for equivalent oil-only and gas-only columns, but less than that predicted for a horizontal membrane cap rock under similar conditions. Where a two-phase column occurs on both sides of a sealing fault (at two-phase seal capacity), the differences in OWCs and GOCs are in part dependent upon the seal continuity in the oil and gas legs.

The developed seal theory has important implications for prospect appraisal and can be directly applied to a series of development geologic problems, involving fault leakage during depletion and analysis of barrier efficiency from RFT profiles.

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