Changes

==Example: Ekofisk field==

==Example: Ekofisk field==

Like many North Sea chalk reservoirs, Ekofisk field has distinctive geochemical and geophysical evidence of gas escape into overlying Cenozoic mudrocks.<ref name=ch11r35>Van den Bark, E., Thomas, O., D., 1981, [http://archives.datapages.com/data/bulletns/1980-81/data/pg/0065/0011/2300/2341.htm Ekofisk: first of the giant oil fields in western Europe]: AAPG Bulletin, vol. 65, p. 2341–2363.</ref> 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.<ref name=ch11r37>Watts, N., L., 1983, [http://archives.datapages.com/data/bulletns/1982-83/data/pg/0067/0002/0200/0201.htm Microfractures in chalks of Albuskjell field, Norwegian sector, North Sea: possible origin and distribution]: AAPG Bulletin, vol. 67, p. 201–234.</ref> Because capillary pressures at the top of the reservoir exceed 180 psi, the intact membrane seal is probably leaking.

Like many North Sea chalk reservoirs, Ekofisk field has distinctive geochemical and geophysical evidence of gas escape into overlying Cenozoic mudrocks.<ref name=ch11r35>Van den Bark, E., Thomas, O., D., 1981, [http://archives.datapages.com/data/bulletns/1980-81/data/pg/0065/0011/2300/2341.htm Ekofisk: first of the giant oil fields in western Europe]: AAPG Bulletin, vol. 65, p. 2341–2363.</ref> 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 top seals|natural hydraulic fracturing]] of the seal is unlikely unless tectonically assisted.<ref name=ch11r37>Watts, N., L., 1983, [http://archives.datapages.com/data/bulletns/1982-83/data/pg/0067/0002/0200/0201.htm Microfractures in chalks of Albuskjell field, Norwegian sector, North Sea: possible origin and distribution]: AAPG Bulletin, vol. 67, p. 201–234.</ref> Because capillary pressures at the top of the reservoir exceed 180 psi, the intact membrane seal is probably leaking.

==Predicting leakage==

==Predicting leakage==