| Abstract |
Knowledge of the in- situ stress field is a key constraint for a variety of sub surface activities and crucial for the safe and sustainable use of the sub surface. However is a lack of available stress magnitude data across the UK. This report assesses legacy stress magnitude data along with new analysis to characterise the UK onshore stress field. To investigate the UK onshore in-situ stress field, three regions were studied. The regions were selected based on the potential availability of information to characterise the stress field and their resource potential for unconventional shale resources, highlighted by Andrews et al. (2013). The study focused on: East Yorkshire and North Nottinghamshire, Cheshire and Lancashire and the Weald. The vertical stress across the UK varies between 23 and 26 MPakm-1 with higher values recorded in Cheshire and Scotland compared to East Yorkshire, North Nottinghamshire and the Weald. Pore pressure measurements from Cheshire, Lancashire, East Yorkshire and North Nottinghamshire are hydrostatic with a gradient of 10.19 MPakm-1. Leak off test and formation integrity test data has been used to estimate the gradient of minimum horizontal stress in Cheshire, Lancashire East Yorkshire and North Nottinghamshire. This estimates show that the minimum horizontal stress gradient is two MPakm-1 higher in Cheshire and Lancashire than East Yorkshire and North Nottinghamshire, which is similar to the differences in vertical stress gradients. Legacy maximum horizontal stress data has been compiled from a variety of techniques from the Coal Authority and peer review publications. This data shows that the maximum horizontal stress > vertical stress, When combined with the leak off test and formation integrity test data (which shows vertical stress > minimum horizontal stress) this indicates that the UK is predominately a strike slip faulting environment. Above 1200 m there are indications of reverse faulting though these are largely confined to igneous rocks in Cornwall, Leicestershire and Cumbria. The available information shows that there are similarities in the stress field across the UK though due to the geographic and stratigraphic constraints on the data more information would help to better characterise the stress field. |
