Abstract This volume showcases recent geological, geophysical and geochemical research on the Carboniferous Bowland Shale Formation. The Bowland Shale is a relatively thick and extensive Palaeozoic black shale unit with a long history of debate and controversy in the UK. The Bowland Shale is proven in many of the key Carboniferous basins in the Midlands, northern England and North Wales, and represents a significant, near-continuous temporal record spanning 16 Myr including the important mid-Carboniferous boundary (Mississippian–Pennsylvanian). Since the first geological surveys in the late nineteenth century, the Bowland Shale has been of interest for a variety of reasons, including the search for Irish-type Pb–Zn base metal mineral deposits, and as a source rock in conventional hydrocarbon systems. In the mid-2000s, attention turned to the Bowland Shale as a target for unconventional hydrocarbon extraction, shale gas, following success in the USA. This placed the Bowland Shale at the centre of a series of interconnected controversies and debates from the local to national scale. The geological credibility of the purported shale gas resource in the UK was – and continues to be – highly contentious. This volume contributes to a more updated view of the Bowland Shale, covering topics such as sedimentary, geochemical and physical properties and processes, basin-forming events, hydrocarbon prospectivity, mineralization and heat and fluid flow in the subsurface. The volume also includes a field guide to some of the key localities in the UK. With the benefit of hindsight offered by the latest generation of research, the early regional shale gas assessments failed to attach sufficient weight to the compositional heterogeneity, structural complexity, compartmentalization and highly variable exhumation, erosion and palaeo-heat flow history of the Bowland Shale. The following topics are identified as promising avenues for future research beyond shale gas: (1) the role of the Bowland Shale in the context of mineral systems; (2) the role of the Bowland Shale as a cap rock in the Lower Carboniferous limestone geothermal play and potentially as an analogue of CO 2 or radioactive waste storage in the UK; and (3) pathways and mechanisms for weathering, alteration and trace element release from the Bowland Shale into the surface and/or subsurface environment.

Abstract During geological carbon dioxide storage in deep saline aquifers, buoyant CO 2 tends to float upwards in the reservoir overlaid by a low permeable formation called a caprock. Caprocks should serve as barriers to potential CO 2 leakage that can happen through diffusion and permeation through faults, fractures or pore spaces. The leakage through intact caprock would mainly depend on its permeability and CO 2 breakthrough pressure and is affected by the heterogeneities in the material. Here, we study the sealing potential of a caprock from the Illinois Basin – Eau Claire shale, with sandy and clayey fractions distinguished via electron microscopy, grain/pore size analyses and surface area characterization. The direct measurements of permeability of sandy shale provide the values on the order of 10 −15 m 2 , while clayey specimens are three orders of magnitude less permeable. The CO 2 breakthrough pressure under in situ stress conditions is 0.1 MPa for the sandy shale and 0.4 MPa for the clayey counterpart – these values are higher than those predicted by the porosimetry methods performed on the unconfined specimens. Sandy Eau Claire shale would allow penetration of large CO 2 volumes at low overpressures, while the clayey formation can potentially serve as a caprock in the absence of faults and fractures.