A number of terms some of which are not usually employed by subsurface geologists and geophysicists, and some which are used with a slightly different meaning than usual are used throughout this report. The following short definitions are not exhaustive, but should preclude most ambiguities arising from the use of a somewhat unconventional terminology.

 

Consolidation: is the process leading to tighter packing of grains during burial of granular materials. Mechanical processes cause consolidation, usually by shear failure that causes collapse of a metastable granular framework. Sudden consolidation of sand is often related to liquefaction.

Depositional sandstones: contain preserved depositional structures, which are unmodified by post-depositional sand remobilisation.

Disequilibrium compaction: is the process by which overpressure is created when the upward escape of pore fluid is restricted by a low permeability unit (a seal).

Dykes: are bedding-discordant intrusive features.

Fluidization: occurs when a fluid flowing vertically through a granular aggregate exerts sufficient drag forces on the grains to lift or suspend them momentarily against the force of gravity.

Lateral transfer: is the transfer of fluid pressure from deeper to shallower reservoirs, either via structurally-inclined aquifers or via sand injectites.

Liquefaction: is the transformation of a saturated granular material from a solid to a liquefied state as a consequence of increased pore-water pressure. Liquefaction is caused mainly by the application of shear stresses, which causes a breakdown of the granular structure and a rapid build-up of interstitial pore-water pressure.

Sand injectites: are the products of forceful remobilisation and injection of sand during burial.

Shale clast breccia: is a descriptive lithological term for a coarse grained sandstone that comprises angular shale/mudstone fragments in sand matrix. A range of textural variations may occur but all tend to be supported by a sand matrix. Sills are approximately bedding-conformable intrusive units.

Source sandbody: is the point of origin for sand injectites. Usually, but not exclusively, a source sandbody is a depositional sandbody or a combination of several depositional sandbodies.

Steep-sided mounds: are seen on seismic as convex upward mounds with steep flank angles, often approaching 10-20+ degrees. Geologically they represent depositional sandbodies whose flank angles are steeper than typical depositional angles due to differential compaction of the encasing shales and in some cases due to post-depositional remobilisation. The width to height ratio of the sands typically ranges from 30:1 to 10:1. Such mounds are particularly common in the distal parts of the Upper Paleocene-Eocene of the northern North Sea.

V-structures: or simply Vs are defined seismically as V-shaped, discordant seismic reflections that emanate upward from an apex, which is not visibly in contact with a source sandbody. Geologically they represent low-angle sand dykes intruded upward from an apex as more or less perfect cone sheets. V-structures are widespread within the Eocene-Oligocene of the northern North Sea.

Wings: are defined seismically as discordant reflections that emanate upward from the margins of steep-sided mounds. Geologically they represent low-angle sand dykes emanating upward from the flanks of in situ source sandbodies. Wings are frequently observed flanking producing fields in the Upper Paleocene-Eocene of the northern North Sea.a