Micropermeable seal leakage
Micropermeable leakage is caused by a variety of seal failure mechanisms, as discussed in the following sections.
Characteristics
Some fine-grained rocks, such as mature source rocks, are oil wet.[1] 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.
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.
Example: Ekofisk field
Like many North Sea chalk reservoirs, Ekofisk field has distinctive geochemical and geophysical evidence of gas escape into overlying Cenozoic mudrocks.[2] The mechanism of seal failure leading to a micropermeable seal is undocumented, but overlying Paleocene shales are immature and therefore are not oil wet. Pore pressures decrease downward into the field from the seal. In both the seal and the reservoir, fluid pressures are less than 75% of overburden stress. This indicates natural hydraulic fracturing of the seal is unlikely unless tectonically assisted.[3] Because capillary pressures at the top of the reservoir exceed 180 psi, the intact membrane seal is probably leaking.
Predicting leakage
Micropermeable leakage is difficult to predict from rock properties because wettability and permeability of seals are poorly known in exploration settings. Micropermeable leakage can be geophysically and geochemically detected where it occurs at a moderately rapid rate in a dynamic basinal environment, as in the preceding example.
Leakage by any mechanism obviously goes through a drainage stage when the seal leaks like a micropermeable seal. Because micropermeable leakage can be slow, it is more likely to destroy old rather than young accumulations. Many fields not filled to the spill point in oil basins with former prolific generation (such as those along the Aylesworth anticline in the Anadarko basin) were probably once filled to the spill point and have since leaked to their present contacts. Marginal seal lithologies such as argillaceous carbonates or silt-stone are more likely to suffer micropermeable leakage than accumulations under salt or claystone seals.[4]
See also
References
- ↑ McAuliffe, C., D., 1980, 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.
- ↑ Van den Bark, E., Thomas, O., D., 1981, Ekofisk: first of the giant oil fields in western Europe: AAPG Bulletin, vol. 65, p. 2341–2363.
- ↑ Watts, N., L., 1983, Microfractures in chalks of Albuskjell field, Norwegian sector, North Sea: possible origin and distribution: AAPG Bulletin, vol. 67, p. 201–234.
- ↑ Grunau, H., 1987, A worldwide look at the cap rock problem: Journal of Petroleum Geology, vol. 10, p. 245–266.
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