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→Evaluating seal capacity
* ''P<sub>dS</sub>'' = brine-hydrocarbon displacement pressure of the seal (in psi)
* ''P<sub>dS</sub>'' = brine-hydrocarbon displacement pressure of the seal (in psi)
* ''P<sub>dR</sub>'' = brine-hydrocarbon displacement pressure of the reservoir rock (in psi)
* ''P<sub>dR</sub>'' = brine-hydrocarbon displacement pressure of the reservoir rock (in psi)
It is difficult to determine the [[Top seal displacement pressure|displacement pressure (P<sub>d</sub>) of a top seal]].
* First, [[seal capacity]] is based on the presence of a single, continuous, high-P<sub>d</sub> layer, not upon an arithmetic average of all P<sub>d</sub> values. The trapping capacity is determined by the highest displacement pressure within a seal, theoretically even if that interval is a layer only one grain thick. A 500-m-thick siltstone may appear incapable of trapping more than [[length::1 m]] of hydrocarbon. The presence of a 1-cm-thick claystone layer, however, may create a seal for thousands of meters of hydrocarbon. Predicting these local seals is difficult and adds a measure of risk to any evaluation of intact top seals using capillary theory.
* Second, experiments demonstrate that hydrocarbon [[migration]] through a seal does not occur along a broad, uniform front but along narrow fingers reflecting local, tortuous pathways of low displacement pressure.<ref name=ch10r17>Dembecki, H., Jr., Anderson, M., J., 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0008/1000/1018.htm Secondary migration of oil: experiments supporting efficient movement of separate, buoyant oil phase along limited conduits]: AAPG Bulletin, vol. 73, no. 8, p. 1018–1021.</ref><ref name=ch10r12>CatalanXiaown, L., F., Chatzis, I., Dullien, F., A., L., 1992, [http://archives.datapages.com/data/bulletns/1992-93/data/pg/0076/0005/0000/0638.htm An experimental study of secondary oil migration]: AAPG Bulletin, vol. 76, no. 5, p. 638–650.</ref>
These tortuous pathways develop even in bead packs of uniform, closely packed glass spheres. Natural seals are even more heterogeneous.
The difficulty of defining these narrow zones during sampling complicates our ability to predict the critical displacement pressure of a seal. Although 99% of the samples from a trap may indicate a top seal capable of trapping a 500-m oil column, the seal may well leak through a narrow pathway only centimeters in diameter.
===Additional applications===
===Additional applications===