Architecture of the Lower Carboniferous Limestone in the Irish Sea Basin, UK

Lower Carboniferous (Visean) limestone is a principle target for geothermal energy production in the UK and northern Europe. In order to constrain its potential to provide low enthalpy heat to UK conurbations, particularly in central and northern England, it is necessary to understand the distribution of the limestone in the subsurface. Since the Lower Carboniferous limestone was deposited on the footwall of normal fault blocks within extensional basins (Fraser and Gawthorpe, 2003), it is not uniformly present across the UK. Furthermore, differential periods of carbonate platform growth, burial and uplift between sub-basins means that it is present at a range of depths and thicknesses, which will have a fundamental impact on the temperature of fluids that it contains.
The Lower Carboniferous Limestone is exposed across north Wales, northern England and the Isle of Man, and has been well described in outcrop (e.g. Hendry et al., 2015; Juerges et al., 2016; Manifold et al., 2020; Breislin et al., 2020). Many of these platforms are partially dolomitized and are mineralized from fluids sourced from juxtaposed hanging wall basins (e.g. Frazer et al., 2014; Breislin et al., 2020) and the platforms have undergone surface and hypogenic karstification (e.g. Farrant and Harrison, 2017). Mapping the presence, thickness and properties of Lower Carboniferous limestone in the subsurface onshore UK is hindered by a lack of good quality seismic data and limited well penetration, however. Although the UK Onshore Geophysical Library provides good coverage of northern England, there are many data gaps around urban areas, and data quality is generally poor. Consequently, predictive models of the distribution, thickness and depth of the Lower Carboniferous Limestone are required to facilitate prediction of its presence and depth close to the market for geothermal energy. In particular, knowledge of the position, angle and architecture of platform margins is critical to the successful identification of targets in NW England. Furthermore, improved methods for confident identification of diagenetic features such as dolomitization and karstification, which are likely to create flow perturbations in the subsurface, are required.
A suite of 2D and 3D seismic data is available for the Irish Sea Basin, with accompanying well data. The distribution of carbonate platforms was mapped across the basin by Pharaoh et al (2018). Recent work at University of Manchester has provided a detailed interpretation of the platform at the southern margin of the basin and has mapped the northernmost extension of the North Wales Platform (Al Brashdi, 2020). This work shows that carbonate platform distribution strongly influenced by the position of isolated fault blocks within the hanging wall basin and that carbonate platform margins are complex, with. With this background, this study will interpret seismic data from the Irish Sea Basin in order to:
• Map the distribution of faults, and determine their control on the position of the North Wales and Manx platforms, as well as the distribution of smaller, isolated carbonate platforms within the basin in order to reconstruct the history of carbonate platform nucleation and growth
• Identify the size, shape, thickness and platform margin geometry of the North Wales and Manx Platforms, and their stratal architecture (patterns of progradation, aggradation or retrogradation)
• Construct a conceptual model for the evolution of Tournasian and Visean carbonate platform growth and demise, and apply this model to the evolution of age-equivalent platforms onshore in order to improve knowledge of the distribution and architecture of Lower Carboniferous Limestone.
In addition, the project will aim to identify large-scale diagenetic features, such as karstification and dolomitization, which might influence the flow behaviour of the carbonate strata.
Data availability
• Seismic lines (via NERC Open Research Archive) and SPEC data, provided by IHS
• Wireline logs, core and petrophysical data from selected wells in East Irish Sea Basin
• Published data from the North Wales and Manx Platforms
Planned methods include:
• Seismic interpretation and mapping using correlation of key, time-equivalent surfaces
• Structural mapping and basin modelling
• Petrophysical log interpretation
• Short periods of fieldwork in North Wales and Isle of Man to view the carbonate platforms onshore
This project requires an upper second class or first class degree BSc or MEarthSci in geological sciences and/or an MSc at merit level or above in a geological discipline. Applicants should have experience of seismic interpretation and knowledge of carbonate sedimentology and diagenesis is an advantage.
Training will be provided in seismic interpretation, petrophysical methods and basin-scale carbonate sedimentology and diagenesis.