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

Highlights

• 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.

Abstract

The orientation of the maximum horizontal compressive stress (S_Hmax) 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 S_Hmax. This is interpreted as being a result of plate boundary convergence. Local deviations in the S_Hmax 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 S_Hmax 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 S_Hmax 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.

Introduction

Knowledge of the in-situ stress field is required to understand the geomechanical response of subsurface systems to the extraction or injection of fluids. In the UK sector of the Southern North Sea (SNS), efforts to develop both conventional and unconventional reservoirs would benefit from an improved understanding of the stress affecting reservoirs and their overburden. Projects include further development of new or marginal exploration targets, as well as plans for seasonal subsurface gas storage (Havard and French, 2009). The prospect of using saline aquifers and depleted gas fields in the SNS for the sequestration of anthropogenic carbon dioxide (Holloway et al. 2006) also necessitates a greater understanding of the in-situ stress conditions. It is well documented that the injection of industrial quantities of carbon dioxide into reservoir rocks will result in increased pore fluid pressures within the connected pore volume (Bachu et al., 2007, NETL, 2008, Noy et al., 2012). This will result in a range of geomechanically-induced deformation processes (Zoback and Gorelick, 2012, Verdon et al., 2013). Important considerations relevant to exploration and the utilisation of subsurface reservoirs include the integrity of reservoir sealing related to fracturing of cap-rocks and pressure-induced fault reactivation (Finkbeiner et al., 2001, Reynolds et al., 2003, Streit and Hillis, 2004). Additionally the production performance of fractured reservoirs, the optimisation of fracture stimulation works, wellbore stability and production/injection induced deformation is particularly relevant in certain operational circumstances (Maury et al., 1992, Hillis and Nelson, 2005, Hennings et al., 2012). A robust geomechanical model is required to address these issues, the basic components of which will comprise knowledge of rock strength, pore pressure, existing fault properties and knowledge of the principal stress magnitudes and orientations (the in-situ stress field).

A brief summary of the current understanding of stress in NW Europe and particularly the North Sea region is given here. Klein and Barr, 1986, Müller et al., 1992, Müller et al., 1997 show that continental NW Europe exhibits a consistent NW–SE, to NNW–SSE orientation of the maximum horizontal stress (S_Hmax), while a more WNW–ESE trend with larger variability is seen in Scandinavia. The generally NW–SE orientation of S_Hmax can be attributed to plate boundary forces resulting from ridge-push along the mid-Atlantic ridge and continental collisional forces along the southern and eastern Eurasian plate margins (Müller et al., 1992, Gölke and Coblentz, 1996, Hillis and Nelson, 2005). The relatively thicker and cooler continental lithosphere in the Scandinavian region results in lower mean compressional stresses and a higher susceptibility to the supposition of stresses related to other factors such as lithospheric flexure and topography (Müller et al. 1992). Despite the almost homogenous orientation of S_Hmax across continental NW Europe, Müller et al. (1997) note that short-scale variations in the tectonic stress regime are seen, with strike-slip stress regimes transitioning to normal or reverse faulting stress states due to the magnitude of S_Hmax being close to that of the vertical stress (S_v). Significant variation is seen in the stress regimes across the North Sea region itself (Hillis and Nelson, 2005), with highly variable S_Hmax orientations occurring in the Central North Sea within a normal faulting stress regime, compared with a transitional strike-slip/reverse faulting stress regime in the Northern North Sea (NNS) where S_Hmax is oriented approximately E–W. Wiprut and Zoback (2000) present data that indicates the magnitude of S_Hmax in this region of the NNS to be greater than S_v, and the minimum horizontal stress (S_hmin) to be nearly equal to S_v; consistent with strike-slip and reverse faulting stress states known from earthquake focal mechanisms (Lindholm et al. 1995). The stresses in the NNS have been affected by lithospheric flexure following deglaciation (Grollimund et al., 2001, Hillis and Nelson, 2005). For offshore Norway, average S_Hmax orientations vary from a mean of N78° in the Viking Graben, N97° in the Central Graben, N127° in the Møre Vøring region to N177° in the Barents Sea area (Gölke and Brudy, 1996), differing markedly from the NW–SE trend. NW–SE orientations of S_Hmax are observed offshore Norway by Lindholm et al. (1995) with a good consistency between those indicators derived from boreholes and deeper earthquake focal mechanisms, however in the Tampen Spur area of the NNS, they observed a rotation of the S_Hmax orientation to roughly NE–SW. Lindholm et al. (1995) associate this to postglacial uplift effects and/or variations in the physical properties of the crust. Unlike the northern and central parts of the North Sea, S_Hmax orientations in the Netherlands sector of the SNS are consistently oriented NW–SE, similar to those seen onshore (Janot et al., 1988, van Eijs and van Dalfsen, 2004). S_Hmax orientations from the Danish sector of the North Sea exhibit a large degree of variation (Ask, 1997) similar to that observed in the Central Graben by Hillis and Nelson (2005).

Despite the long history of hydrocarbon development in the region, current compilations of S_Hmax orientation data (Klein and Barr, 1986, Cowgill et al., 1993, Heidbach et al., 2008) contain few direct indications of the S_Hmax orientations in the UK SNS, while future development opportunities may require detailed information regarding the in-situ stresses. An assessment of the S_Hmax orientation in the UK sector of the SNS is presented here, based on the analysis of borehole breakouts. The observations are discussed in terms of the prevailing crustal stresses and local perturbations resulting from faulting and potential stress detachment above salt-cored anticlines. Relative stress magnitudes are also discussed.