OR/17/048 Conclusions

Fellgett, M W, Kingdon, A, Williams, J D O, and Gent, C M A. 2017. State of stress across UK regions. British Geological Survey Internal Report, OR/17/048.

Despite some variability in S_hmin and S_HMax due to the strata and measurement techniques the available data indicates that the UK landmass is predominantly a strike slip faulting environment with possible indicators of reverse faulting above 1200 m (though these are largely confined to igneous rocks in Cornwall, Cumbria and Leicestershire).

Pore pressure observations across all regions studied, largely support the assumption of hydrostatic pore pressure with a gradient of 10.19 MPakm^-1, with only a small number of measurements indicating overpressure e.g. Marishes 1 (1700 m). However, given that there data is constrained both spatially and stratigraphically these may have be from isolated or impermeable units. These data do not show significant or under pressure in Cheshire, Lancashire, East Yorkshire or North Nottinghamshire.

Vertical stress gradients (Figure 20) and values from LOT’s and FIT’s across the different regions show that the gradient of both S_v and S_hmin is two MPakm^-1 higher in Cheshire and Lancashire when compared to East Yorkshire or North Nottinghamshire. However, XLOT’s or laboratory test data are required to validate the estimates of S_hmin.

The general trend of S_hmin from the leak off tests appears to show the S_hmin < S_v < S_HMax indicating a predominantly strike-slip regime. There is a larger variation in the recorded values of S_hmin in the Permo–Triassic strata of the UK than the Carboniferous, with S_hmin approaching the value of S_v and in some cases ≥ S_v.

There are observations from hydraulic fracturing and overcoring which may suggest greater variation in stress field orientation than recorded in Kingdon et al (2016). However this study focused on borehole imaging datasets which are stratigraphically limited. Borehole imaging data from within the Carboniferous succession indicates that across the areas of interest the S_HMax orientation is likely to be NW–SE.

The S_HMax data from hydraulic fracturing, overcoring and borehole breakouts data plot above S_v (23 MPakm^-1) with a three exceptions. This is indicative of a strike-slip regime. However, there are substantial variations in the overcoring data and there are errors associated with hydraulic fracturing data, particularly when used to estimate S_HMax (Zoback, 2010).

From the available data there appears to be similarities in the stress field across the UK, pore pressure is hydrostatic with a gradient of 10.19 MPakm^-1. The vertical stress gradient for the Weald, East Yorkshire and North Nottinghamshire is 23 MPakm^-1 and varies from values of 25–26 MPakm^-1 recorded for areas of Cheshire and Lancashire and Scotland. Data derived from LOT, hydraulic fracture data and S_HMax calculated from borehole breakouts, collectively indicate a strike-slip faulting regime (S_hmin < S_v < S_HMax). However at depths of <1 km there is greater uncertainty in the relation between S_v, S_hmin and S_HMax. Stress magnitude data from the Triassic appears to show a greater variation than data from Carboniferous successions.

Much of the data is geographically constrained and sourced from legacy data, and more information will help to better characterise the UK stress field.