Research paperIn-situ stress orientations in the UK Southern North Sea: Regional trends, deviations and detachment of the post-Zechstein stress field

- Horizontal stress orientations across UK Southern North Sea derived from borehole breakouts.
- Four-arm caliper and image log data used to determine stress orientation.
- Regional trend of maximum horizontal stress orientation is NW-SE.
- Local deviations in stress orientation associated with detachment due to thick evaporites.
- Orientation of maximum horizontal stress aligned along Dowsing Fault Zone.

The orientation of the maximum horizontal compressive stress (SHmax) in the UK Southern North Sea has been determined using data derived from borehole breakout analysis of four-arm caliper logs. The results agree with existing stress models for NW Europe, confirming that horizontal stresses in the region have an approximately NW-SE orientation of SHmax. This is interpreted as being a result of plate boundary convergence. Local deviations in the SHmax orientations are observed spatially and also vertically within some wells. Some of these deviations are attributed to rotations of the stress field adjacent to faults or between different fault blocks. The data also suggest detachment of the stress regime in the post-Permian cover rocks, caused by the presence of a thick underlying Permian-aged evaporite sequence and associated halokinesis. Analyses of borehole resistivity image logs have been used to verify the SHmax orientations in some wells. These image logs validate some of the stress indicators whilst highlighting a number of deficiencies in the use of four-arm caliper data to characterise borehole breakouts. From the available data it is difficult to unambiguously define the nature of variations from the mean SHmax orientations observed. Further analyses of image log data over greater depth-ranges are therefore required in order to investigate more fully the effects of stress rotations near faults and apparent stress detachment above salt-cored anticlinal structures.