Multiscale Analysis of Mississippian Carbonate Platforms in the Irish Sea and Adjacent Areas

Abstract

This thesis investigates the tectonostratigraphic and structural controls influencing the development and geothermal potential of Mississippian carbonate platforms in the East Irish Sea Basin (EISB) and Solway Basin, United Kingdom. The study is based on seismic interpretation and integrates multiscale datasets, including well interpretation, outcrop analogue observations, and fracture network modelling. Together, these approaches provide new insights into the spatial and temporal variability of carbonate platform growth and demise, and their implications for geothermal energy exploration. In the Upper Mississippian mixed carbonate-clastic successions of the Solway Basin, located in the northern part of the study area, clastic influx associated with major northeast–southwest trending faults disrupts the continuity of carbonate platforms. This tectono-sedimentary complexity introduces significant reservoir heterogeneity and increases exploration risk, owing to compartmentalisation and uncertain permeability pathways. By contrast, in the southern EISB, the growth and subsequent drowning of discrete carbonate platforms are linked to footwall uplift, differential subsidence, and sea-level fluctuations during the Late Visean to early Namurian. These platforms preferentially nucleated on structurally elevated highs within clear-water, low-nutrient environments. Seismic facies analysis reveals features such as localised karstification and margin collapse, which suggest enhanced secondary porosity. These platforms, typically several kilometres wide and buried at depths between 1 and 3 kilometres, represent promising geothermal targets, provided that fracture connectivity is sufficient to support fluid circulation. By combining seismic data with outcrop analogues and applying quantitative fracture analysis using FracPaQ, this study evaluates the scale-dependent connectivity of fracture systems within the Mississippian carbonate platforms. At the bed and outcrop scales, fracture networks exhibit high connectivity indices (CB values greater than 1.5), with dominant north–south orientations that align with regional fault trends. These sub-seismic fracture systems may sustain geothermal fluid flow even in intervals characterised by limited matrix porosity. Slip tendency and dilation tendency analyses indicate that many of these fractures are favourably oriented for reactivation and dilation under the current stress regime, thereby enhancing their potential for fluid conductivity. However, seismic-scale interpretations tend to underestimate fracture connectivity due to resolution constraints, underscoring the importance of multiscale fracture modelling in evaluating geothermal reservoir performance. This study demonstrates that the geothermal viability of Mississippian carbonate reservoirs in the United Kingdom is highly site-specific, controlled by factors such as structural elevation (which governs carbonate platform development), fracture intensity, and burial depth. While the southern EISB platforms present viable pilot targets for drilling and heat extraction, the northern provinces, including the Solway and Northumberland areas, present greater geological risks due to clastic contamination, thinner carbonate sequences, and variable thermal gradients. The thesis concludes by proposing a strategic roadmap for future geothermal exploration. Key recommendations include prioritising structurally elevated platforms with evidence of karstification and high fracture connectivity, incorporating analogue-based models, and adopting phased drilling campaigns supported by diagnostic fracture testing. These findings contribute to national energy diversification goals and provide a foundation for the advancement of deep geothermal development in the Carboniferous carbonate provinces of the United Kingdom.

Date of Award	9 Oct 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Catherine Hollis (Main Supervisor) & Mads Huuse (Co Supervisor)