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 The recent shale gas revolution originated in the USA in the late 1990s with the exploration of the Carboniferous Barnett Shale in Texas. Success in a number of additional basins in North America, such as the Marcellus, Eagle Ford and Bakken basins, stimulated a search for similar opportunities elsewhere around the world. Among the shales and basins targeted by industry was the Carboniferous Bowland Shale (and equivalents) in northern England. The initial premise that the Barnett Shale represented an excellent analogue for the Bowland Shale led to over-optimistic reserve estimates that have since been shown to be largely incorrect. On the basis of visual inspection of wellbore cores, the Carboniferous Barnett and Bowland shales appear to be very similar. Unfortunately, it is there that the similarity ends. Research carried out for the UK Unconventional Hydrocarbons project has highlighted important differences adversely impacting prospectivity. These can be summarized as basin type/continuity and structural complexity. The total organic carbon, maturity, mineralogy and thickness of the Bowland Shale and equivalents are broadly similar to the successful US examples. Our conclusion is that the Bowland Shale in the UK does not represent a technically significant resource, and in hindsight did not merit the considerable industry and media attention that has been associated with it. One key learning is that fundamental research based on heritage data and modern analytical and modelling techniques should have preceded drilling and fracking operations in northern England.

Abstract Once highlighted for having significant shale gas resource potential, the Bowland Basin has been at the centre of both scientific and political controversy over the last decade. Previous shale gas resource estimates range from 10 3 to 10 1 TCF. Repeated events of induced seismicity following hydraulic fracturing operations led to an indefinite government moratorium and abandonment of operations across the mainland UK. We use apatite fission-track analyses to investigate the magnitude and timing of post-Triassic uplift and exhumation. Results indicate that maximum palaeotemperatures of 90–100°C were reached in the stratigraphically younger Sherwood Sandstone. We combine palaeotemperature predictions to constrain palaeo heat flow and erosion in regional basin models for the first time. Our results indicate variable maximum Late Cretaceous palaeo heat flow values of 62.5–80 mW m −2 and the removal of 800–1500 m of post-Triassic strata at wells across the basin. Regional 2D basin modelling indicates a gas-in-place estimate of 131 ± 64 TCF for the Bowland Shale. This reduces to a resource potential of 13.1 ± 6.4 TCF, assuming a recovery factor of 10%. These values are significantly lower than previous resource estimates and reflect the highly complex nature of the Bowland Basin and relatively unknown history of post-Triassic uplift, exhumation and erosion.

Abstract We conducted a high-resolution multi-disciplinary analysis of two core sections in the borehole Ellesmere Port-1, Cheshire, UK. Biostratigraphic analysis indicates that the core sections are Kinderscoutian and late Arnsbergian–Chokierian in age, respectively. Both cores are assigned to the Bowland Shale Formation (Holywell Shale). Coupled core scan and discrete geochemical analysis enables interpretation of syngenetic processes at a high stratigraphic resolution. Both cores exhibit the classic cyclicity of limestones, calcareous to non-calcareous mudstones and siltstones, interpreted to represent sediment deposition during fourth-order sea-level fluctuation. Machine learning of the well log data coupled to the core scan data enabled prediction of the key lithofacies through the entire Bowland Shale interval in Ellesmere Port-1. The machine predictions show that the Bowland Shale is interfingered with three turbiditic leaves of the Cefn-y-Fedw Sandstone Formation and contains at least 12 complete fourth-order cycles. The Bowland Shale exhibits high radiogenic heat productivity in comparison with other sedimentary rocks, due primarily to relative U enrichment under intermittently euxinic conditions. Thermal modelling, however, shows that the radiogenic heat productivity of the Bowland Shale contributes a negligible source of additional heat at the scale of hundreds of metres.

Abstract We evaluated the unconventional hydrocarbon potential of the Holywell Shale Formation, a lateral equivalent of the Bowland Shale Formation deposited in the Blacon Basin. Two cores with Arnsbergian–Chokierian and Kinderscoutian (Namurian, Late Mississippian–Early Pennsylvanian) ages from the Ellesmere Port 1 borehole were sampled for palynological, stable isotope, Rock-Eval 6 pyrolysis and ichnofacies analyses. The study was designed to provide boundary conditions for parameters that are under-represented in the public domain and hamper accurate resource assessments: thermal maturity (through mean random vitrinite reflectance, R r), present-day organic matter content, kerogen type, original hydrogen index and original organic matter content. Our results show that the Arnsbergian Core 2 has been buried to a depth equivalent to the bottom of the oil window to the top of the gas window (% R r = 1.15%–1.29%). The Kinderscoutian Core 1 is too immature to have generated any natural gas (% R r = 0.91–1.03%). Furthermore, kerogen typing and ichnofacies analysis show that the Bowland Shale Formation is very heterogeneous, with organic matter originating from terrestrial and marine sources. Five palynofacies assemblages are described that range in basin setting from proximal and oxic to distal and anoxic with evidence of episodic connections to the open ocean. The combination of heterogeneity and low thermal maturity restricts the thickness of the Bowland Shale Formation in the Blacon Basin that is prospective for unconventional hydrocarbons. Our results show that these Carboniferous mudstones should not be treated as uniform units with uniform composition and maturity in basin modelling and resource estimates. This undoubtedly has repercussions for future exploration because the contrasting composition and density of the materials making up the Bowland Shale Formation may complicate extraction, while the thermal maturity window significantly narrows the prospective interval.

Abstract The regional character of organic matter types and depositional conditions of Pendleian, Brigantian and Arnsbergian mudstones between the Craven Basin and the Widmerpool Gulf was compared through interpretation of biomarker and pyrolysis data from 201 samples recovered from nine boreholes. The Carboniferous seaways have been determined to commonly host dysoxic conditions, enabling the preservation of a mixture of marine and terrestrial organic matter types. Photic zone anoxia, established by the presence of aryl isoprenoids, was determined to be persistent during ‘marine’ conditions represented by marine band, high-sea-level and carbonate facies. The observation and correlation of diasteranes and trisnorneohopane/trisnorhopane ratios within the samples and to other maturity parameters highlighted a significant clay mineral catalytic and/or hydrocarbon retention effect in the samples. This influenced both biomarkers and programmed pyrolysis thermal maturity indices such as T max , reducing the reliability of such results for interpreting the burial depth, and ultimately reserve potential.

Abstract The Bowland sub-basin is a target for hydrocarbon exploration, but to a large extent it remains unexplored. To determine the economic potential of the Bowland sub-basin, it is important to identify the oceanographic processes involved in the deposition of the Bowland Shale Formation in the Late Mississippian (c. 330 Ma). Palaeoceanographic processes are known to be a major control on the development of hydrocarbon source rocks. This study investigates core (Preese Hall-1 and Becconsall-1Z) materials from the Upper Bowland Shale, and makes a comparison with previously published data (outcrop Hind Clough), all from the Bowland sub-basin, Lancashire, UK. The sedimentology and geochemistry of this formation were determined via a multi-technique approach including X-ray fluorescence, sedimentology, gamma-ray spectra, X-ray diffraction and Rock-Eval pyrolysis. Key trace metal abundances and enrichment factors were used to assess sediment provenance and to determine the bottom-water redox conditions during the deposition of the Upper Bowland Shale. Our results support interpretations of contemporaneous anoxia developing in bottom waters in at least three sites in the Bowland sub-basin. In a comparison with the Fort Worth Basin (Barnett Shale, USA), the Bowland sub-basin was apparently less restricted and deposited under a much higher mean sediment accumulation rate. Knowledge from this study will improve future resource estimates of the Bowland Shale Formation, and challenge the early assumptions that the Barnett Shale is an analogue of the Bowland Shale.

Abstract Shale rock core from the Bowland Shale Formation, UK, was analysed in the laboratory using Rock-Eval pyrolysis and Fourier transform infrared spectroscopy (FTIR). These methods are used to characterize the organic constituents of soil and rock. This research is a proof-of-concept study to investigate whether regression models developed using FTIR and Rock-Eval data for the same length of core can be used to estimate selected Rock-Eval parameters. The accuracy of the regression models was assessed using statistical methods, the results of which were used to choose preferred models for each Rock-Eval parameter. The models produced were shown to have an acceptable level of uncertainty for total organic carbon and S1, S2 and S3 outputs, which led us to conclude that these are potentially suitable for estimating unknown down-core Rock-Eval parameter values. Conversely, the model for the temperature of the maximum rate of hydrocarbon generation (T max) had higher variability in the cross-validation data above the acceptable level of uncertainty, which could lead to erroneous estimates. Down-core interpolations of selected Rock-Eval parameters could be practically achieved by modelling FTIR data by maintaining standard sample frequencies for Rock-Eval while supplementing with higher frequencies for FTIR and chemometric analysis.

Abstract Shale rocks are highly structurally and chemically heterogeneous, such that the pore structure–transport relationship is complex. Shales typically have porosity over many length-scales from the molecular up to macroscopic fractures. This work utilizes gas overcondensation to probe pore sizes from micropores to very large macropores all in the same experiment without the potential for damage due to high pressures when conducting mercury porosimetry. Indeed, the Bowland Shale samples studied here are generally inaccessible to mercury intrusion. The gas overcondensation method can also be augmented using scanning loops to assess the spatial juxtaposition of very different pore sizes, and this has been used to determine that some large macropores are shielded by pore necks less than 4 nm in size in the Bowland Shale. In addition, the adsorption calorimetry method has been used to assess the accessibility of the void space. It has been found that mass transport is limited by particular ‘hour-glass’-like pore necks that fill at quite low saturation, and thus present a barrier to molecular migration. The shielding of macroporosity by narrow necks was particularly significant for the Above Marine Band sample, with lower shielding observed in the Marine Band and Below Marine Band materials.

Abstract Mineral authigenesis in mudstones responds differently across the compositional spectrum of fine-grained rocks, resulting in a complex array of possible rock fabrics that affect the mechanical capacity of mudstones. Within carbonate-rich mudstones, geochemical controls on silicate and carbonate mineral reactions are variable and poorly understood. Here we analyse the diagenetic minerals of the carbonate-rich Hodder Mudstone from the Carboniferous Bowland Basin, UK, using SEM petrography combined with X-ray-based inorganic geochemical data. Our findings show that up to 90% of quartz cements in the samples are diagenetic, originating mainly from biogenic silica dissolution and clay mineral reactions. It is also evident that due to the varying diagenetic silica-yielding reactions, authigenic silica overgrowths and isolated authigenic quartz crystals are localized in argillaceous samples, while calcareous samples are characterized by silica replacement textures and quartz/calcite intergrowths. Moreover, euhedral dolomite crystals are concentrated within argillaceous units, whereas calcareous units are characterized by anhedral dolomite precipitation and replacement textures. These finding presents a case for facies-selective cementation, as both early and burial diagenetic alterations were observed to be controlled by primary depositional components of biogenic debris and extrabasinal silicate detritus that resulted in complex and variable precipitation of authigenic minerals.

Abstract A sulfurous spring emerges at an outcrop of Bowland Shale Formation on Crimpton Brook, Lancashire. The spring is marked by the precipitation of white and yellow jarosite. Comparable light sulfur isotope compositions suggest that the jarosite was derived from surface oxidation of pyritic shale. The shale contains anomalous concentrations of selenium and other trace elements, in common with other exposures of the Bowland Shale Formation in the north of England. The selenium is resident in the pyrite, and was released during oxidation and incorporated into the jarosite in Crimpton Brook. The small scale of the water flow at the site and the remote situation pose no environmental problem, but the data exemplify the potential danger of pyrite weathering into groundwater on a larger scale.

Abstract Samples of the Bowland Shale and equivalents in Ireland that represent a range of thermal maturities from the oil window to gas window contain trace elements distributed through multiple residences. The trace element distribution reflects the depositional environment, and influences subsequent element release. Measurements of Se, Cu, As, U and Mo indicate variable retention in loosely bound and strongly bound forms. The trace elements are especially associated with diagenetic pyrite. Shales from Ireland, at higher thermal maturity, have relatively high proportions of loosely bound Se, Cu and As, which may reflect expulsion from the pyrite during recrystallization. The shale samples consistently contain high organic matter contents, reflecting deposition in an anoxic environment. The Se, Mo and U in particular may be bound to organic matter, which may have adverse environmental impacts upon weathering of the shale.

Abstract This is a guide to the key exposures of the Bowland Shale Formation present within the Craven and Edale basins of England, providing contrasting settings from respectively the northern and southern parts of the Craven Group extent. It provides a description of the evolution of the deposition of the hemipelagic mudstone-dominated components of the Craven Group during the Visean to early Namurian (Mississippian to early Pennsylvanian). It explains the significance of the chosen localities in understanding the development of the Bowland Shale Formation and gives details on the sections visible at the time of compilation, providing useful guidance for field visits.

This volume showcases recent geological, geophysical and geochemical research on the UK Mississippian Bowland Shale Formation. Here, the Bowland Shale and equivalent units are described and interpreted in terms of sedimentary, geochemical, and mechanical properties and processes, basin-forming events, hydrocarbon prospectivity, mineralization, and heat and fluid flow in the subsurface.