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  | isbn    = 0-89181-602-X

  | isbn    = 0-89181-602-X

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An intact membrane seal fails when the [[capillary pressure]] (created by the height of an underlying petroleum column) exceeds the seal capillary [[displacement pressure]]. This type of seal does not fracture during [[deformation]].

An intact membrane seal fails when the [[capillary pressure]] (created by the height of an underlying petroleum column) exceeds the seal capillary [[displacement pressure]]. This type of seal does not [[fracture]] during [[deformation]].

==Ductility and capillary displacement pressure==

==Ductility and capillary displacement pressure==

Most old accumulations have seals of this type.

Most old accumulations have seals of this type.

[[Claystone]]s, [[Halite|salt]], and sulfates ([[gypsum]] and [[anhydrite]]) make seals of this sort because they are ductile under most geological strain rates and confining pressures. Ductility is important; if fracturing occurs, oil can leak through the fractures without invading the matrix porosity.

[[Claystone]]s, [[Halite|salt]], and sulfates ([[gypsum]] and [[anhydrite]]) make seals of this sort because they are ductile under most geological strain rates and confining pressures. Ductility is important; if [[Fracture|fracturing]] occurs, oil can leak through the [[fracture]]s without invading the matrix porosity.

Deeply buried claystones, salts, and anhydrite have capillary displacement pressures great enough to exceed the buoyancy pressure from any reasonable oil column height (hundreds to thousands of feet). Conversely, [[silty mudrock]]s, shallow-buried claystones, and [[argillaceous siltstone]]s have displacement pressures low enough to allow leakage even where the petroleum column has not filled to the structural spill point.

Deeply buried claystones, salts, and anhydrite have capillary displacement pressures great enough to exceed the buoyancy pressure from any reasonable oil column height (hundreds to thousands of feet). Conversely, [[silty mudrock]]s, shallow-buried claystones, and [[argillaceous siltstone]]s have displacement pressures low enough to allow leakage even where the petroleum column has not filled to the structural spill point.