Abnormal Formation Pressure and Shale Porosity

An abnormally pressured Jurassic shale from the North Sea Viking graben has a fluid pressure-overburden ratio close to 0.8 at a depth of more than 4,000 m and is associated with low porosity and high density. The porosity and fluid pressure history of the shale involves three successive stages: (1) normal gravitational compaction prevailed during the first few hundred meters of burial; (2) the shale porosity was severely reduced by carbonate cementation in conjunction with a major break in sedimentation, but the fluid pressure remained hydrostatic; and finally, (3) the porosity was further reduced by “compaction” and possibly cementation, and the fluid pressure increased to above hydrostatic in response to renewed subsidence and the Cretaceous and Tertiary overburden load.