Structural Style of Fault-Propagation Folding in Salt-Influenced Rift Basins; the Stavanger Fault Zone, North Sea
Fault-propagation folding is an important process during the early stages of fault growth; this is especially true in salt-influenced rift basins, where the presence of mechanically-weak, evaporite-rich units within the pre-rift succession may serve to decouple folded supra-salt strata from faulted sub-salt strata. Cover structures and their underlying fault blocks may be associated with hydrocarbon accumulations, although accurate identification of such structures can be difficult. In this study we aim to document the along-strike variability of and the controls on the structural styles that are associated with salt-influenced fault-propagation folds.
A 3600 km2, high-quality, 3D seismic reflection survey has been used to document the structural style and evolution of the Stavanger Fault Zone (SFZ), a 40 km long normal fault, located in the Egersund Basin, North Sea. The SFZ is a, sub-salt, basement-involved, NW-SE striking, SW-dipping normal fault, which has up to c. 1.5 km of displacement in the north of the study area. Displacement decreases progressively towards its tip in the south, and this decrease in displacement is expressed in the cover strata by the transition from a SW-dipping normal fault and half-graben in the north, to an un-breached, SW-dipping monocline in the south.
Five distinct supra-salt cover structures are recognised along the SFZ and from north to south these comprise: (i) a WNW-ESE striking, basement-detached, hangingwall fault complex; (ii) a low-amplitude (c. 250 ms TWT), sub-circular, footwall diapir; (iii) a WNW-ESE striking footwall graben; (iv) a intensely-deformed, basinward-dipping monocline; and (v) a WNW-ESE striking, low-amplitude (c. 750 ms TWT), hangingwall salt wall. In addition to along-strike variations in total fault offset at basement level, we also document a southerly increase in salt thickness, and presence of marginal evaporates facies within the salt layer.
This study indicates a wide range of trapping styles are associated with fault-propagation folds, and the degree of coupling between sub- and supra-salt deformation in salt-influenced rift basins is controlled by: (i) salt thickness; (ii) salt composition; and (iii) fault displacement. Our observations support some of the predictions made by numerical and scaled physical models. Moreover, we illustrate how inherited geological in-homogeneities and halokinesis can also impact the style and variability of such structures.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California