Abstract: Diagenetic porosity and permeability enhancement and palaeohydrogeology in giant North Sea fields, UK

Stuart Haszeldine, R. S. Haszeldine

Three contrasting settings show how palaeohydrogeology links to reservoir engineering and to regional-scale porosity and permeability in this rift basin.

Porosity at the crest of each tilted fault block trap in the Brent Sandstone province lies on a normal compaction curve. However porosity within each trap decreases rapidly downflank due to quartz cementation. Fluid-inclusion temperatures from these quartz cements are reset, and do not indicate growth from hot water. Isotopic data from the whole suite of cements indicates meteoric water, and hydrogeological computer modelling shows that regional meteoric flows were physically feasible. These flows facilitated uniform diagenesis, which closed down gradationally on structure crests when each trap charged with oil.

South Brae is a submarine fan off the Viking Graben bounding fault. The 400 m-thick reservoir lost porosity by calcite cementation, mediated by inflowing meteoric water. Flow reversed during deep burial, so that compactional water outflows from the interfingering source rocks were focused along the top reservoir. This has produced additional strata-bound effective secondary porosity of up to 8%, forming a diagenetic fairway extending 30 km basinward.

Superpressured oil is trapped in the Upper Jurassic of the Central Graben within poor to excellent reservoirs. Porosity is secondary, due to feldspar and dolomite dissolution at depth. In detail, this focused dissolution has resurrected the most permeable facies, often along sequence boundaries. Regionally, clay loss has enhanced porosity and permeability in structures where super-pressures leak off vertically through microfractures. These highest structures have lost their oil, however deeper levels can have oil and good porosity.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994