Structural Geology and Fracture Characterisation in the Mesozoic Formations of the Somerset Coast

The Bristol Channel coastline is world-renowned for the quality of its’ exposures which display fascinating structural geology and fracture patterns in a range of formations including Permian mudstones and Jurassic micritic carbonates. Localities such as Lilstock and Kilve on the Somerset coast, and many parts of the Glamorgan coast, have been long studied with a series of detailed publications available. 

This trip is based around a fracture characterisation training course that is held locally and uses the North Somerset coast as an area of ‘ground-truthing’ for geoscientists and reservoir engineers dealing with the complexities of fracture characterisation and modeling in the sub-surface.  The Bristol Channel Basin originated by N-S extension of the Variscan landmass during the Permo-Triassic, controlled by mainly E-W trending faults. A succession of continental red-beds / alluvial fan deposits were deposited, passing upwards into the Sherwood Sandstone Group and then the Mercia Mudstones which include major evaporite intervals. Renewal of fault activity in the late Triassic / early Jurassic resulted in a transition to open marine conditions and the cyclic deposition of mudstones and micritic carbonates (the Blue Lias of the early Jurassic). Younger rocks are not preserved locally. 

Following the extensional phase, which involved oblique or transtensional movements controlled by major NW-SE trending transfer structures rooted in the Variscan basement, the region underwent at least one, and possibly two, phase of inversion.  These took place in the early Cretaceous (possibly) and in the Tertiary (the latter tends to overprint the former). Many of the extensional faults were-reactivated in transpression and original roll-over folds were accentuated. Much of the fracturing observed at outcrop is thought to be associated with uplift and inversion although earlier fracture events can be identified too. There remain many uncertainties despite the wealth of structural studies in the region and at these localities.

On this trip we will be able to examine the fracture system and its attributes in relation to the larger-scale fold and fault structures, as well as the relationships with a range of lithologies. There will be discussion of fracture dynamics, fracture scale and the controls of fracture distribution such as mechanical stratigraphy, fold growth and fault proximity.  Participants will be encouraged to share their experiences of investigating heterogeneous fracture systems during reservoir characterisation and development.