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Caprock seal capacity is a function of pore size, rock integrity, regional continuity, and thickness. Calculations of the oil or gas the pore system of a rock can seal can be made by Smith's 1966 formula:

H = <fr>PdB - PdR</>(pw-ph) × 0.433</fr>

Data necessary to make these calculations are: pore size, oil and water density, oil-water interfacial tension, and wettability.

Rock integrity, or lack of open fractures, controls whether or not the pore system of the rock will be dominant in controlling seal capacity or of minor importance. Rock integrity is a function of mechanical properties, structural stress and fluid pressures in the subsurface. Mechanical properties can be measured in the laboratory and qualitatively used to estimate rock integrity when incorporated with subsurface stress conditions.

Samples from seals for 27 reservoirs were collected and analyzed for detailed lithology, pore size, and rock mechanical properties. On the basis of this sampling, all major rock types can act as local caprock seals for hydrocarbons. Evaporites and ductile clay shales are the most likely rock types to act as regional caprock seals because of their small pore size and ductile mechanical properties. Limestones, dolomites, siltstones, and sandstones can act as local caprock seals based on this sampling.

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