Tectonic inversion and halokinesis are well documented as mechanisms for
generating structural traps for hydrocarbons. Many sedimentary basins that contain
kinematically active halite deposits have also experienced deformation related to
positive tectonic inversion (contractional reactivation of pre-existing structures). In
such cases, patterns of uplift are often complex and the relative role of competing
deformation processes and their influence on structural style is poorly understood.
The focus for this study is the Norwegian sector of the Central North Sea. A major
petroleum play comprises Chalk Group reservoirs, where trap development has
previously been attributed to halokinesis of the Permian Zechstein Supergroup, to
Mesozoic and Cenozoic tectonic inversion events, and to a combination of both, but
has never been well understood. Interpretation of high resolution 3D seismic data
from a 5000 km2 area has revealed new insights into the relationship between
tectonic inversion and halokinesis.
Halite of the Zechstein Supergroup became mobile during the Triassic, with the
creation of minibasins and adjacent salt highs. Salt movements continued until
Miocene times. Tectonic inversion was driven by far-field plate margin forces and
occurred during comparatively discrete intervals; the principal events are dated
Maastrichtian - Danian (contemporaneous with Chalk deposition) and Eocene -
Middle Miocene. 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 deep-seated,
more asymmetric and localized over the site of a pre-existing structural trend.
Tectonic inversion and halokinesis have affected the porosity and permeability
characteristics, and therefore the reservoir quality, of the Upper Cretaceous Chalk
Group. Syn-depositional uplift exerted a strong influence over Chalk Group
thickness distribution and depositional facies type. Sedimentological studies suggest
initial (facies-related) matrix porosity variations were preserved or even enhanced
during subsequent diagenesis.
The physical characteristics of internal fracturing are a major control over Chalk
Group reservoir quality. Historically, it has been difficult to characterize sub-seismic
scale 3D heterogeneities within the Chalk Group. This study has addressed the
problem of fracture development in response to fold growth through integration of
theoretical considerations, subsurface data and outcrop observations, using suitably
chosen structural analogues. It is probable that both inversion and halokinesis
directly affected and enhanced the fracture characteristics of the reservoir.