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A low-porous permeability sealing layer that impedes fluid escape from a reservoir. Seal effectiveness depends on continuity, connected fractures, capillary entry pressure and effective stress, rather than simply the geological name of the layer.

Potential leakage paths are permeable or reactivated faults and connected fractures cutting the sealing sequence; lateral migration through connected reservoir layers into an unsealed outcrop or spill point; and natural or artificial penetrations such as poorly sealed abandoned wells or a compromised injection well. Injection raises pore pressure, potentially overcoming the capillary entry pressure of a seal, hydraulically opening fractures, or causing fault reactivation through reduced effective stress.

Seal continuity and porous permeability matter more than the rock name alone: a fractured or porous limestone need not be an effective caprock. Dissolution, chemical alteration or well-cement degradation can change leakage pathways over time. The underlying salt may be an effective barrier beneath the reservoir, but it does not seal the roof. Conversely, dissolved or residually trapped carbon dioxide is less mobile than a connected buoyant free-fluid phase, so connected porosity and fluid state also affect leakage.