The effect of lithology, sub-bed scale heterogeneities, and mechanical stratigraphy on fault and fracture properties in coal bearing sequences

While sub-surface extraction of coal in the UK has ceased, renewed interest into the internal structure and growth of faults cutting coal measures exists due to applications for mine geothermal projects and analogues for natural gas plays in the Southern North Sea. In this study three field sites are used with detailed field observations, geological mapping, and sedimentological classification undertaken to understand the role lithology and subbed scale heterogeneity plays in the deformation of UK Carboniferous Coal Measure. This study demonstrates, and suggests methods to limit, geologists’ own biases during fracture data collection that can influence the data collected, and hence the derived statistics used for fracture modelling. Biases are also introduced when the temporal evolution and connectivity of individual sets of fault-fracture networks are not considered when assessing connectivity. Unlike binary mechanically layers sequences (e.g. limestone/marl), we find the presence of shale inter-beds and the abundance of sub-bed scale sedimentary heterogeneities (e.g. channel coals) to strongly affect the development fault and fractures. Where jointing exists at the time of faulting, fault-growth was found to be restricted by favourably orientated structures. The thickness and composition of fault-rock is controlled by lithological juxtaposition, with organic fragments found along principle displacement zones of faults of all sizes, even where organic rich layers have apparently not been cut. This study also shows faults that form following the collapse of Pillar and Stall mine workings are strongly affected by mechanical stratigraphy, with the height disruption controlled by a combination of the width of the stall, and the distribution and thickness of competent sandstone layers. Collapse leaves a clay-rich anthropogenic sedimentary layer which will retard the flow of groundwater in abandoned pillar and stall workings. These findings will improve our ability to assess geo-technical risk in ex-coal mining areas and de risk shallow mine geothermal projects.While sub-surface extraction of coal in the UK has ceased, renewed interest into the internal structure and growth of faults cutting coal measures exists due to applications for mine geothermal projects and analogues for natural gas plays in the Southern North Sea. In this study three field sites are used with detailed field observations, geological mapping, and sedimentological classification undertaken to understand the role lithology and subbed scale heterogeneity plays in the deformation of UK Carboniferous Coal Measure. This study demonstrates, and suggests methods to limit, geologists’ own biases during fracture data collection that can influence the data collected, and hence the derived statistics used for fracture modelling. Biases are also introduced when the temporal evolution and connectivity of individual sets of fault-fracture networks are not considered when assessing connectivity. Unlike binary mechanically layers sequences (e.g. limestone/marl), we find the presence of shale inter-beds and the abundance of sub-bed scale sedimentary heterogeneities (e.g. channel coals) to strongly affect the development fault and fractures. Where jointing exists at the time of faulting, fault-growth was found to be restricted by favourably orientated structures. The thickness and composition of fault-rock is controlled by lithological juxtaposition, with organic fragments found along principle displacement zones of faults of all sizes, even where organic rich layers have apparently not been cut. This study also shows faults that form following the collapse of Pillar and Stall mine workings are strongly affected by mechanical stratigraphy, with the height disruption controlled by a combination of the width of the stall, and the distribution and thickness of competent sandstone layers. Collapse leaves a clay-rich anthropogenic sedimentary layer which will retard the flow of groundwater in abandoned pillar and stall workings. These findings will improve our ability to assess geo-technical risk in ex-coal mining areas and de risk shallow mine geothermal projects