Changes

==Characteristics==

==Characteristics==

Some fine-grained rocks, such as mature source rocks, are oil wet.<ref name=ch11r23>McAuliffe, C., D., 1980, [http://archives.datapages.com/data/specpubs/geochem1/data/a029/a029/0001/0050/0089.htm Oil and gas migration: chemical and physical constraints], in Roberts, W., Cordell, R., eds., Problems of Petroleum Migration: AAPG Studies in Geology 10, p. 89–108.</ref> Leakage through these seals does not require that [[capillary pressure]] exceed displacement pressure because oil spontaneously imbibes into oil-wet rocks. Likewise, some water-wet seals have petroleum column heights that may exceed the capillary displacement pressure of matrix [[porosity]]. The effective [[permeability]] to petroleum is no longer zero, but it may be small. Finally, where fractures are few or where fracture apertures are very small, fracture porosity may be invaded, but the leakage rate may be small.

Some fine-grained rocks, such as mature source rocks, are oil wet.<ref name=ch11r23>McAuliffe, C., D., 1980, [http://archives.datapages.com/data/specpubs/geochem1/data/a029/a029/0001/0050/0089.htm Oil and gas migration: chemical and physical constraints], in Roberts, W., Cordell, R., eds., Problems of Petroleum Migration: AAPG Studies in Geology 10, p. 89–108.</ref> Leakage through these seals does not require that [[capillary pressure]] exceed displacement pressure because oil spontaneously imbibes into oil-wet rocks. Likewise, some water-wet seals have petroleum column heights that may exceed the capillary displacement pressure of matrix [[porosity]]. The effective [[permeability]] to petroleum is no longer zero, but it may be small. Finally, where [[fracture]]s are few or where fracture apertures are very small, fracture porosity may be invaded, but the leakage rate may be small.

In these cases, accumulations can last for a geologically significant amount of time if the permeability of the seal to petroleum is low enough. These seals most likely occur in young basins where traps are still actively charged. Because the seals leak, the height of the petroleum column decreases with time since charging. [[Permeability]] and relative permeability of fine-grained rocks are difficult to analyze; however, accumulations apparently sealed by oil-wet source rocks have existed for tens to hundreds of millions of years, so at least in some settings the leakage rate is low enough to ignore.

In these cases, accumulations can last for a geologically significant amount of time if the permeability of the seal to petroleum is low enough. These seals most likely occur in young basins where traps are still actively charged. Because the seals leak, the height of the petroleum column decreases with time since charging. [[Permeability]] and relative permeability of fine-grained rocks are difficult to analyze; however, accumulations apparently sealed by oil-wet source rocks have existed for tens to hundreds of millions of years, so at least in some settings the leakage rate is low enough to ignore.