− | 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. | + | 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 W. Roberts, and R. Cordell, 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 [[Calculating charge volume|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 [[Calculating charge volume|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. |