An introduction to the Bowland Shale Formation, UK: processes and resources Free

The Bowland Shale Formation spans c. 16 Myr from the upper Visean (c. 332–334 Ma; Menning et al. 2006; Kryza et al. 2011) to middle Bashkirian (c. 319 Ma; Menning et al. 2006; Davies et al. 2011; Waters and Condon 2012) (Fig. 1), and is perhaps the most controversial black shale unit in the UK. The volume focuses on the key Carboniferous basins in the Midlands, northern England and North Wales (Fig. 2). The main basins assessed in the volume are the Craven Basin, including the Bowland sub-basin, the Blacon Basin, the Edale Basin, the Widmerpool Gulf and the Gainsborough Trough. The Bowland Shale and equivalent units are described and interpreted in terms of sedimentary, geochemical and physical properties and processes, basin-forming events, hydrocarbon prospectivity, mineralization and heat and fluid flow in the subsurface.

Early geological surveys on the Bowland Shale (historically referred to as the ‘Bolland Shales’) include Hull et al. (1873), Moseley (1953) and Earp et al. (1961). One early controversy concerned the origin – depositional or structural – of the Cracoean reef knolls, a debate that was first documented by Marr (1899) and Tiddeman (1901). The reefs occur locally along the Craven fault system and are onlapped by, and grade into, the Bowland Shale in multiple directions. Subsequent contributions by Hudson (1932), Bond (1949) and Black (1958), among others, sought to constrain the extent to which flank collapse and tectonism control the present-day bed geometries of the knoll reefs. Over a century after the first documented observations on the Cracoean reefs, we now possess a much improved understanding of the geometry, facies assemblages and palaeoecology of the reef knolls (see Mundy 1994; Rigby and Mundy 2000; Waters et al. 2017 and references therein). Despite this, the precise environmental conditions required to sustain the niche microbial framework community and the relationship with euxinic conditions in adjacent bottom waters remain unresolved, particularly as there are few modern or ancient analogues of this setting (Mundy 1994).

The Bowland Shale is of particular interest from a stratigraphic perspective, because it represents a near-continuous temporal record across the mid-Carboniferous boundary to the Mississippian and Pennsylvanian subsystems (Saunders and Ramsbottom 1986) (Fig. 1). The development of ammonoid biostratigraphy by the early workers Bisat (1923), Hudson (1945) and Ramsbottom and Saunders (1985), deriving in part from observations of ammonoids in the Bowland Shale, ultimately contributed to this international chronostratigraphic framework (Lucas et al. 2022). However, the extent to which the UK substages used to subdivide the Bowland Shale are correlative beyond western Europe remains unresolved and a subject of debate today (Cózar and Somerville 2021; Lucas 2021; Lucas et al. 2022).

The Namurian is well known for the development of strong eustatic cyclicity in both shelfal and basinal successions, including within the Bowland Shale (e.g. Ramsbottom et al. 1962), but the relative importance of autocyclicity remains a subject of debate (e.g. Ramsbottom 1977; Holdsworth and Collinson 1988; Martinsen et al. 1995). The most striking feature of cyclicity in the Namurian portion of the Bowland Shale is the classic ‘marine bands’; these are macrofauna-bearing, commonly carbonate-rich sedimentary packages (e.g. Emmings et al. 2020a) deposited during marine transgressions (e.g. Waters and Condon 2012). Cyclicity is also manifested in the record of water column salinity and bottom water redox conditions through the Bowland Shale (e.g. Leeder et al. 1990; Gross et al. 2015; Emmings et al. 2020c), although the presence of cyclicity in all basins and the specific water column configuration – deep, ponded hypersaline waters or productive, thermally stratified normal marine waters – also remains debated (Emmings et al. 2020c; Li et al. 2023).

In the late 1970s and into the 1980s, the Bowland Shale in the Craven Basin was of particular interest in the search for Irish-type Pb–Zn sulfide deposits (Wadge et al. 1983) and as a source of fluids in the Pennine-type Pb–Zn–F mineral system (Plant and Jones 1989). The Craven Basin does contain small occurrences of base metal mineralization within reefal Visean carbonate facies, which have been mined at a small scale since the sixteenth century (Wadge et al. 1983). However, large, economic-grade Irish-type mineral deposits were never discovered in the Craven Basin. Wadge et al. (1983) concluded the Craven Basin might nonetheless host Irish-type mineralization, but that exploration should focus within the Visean reef carbonates located along the margin of the Askrigg Block, which are not exposed at surface. In terms of Irish-type Pb–Zn mineralization, the Bowland Shale was thought to be important either as a cap rock to mineralizing fluids in the underlying carbonates and/or as a source of metal-bearing fluids (Wadge et al. 1983), but our understanding of this mineral system has evolved significantly since the 1980s. Irish-type mineralization specifically implicates the mixing of syngenetic to early diagenetic euxinic bottom waters with oxidizing metal-bearing brines (Ashton et al. 2016). Thus, the development of Irish-type Pb–Zn mineralization in the Craven Basin depends upon the co-existence and intersection of deep-seated faults with euxinic bottom waters. Euxinia is now proven in multiple intervals and basins in the Bowland Shale (Emmings et al. 2020c; Li et al. 2023), but the intersection with deep-seated faults at or near the seabed remains unresolved. Regarding Pennine-type Pb–Zn–F deposits, the evidence is more compelling: dewatering Namurian mudstones, within overpressured and compartmentalized basins, were a key source of metal-bearing fluids supplied to the adjacent blocks (Wadge et al. 1983; Gawthorpe 1987; Plant and Jones 1989; Parnell and Swainbank 1990; Kendrick et al. 2002; Bouch et al. 2006; Frazer et al. 2014).

The Bowland Shale is an important conventional hydrocarbon source rock in the North Sea, the Irish Sea and onshore in the UK (e.g. Armstrong et al. 1997; Smith et al. 2010; DECC 2013; Monaghan et al. 2017, 2019). In the mid-2000s, attention turned to the Bowland Shale as a target for unconventional hydrocarbon exploration in the UK following success in the USA (e.g. Selley 2005; Smith et al. 2010). 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 (Andrews 2013; Yang et al. 2016; Whitelaw et al. 2019; Anderson and Underhill 2020). That said, the prospectivity question was overshadowed by other important factors such as the changing societal perceptions of shale gas and the related economic, policy and political factors (e.g. Bradshaw et al. 2022), and the uncertainty regarding the hazards to the environment such as groundwater contamination and seismicity.

As of 2024, our understanding of the Bowland Shale in terms of geological processes and hydrocarbon resources is considerably more advanced than at the time of the Andrews (2013) report. In addition to the contributions in the volume, the key research publications from the perspective of unconventional hydrocarbon prospectivity are, by each main basin (Fig. 2): the Craven Basin including the Bowland sub-basin (Fauchille et al. 2017; Clarke et al. 2018; Fellgett et al. 2018; Herrmann et al. 2018; Newport et al. 2018; Whitelaw et al. 2019; Anderson and Underhill 2020; Emmings et al. 2020b; de Jonge-Anderson et al. 2021, 2022; Ma et al. 2021; Nantanoi et al. 2022), the Widmerpool Gulf (Gross et al. 2015; Könitzer et al. 2016; Yang et al. 2016; Whitelaw et al. 2019), the Blacon Basin (Newport et al. 2016), the Edale Basin (Hennissen et al. 2017; Waters et al. 2020), the Cleveland Basin (Słowakiewicz et al. 2015; Hughes et al. 2018) and the Gainsborough Trough (Palci et al. 2020). Additionally, Loveless et al. (2018, 2019) and Bell et al. (2017) provided important insights into the baseline groundwater conditions and vulnerability across these basins.

Favourable reservoir conditions and successful gas flows were ultimately demonstrated by the company Cuadrilla Resources in the Craven Basin (e.g. Clarke et al. 2018), indicating that the Bowland Shale is a viable shale gas target at least in terms of composition and thermal maturity in the Craven Basin. However, in 2011, induced seismicity caused by hydraulic fracturing of the Bowland Shale at Preese Hall-1 demonstrated the critical importance of understanding the risk of fault reactivation (Green et al. 2012). The multiple microseismic events at Preese Hall-1 (Clarke et al. 2014), and later at Preston New Road (Clarke et al. 2019; Kettlety et al. 2020; Nantanoi et al. 2022), ultimately led to a moratorium on shale gas in the UK (e.g. Bradshaw et al. 2022). Green et al. (2012), Kettlety et al. (2020) and Nantanoi et al. (2022) provided up-to-date perspectives on the seismicity risk associated with hydraulic fracturing in Lancashire.

With the benefit of hindsight offered by the latest generation of research, it is clear that the regional geological assessments by the British Geological Survey (BGS) (Smith et al. 2010; Andrews 2013) failed to attach sufficient weighting to the structural complexity, compartmentalization and the highly variable exhumation, erosion and palaeo-heat flow history of the Bowland Shale. All of these factors contrast to the generally undeformed nature of many successful US shale gas plays (Anderson and Underhill 2020). Additionally, the use of open system pyrolysis for resource estimation potentially led to overestimation in the size of the resource (Whitelaw et al. 2019). Overall, the regional perspective as of 2024 is one of uncertainty and risk. Some areas are demonstrably too structurally complex, particularly in the relatively deformed Craven Basin (e.g. Arthurton 1984; Pharaoh et al. 2020). Nonetheless, the viability of shale gas from the Bowland Shale in the less-deformed basins remains uncertain and is largely untested.

Finally, we reflect on the funding that supported the aforementioned key papers, the majority of which was initiated in the decade that followed the seminal Andrews (2013) report (Table 1): three papers were led by a Natural Environment Research Council (NERC) Oil and Gas Centre for Doctoral Training (CDT) PhD student, one paper was supported by a NERC standard grant, one paper was led by a NERC Fellow, five papers were led by PhD students supported by the British Universities Funding Initiative (BUFI) BGS scheme, six papers were made possible under the NERC directed Unconventional Hydrocarbons programme, one paper was led by a NERC Central England Training Alliance (CENTA) Doctoral Training Partnership (DTP) PhD student, four papers were led by industry (Cuadrilla Resources, IGas Energy and Third Energy), two papers derive from industry consortia (GASH, supported by Statoil, ExxonMobil, GDF Suez, Wintershall, Vermillion, Marathon Oil, Total, Repsol, Schlumberger and Bayerngas, and the BGS Onshore Carboniferous Basins project, supported by Centrica PLC, DECC/BEIS, GDF Suez E&P UK and Total E&P UK), two papers were supported by NERC–BGS National Capability funding, two papers were supported through the European Union's Horizon 2020 Research and Innovation Programme under the ShaleXenvironmenT project, one paper was led by a University of Manchester NERC DTP PhD student, one paper was led by a University of Leicester NERC DTP PhD student, one paper was funded by a NERC follow-on award, one paper was led by a researcher supported by the China Scholarship Council, one paper was supported by the Mobility Plus programme postdoctoral fellowship of the Ministry of Science and Higher Education of Poland, and finally three papers were supported by combined NERC National Capability (BGS) and Environment Agency funding.

Regarding the funding of papers in the volume (Table 1), Fraser et al. (2024) , Hennissen et al. (2024) , Lodhia et al. (2023)  and Beriro and Vane (2023)  were funded through the NERC Unconventional Hydrocarbons directed research programme. Emmings et al. (2023)  were funded via a NERC National Capability award to the BGS, and were also supported by the National Environmental Isotopes Facility (NEIF) Steering Committee and the NERC Unconventional Hydrocarbons programme. The lead authors of Sims et al. (2024) , Walker et al. (2023)  and Pitcher et al. (2022)  were PhD students funded by the NERC CDT in Oil and Gas. Sims et al. (2024)  were also partially supported by the NERC Unconventional Hydrocarbons directed research programme. Parnell et al. (2022)  and Armstrong et al. (2023)  were funded through the NERC Minerals directed research programme. Armstrong et al. (2023)  were also supported by the NERC Highlights CuBES programme. The Waters et al. (2023)  field guide was funded through the Onshore Carboniferous Basins Consortium.

Fraser et al. (2024)  provide an updated holistic assessment of the shale gas history and resource potential of the Bowland Shale in the UK. The authors combine the results of 2D regional seismic mapping, well correlations, new mineralogy and pyrolysis analyses, and an extensive literature review, to derive play risk maps across the volume area of interest. The authors conclude that the resumption of unconventional shale gas exploration in the UK is generally not supported by the geology at a regional scale. The regional screening results show that parts of the Bowland sub-basin and Gainsborough Trough are the most prospective for shale gas, but overall, the size of the resource is not likely to be significant. A high degree of compartmentalization, a relatively limited spatial extent to gas mature shales and structural complexity and deformation are key geological factors that differentiate the Bowland Shale from the proven US shale gas plays. Additionally, burial modelling by Lodhia et al. (2023)  shows that a significant fraction of hydrocarbons from the gas mature shales was probably lost through expulsion during peak burial. In hindsight, the authors find that fundamental research based on legacy data, new analyses and modelling should have preceded drilling operations in northern England.

Lodhia et al. (2023)  utilize 1D basin modelling combined with apatite fission track (AFTA) analyses to refine our understanding of the history of hydrocarbon generation and expulsion from the Bowland Shale. The authors show the importance of accounting for strata that were eroded during the Variscan Orogeny and Late Cretaceous–Cenozoic uplift (‘missing stratigraphy’). Similar to other chapters in the volume, the authors' key conclusion is that the shale gas resource of the Bowland Shale is probably significantly lower than the initial regional estimates of Andrews (2013). When accounting for missing stratigraphy, their burial models suggest that most thermogenic hydrocarbons were generated and expelled from the lower Bowland Shale in the mid- to late Carboniferous. Thus, the in-place unconventional hydrocarbon resource in the lower Bowland Shale is reduced. Similarly, the present-day unconventional hydrocarbon resource in the upper Bowland Shale was also reduced by hydrocarbon expulsion in the mid- to Late Cretaceous. The conventional hydrocarbon perspectivity associated with expulsion from the Bowland Shale, into key reservoir intervals such as the Sherwood Sandstone and Kinderscout Grit, were also compromised owing to widespread exhumation and erosion in the Cenozoic. Finally, the authors explore two promising areas for future research: CO₂ injection and sorption within the Bowland Shale (e.g. Busch et al. 2008; Liu et al. 2021; Ma et al. 2021), and geothermal resources in Lancashire, including where the Bowland Shale is a cap to the Lower Carboniferous limestone geothermal play (e.g. Busby 2014; Gluyas et al. 2018).

Emmings et al. (2023)  and Hennissen et al. (2024)  present a dual analysis of the processes, stacking patterns, thermal properties and unconventional hydrocarbon prospectivity in the Bowland Shale in the Blacon Basin, in North Wales and Cheshire. The authors provide a high-resolution analysis of two cores from the well Ellesmere Port-1, drilled by IGas Energy in 2014. Nearly a decade since the core was recovered from this well, these chapters provide the first published observations through these cores. In part 1, Emmings et al. (2023)  couple sedimentological core logging, high-resolution core scan X-ray fluorescence (XRF), bulk geochemistry and isotopic analysis, cluster analysis and machine learning to delineate the key lithofacies, chemofacies and stacking patterns through the Bowland Shale. The authors show that the Bowland Shale generally comprises argillaceous to mixed argillaceous–calcareous–siliceous mudstone and siltstone lithologies, deposited under fluctuating redox conditions. On the basis of assessment of Si/Al and Si/Zr, the Bowland Shale sampled is dominantly siliciclastic and is not siliceous, in contrast to the Bowland Shale in the Craven Basin (Emmings et al. 2020b) and in contrast to the Hodder Mudstone (Ohiara et al. 2023). The authors apply k-nearest neighbour (kNN) machine learning of the core scan chemofacies clusters to predict key lithologies through the entire Bowland Shale span in Ellesmere Port-1. Finally, Emmings et al. (2023)  explore the radiogenic heat productivity (RHP) and 1D heat flow through the Bowland Shale in the Blacon Basin to help improve understanding of the Bowland Shale in the context of the Carboniferous limestone play.

In part 2, Hennissen et al. (2024)  present the results of an integrated palynological and bulk organic geochemical study through the Ellesmere Port-1 cores. The authors show that the Bowland Shale comprises a mixture of Type II and Type III organic matter, consistent with Sims et al. (2024) . Vitrinite reflectance (VR) analysis shows that the cores span the top of the oil window to the top of the gas window. Using the method of Jarvie (2012) and the observed organic matter assemblages, the authors correct total organic carbon (TOC) for thermal maturation and thus estimate the original TOC (TOC_o) content. The calculated original hydrogen index (HI_o) and TOC_o data are relatively low at c. 200–300 HI_o and 1.5–3.0 wt% TOC_o. These HI_o data in the Blacon Basin are c. 100–150 mgHC g⁻¹ TOC lower than the present-day HI reported by Könitzer et al. (2016) and Hennissen et al. (2017) from the Widmerpool Gulf and Edale Basin, and also lower than the immature to marginal oil mature, high TOC samples from the Craven Basin (Emmings et al. 2020c). In the Craven Basin, one sample exhibits a HI of 472 mgHC g⁻¹ TOC at 9.61 wt% TOC (Emmings et al. 2020c), meaning the HI is robust since matrix effects are likely to be negligible (Yang et al. 2016). The calculated HI_o data in the Blacon Basin are also c. 50–100 mgHC g⁻¹ TOC lower than low maturity kerogen isolates from the Widmerpool Gulf and Edale Basin (Yang et al. 2016). Thus, Hennissen et al. (2024)  show that the generative potential of the Bowland Shale in the Blacon Basin is low, in contrast to many sections in the other key basins that comprise a larger proportion of Type II organic matter. A high proportion of Type III organic matter in the Blacon Basin is attributed by the authors to proximity to the Cefn-y-Fedw delta system.

To conclude, Hennissen et al. (2024)  assess 5 of the 13 key unconventional hydrocarbon prospectivity criteria (Andrews 2013), which show that the Bowland Shale in Ellesmere Port-1 exhibits low TOC_o, poor to moderate present-day TOC, low generative potential, dominance of Type III to mixed Type II/III kerogen, generally low gamma ray response and low HI_o. Additionally, Emmings et al. (2023)  show that the studied sections are generally clay-rich and are rarely cemented. The Bowland Shale in Ellesmere Port-1 is not silica-cemented and is rarely carbonate-cemented (marine band facies only). Both core sections are also faulted. While the fault displacements are unknown, similarity of sedimentary facies across the faults suggests minimal displacement and that the faults are likely to be subseismic in scale. The net shale thickness of the Bowland Shale in Ellesmere Port-1 is relatively high, but the section is expanded (i.e. diluted) by a large proportion of detrital components linked to the adjacent delta system (Emmings et al. 2023). The Bowland Shale (Holywell Shale) is a known oil source in the Irish Sea (Armstrong et al. 1997). Thus, of the 13 criteria described by Andrews (2013), the Bowland Shale in Ellesmere Port-1 potentially fails four or five tests (kerogen, TOC_o and HI_o, mineralogy/clay content, thermal maturity, potentially tectonics and burial history) and passes two or three tests (thickness, depth minimum of >1500 m, potentially shale oil precursor). Their analysis suggests that the Bowland Shale in Ellesmere Port-1 is not a robust unconventional hydrocarbon resource.

Sims et al. (2024)  present the results of 201 biomarker analyses through the Bowland Shale spanning nine wells in the Craven Basin and Widmerpool Gulf. The results contribute to an important pool of research on the organic geochemistry of the Bowland Shale (Gross et al. 2015; Słowakiewicz et al. 2015; Yang et al. 2016; Whitelaw et al. 2019). The study is the largest organic geochemical study on the Bowland Shale to date. Sims et al. (2024)  offer several key insights. Firstly, the authors show that the Bowland Shale comprises a wide mixture of Type II, II/III, III and IV organic matter, consistent with multiple studies over the last decade that demonstrate organic matter heterogeneity in the Bowland Shale (e.g. Könitzer et al. 2016; Newport et al. 2016; Yang et al. 2016; Hennissen et al. 2017). The Bowland Shale in the Craven Basin exhibits a relatively low HI compared with the Widmerpool Gulf, which is interpreted by the authors as a dominance of Type III organic matter and/or represents an artefact due to clay mineral matrix effects (e.g. Yang et al. 2016). A mixed origin to the organic matter is supported by the proportions of C₂₇, C₂₈ and C₂₉ steranes, which the authors interpret as a mix of planktonic, bacterial and higher land plant precursors (Volkman 1986). A strong bacterial signal is consistent with palynological observations, which show that the Bowland Shale generally lacks organic-walled algal tests and usually contains abundant amorphous organic matter (Könitzer et al. 2016; Hennissen et al. 2017, 2024 ; Emmings et al. 2019).

Next, Sims et al. (2024)  provide new insights into the bottom water redox conditions during deposition of the Bowland Shale. Pristane/phytane ratios indicate generally mixed, fluctuating or transitional conditions, findings that are broadly consistent with Gross et al. (2015), Słowakiewicz et al. (2015) and Emmings et al. (2020c). Low concentrations of gammacerane (Sinninghe Damsté et al. 1995) suggest that bottom water conditions were weakly rather than strongly stratified. A low aryl isoprenoid ratio (AIR) in the marine band and hemipelagic facies suggests intermittent intra- and inter-basin water column colonization by green sulfur bacteria (Summons and Powell 1987; Schwark and Frimmel 2004). Sims et al. (2024)  interpret these data to indicate the development of photic zone euxinia in the Bowland Shale, particularly during marine transgressions and highstands, reaching a conclusion similar to Gross et al. (2015). Overall, the biomarker palaeoredox proxies are consistent with the interpretation of fluctuating and at least intermittently euxinic redox conditions based on inorganic geochemistry (Riley et al. 2018; Emmings et al. 2020c; Li et al. 2023). Additionally, the Re and Mo analyses of Parnell et al. (2022)  support weakly, but not strongly, euxinic conditions at the regional scale, and Emmings et al. (2023)  and Walker et al. (2023)  provide further evidence for fluctuating Fe–Mn particulate shuttle redox conditions. The palynological observations of Hennissen et al. (2024)  also support a model of fluctuating redox conditions. Finally, Sims et al. (2024)  explore a variety of organic geochemical proxies for thermal maturity, including the diasterane and C₃₁ hopane isomerism ratios, which show that most samples across the Craven Basin and Widmerpool Gulf are within the oil expulsion window.

Walker et al. (2023)  summarize an analysis of Mo and U contents in the Bowland Shale across three sites in the Craven Basin, including two wells drilled by Cuadrilla Resources (Preese Hall-1 and Becconsall-1Z) and the outcrop at Hind Clough (Emmings et al. 2020c). The authors offer a suite of new analyses including bulk mineralogy X-ray diffraction (XRD), XRF and Rock-Eval pyrolysis coupled with well log data and statistical analysis. Walker et al. (2023)  focus on the extent to which the Barnett Shale Formation, USA, represents an appropriate analogue in terms of syngenetic processes. Based on Mo, U and TOC_o analysis, the authors conclude the Barnett Shale is a poor analogue to the Bowland Shale, and the units may instead represent near end-members. Walker et al. (2023)  calculate TOC_o using two approaches, including a classic approach (Jarvie 2012) and a tailored equation based on observations in the adjacent Blacon Basin (Emmings et al. 2023 ; Hennissen et al. 2024). A generally high fraction of refractory organic matter in the Bowland Shale, even in low maturity sections, could indicate only modest increases in TOC_o compared with present-day TOC, despite the high thermal maturity in the studied wells. Understanding the proportion of pyrolysable and refractory organic matter fractions – and by extension the way TOC concentrations are corrected in thermally mature rocks – is critically important to understanding and modelling the genesis of hydrocarbons (e.g. Lodhia et al. 2023 ), the diagenetic history and the interpretation of many proxies such as Mo/TOC_o in the Bowland Shale (e.g. Hart and Hofmann 2022). In the Bowland Shale, deciding which TOC correction equation to use is made complex owing to an apparently large fraction of refractory organic matter (Hennissen et al. 2017). This phenomenon might simply relate to matrix effects (e.g. Yang et al. 2016), but could represent a genuine signal related to the poor preservation potential of bacterial organic matter (Sims et al. 2024 ).

Beriro and Vane (2023)  summarize the results of a proof-of-concept study into the chemometric modelling of Fourier transform infrared spectroscopy (FTIR) (e.g. Leach et al. 2008; Chen et al. 2014) for rapid non-destructive measurement of key bulk organic geochemical indices through the Bowland Shale. The advantage of FTIR compared with Rock-Eval pyrolysis is that it is non-destructive, it is fast, it can be conducted in the field or the laboratory and it is an inexpensive technique per sample (Washburn and Birdwell 2013). Beriro and Vane (2023)  modelled selected Rock-Eval parameters (T_max, S1, S2, S3, TOC) using partial least squares regression (PLSR) of the principal components derived from the FTIR spectra. The authors demonstrate an ability to effectively interpolate selected Rock-Eval parameters down-core via modelling of FTIR data. The approach has the potential to improve our ability to resolve high spatial and temporal bulk organic geochemical signals through mudstone successions. Finally, the authors identify wider applicability including radioactive waste disposal.

Pitcher et al. (2022)  studied three samples, below, within and above the E_1a1 marine band, from the Bowland Shale in Preese Hall-1 in the Craven Basin. They applied novel gas sorption experiments to probe the nanometre-scale to millimetre-scale pore size distributions. Pore volumes range from c. 0.001, 0.003 to 0.011 ml g⁻¹. The sample overlying the E_1a1 marine band package exhibits an order of magnitude greater pore volume and BET surface area than the other samples. This phenomenon is potentially explained by downward diffusion of acidity into the top of the marine band package during early diagenesis (Emmings et al. 2020b). Overall, the findings of Pitcher et al. (2022)  echo the other chapters in the volume and elsewhere: the Bowland Shale is highly heterogeneous at the microscale. To conclude, Pitcher et al. (2022)  develop a conceptual model of the adsorption process in the Bowland Shale. Critically, the authors identify the presence of narrow pore necks in the Bowland Shale, which severely restrict the accessibility of the interior void spaces in this shale, with implications for gas flow through this unit.

Ohiara et al. (2023)  offer a comprehensive sedimentological and diagenetic assessment of the Lower Carboniferous Hodder Mudstone Formation. While the Hodder Mudstone and Bowland Shale are formally recognized as different formations, with the Hodder Mudstone being older (Fig. 1), together the units define the ‘Bowland–Hodder’ shale gas play of Andrews (2013). The authors utilize optical and scanning electron microscopy (SEM) coupled with bulk geochemical and mineralogical analyses to develop a paragenetic framework for the Hodder Mudstone in the Craven Basin. The authors show that the Hodder Mudstone is dominated by mixed argillaceous–siliceous–calcareous and carbonate-dominated lithofacies. Critically, the authors demonstrate through textural observations that most of the quartz in the Hodder Mudstone is authigenic in origin, and provide evidence for significant Al mobility during diagenesis. The authors explain how consistently low to modest Si/Al and Si/Zr indicates the authigenic quartz in the Hodder Mudstone: (1) largely derives from the transformation of allochthonous phases under closed system diagenetic conditions (e.g. feldspar dissolution, detrital clay mineral transformation); and (2) derives partially from early diagenetic biogenic silica mobility, at least within the carbonate-bearing facies. This contrasts the upper Bowland Shale in the Craven Basin, where strong Si/Al and Si/Zr excursions coupled to petrographic evidence show the dominance of a biogenic silica source (Emmings et al. 2020b). Based on mineralogical and textural observations, the authors conclude that the Hodder Mudstone generally exhibits favourable characteristics for hydraulic fracturing within the Bowland–Hodder shale gas play.

Armstrong et al. (2023)  and Parnell et al. (2022)  build on previous research by Armstrong et al. (2019), Parnell et al. (2016) and Parnell et al. (2018) who observed the Bowland Shale is enriched in a suite of redox sensitive and potentially toxic metals and metalloids, including anomalously high selenium (Se) concentrations compared with other shales (Stüeken et al. 2015). Taken with the observations of Riley et al. (2018), Emmings et al. (2020c), Emmings et al. (2023) , Walker et al. (2023)  and Li et al. (2023), the Bowland Shale is evidently enriched in redox sensitive metals across multiple basins and ages, including in the Craven Basin, Blacon Basin and the Edale Basin. The enrichment of redox sensitive metals in the Bowland Shale is linked to deposition under at least intermittently sulfidic bottom water conditions. Importantly, many of these redox sensitive elements are potentially toxic to livestock where the Bowland Shale crops out (e.g. Fleming and Walsh 1956), but the mechanisms for transfer of elements from the Bowland Shale (donor) into soil and vegetation (receivers) during weathering are complex. Two important factors are: (1) the speciation of elements in the host rock; and (2) affinities for element fixation in secondary mineral precipitates generated during weathering. The precipitation of secondary phases, including Fe-(oxyhydr)oxides and sulfate species, can potentially buffer the mobility of potentially harmful elements in the surface environment. Additionally, the study of weathering products in the vicinity of the Bowland Shale potentially serves as a useful analogue to element mobility during subsurface drilling operations.

To help address this complexity, Parnell et al. (2022)  report the speciation of Se, Cu, As, U and Mo in the Bowland Shale, spanning a wide range of thermal maturities (0.9–4.5% VR_o) and basin localities. Armstrong et al. (2023)  report bulk, microscale and isotopic geochemical analyses from an ochreous ‘sulfur spa’ precipitate at Crimpton Brook, Lancashire. Here, Armstrong et al. (2023)  compare the concentrations of redox sensitive elements in the Crimpton Brook precipitates with analyses with adjacent unweathered Bowland Shale samples. Critically, Armstrong et al. (2023)  show that As and Se are hosted primarily within pyrite in the unweathered Bowland Shale. These findings are supported by Parnell et al. (2022)  who demonstrate that pyrite and organic matter are important hosts for Se, Cu, As, U and Mo. This is consistent with a syngenetic fixation pathway for these elements. Finally, Armstrong et al. (2023)  used δ³⁴S_py to demonstrate that the source of S in jarosite within the Crimpton Brook precipitates derives from the oxidation of pyrite in the Bowland Shale, rather than deep evaporites. Thus, while localized in extent, the Armstrong et al. (2023)  study at Crimpton Brook together with the Parnell et al. (2022)  speciation experiments emphasize the importance of the Bowland Shale as a reservoir for reduced metals and metalloids. The potential for trace-element mobilization coupled to pyrite oxidation is relevant to any subsurface drilling operations through or within the Bowland Shale.

Waters et al. (2023)  present a guide to selected exposures of the Bowland Shale in the Craven Basin and Edale Basin. The authors provide detailed descriptions of the lithofacies and stacking patterns observed at each outcrop, with particular attention given to the positions of key correlative marker beds (e.g. marine bands). They describe the significance of the chosen localities in understanding the development of the Bowland Shale and give sufficient details on the sections visible at the time of compilation. The purpose of the field guide is to supplement the BGS memoirs, to provide useful guidance for field visits and to stimulate further research on this enigmatic and controversial black shale unit.

In terms of processes and resources associated with the Bowland Shale, we suggest that the following topics may be promising avenues for future research:

1. The Bowland Shale is a borderline highly enriched metalliferous black shale, containing the redox-sensitive metal V (Emmings et al. 2020c) and the reactive semi-metal Se (Parnell et al. 2016).

2. Dewatering during diagenesis of the Bowland Shale is potentially implicated in the genesis of adjacent base metal (Pb–Zn) deposits (Kendrick et al. 2002; Juerges et al. 2016; Emmings et al. 2020b).

3. The Bowland Shale is a component of at least one geothermal play in the UK where it is a seal for interdigitating sandstone aquifers (Gluyas et al. 2018).

4. The Bowland Shale caps and buries several granitic intrusions in the UK (Leeder 1982; Busby and Terrington 2017), potentially enhancing the prospectivity of this geothermal play.

5. The Bowland Shale is also either a direct cap rock or an analogue for deeper shales overlying the Lower Carboniferous limestone geothermal play (Gluyas et al. 2018).

6. The Bowland Shale remains an important conventional hydrocarbon source rock, for example in the North Sea (Monaghan et al. 2019).

7. High U content implicates the Bowland Shale as a significant source of Rn, derived primarily from the decay of ²³⁸U, which may contribute to the known Rn hazard in areas such as in the Ribble Valley (Miles et al. 2007) or in deep groundwater aquifers.

8. Weathering of the Bowland Shale releases potentially toxic metals into the environment, impacting groundwater, soil quality and the health of livestock (Brogan et al. 1973; Parnell et al. 2016).

9. As a widely distributed fine-grained sedimentary package, the Bowland Shale is potentially of interest as a seal (or analogue) for CO₂ or radioactive waste storage, or might represent a direct target for CO₂ sorption coupled to enhanced CH₄ recovery (e.g. Ma et al. 2021).

10. The Bowland Shale spans the mid-Carboniferous (Mississippian–Pennsylvanian) boundary and extinction event (Saunders and Ramsbottom 1986), representing a period of extreme climatic and/or oceanic conditions of interest to a range of deep time palaeoenvironmental and modern climate research questions.

The volume started life during the COVID pandemic of 2020–21. We gratefully thank all contributors, reviewers and the Geological Society of London for their support and patience. We thank Colin Waters who helped to shape the stratigraphic column included in this introduction.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

JFE: conceptualization (lead), writing – original draft (lead); JP: conceptualization (supporting), writing – review & editing (supporting); MHS: conceptualization (supporting), writing – review & editing (supporting); BHL: conceptualization (supporting), writing – review & editing (supporting).

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

Data sharing is not applicable to to this article as no datasets were generated or analysed during the current study.