Role of Tectonic Inversion and Salt Mobility in Controlling Trap Geometries and Chalk Group Reservoir Quality in the Norwegian Central Trough

Steven Sawyer¹, John R. Underhill¹, and Hugh S. Beeley^2
1School of Geosciences, University of Edinburgh, Edinburgh, United Kingdom
²Scandinavia Exploration, ConocoPhillips Norge, Tananger, Norway

Whilst it has long been recognised that the main petroleum play in the Norwegian sector of the North Sea Central Trough comprises Chalk Group reservoirs, the relative role of the deformation processes has never been well understood. Interpretation of 3D seismic data from a 5000km² area of the Norwegian Central Trough provides important new insights into the relationship between inversion (compression) and halokinesis.

Zechstein Group evaporites became mobile during the Triassic, with the creation of minibasins and adjacent salt highs. Salt movements continued until Miocene times. Inversion of normal faults occurred during comparatively discrete intervals; the principal events are dated to Campanian (contemporaneous with Chalk deposition) and Eocene-Oligocene. The timing and extent of salt movement prior to inversion is a major control on structural style associated with that inversion: there are consistent and predictable differences between salt-free areas as opposed to salt-prone areas. Where there is no salt (or salt has been expelled) structural styles are more asymmetric and localized over the site of a pre-existing structural trend. The other key controls on structural style are the geometry of the basin undergoing compression and its orientation relative to the regional stress field.

Permeability and porosity characteristics of the Chalk Group are influenced by primary sedimentology, by diagenesis and by the physical characteristics of internal fracturing. Subsurface salt movements and/or Campanian inversion influenced Chalk Group thickness distribution and depositional facies type. Later salt movements and/or post-Palaeocene inversions may have directly affected and enhanced the fracture characteristics of the reservoir: this is supported by the assessment strain distribution maps created using 3DMove structural restoration software (Midland Valley Ltd).

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas