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

The growth of faults and fracture networks in a mechanically evolving, mechanically stratified rock mass : a case study from Spireslack Surface Coal Mine, Scotland

Fault architecture and fracture network evolution (and resulting bulk hydraulic properties) are highly dependent on the mechanical properties of the rocks at the time the structures developed. This paper investigates the role of mechanical layering and pre-existing structures on the evolution of strike–slip faults and fracture networks. Detailed mapping of exceptionally well exposed fluvial–deltaic lithologies at Spireslack Surface Coal Mine, Scotland, reveals two phases of faulting with an initial sinistral and later dextral sense of shear with ongoing pre-faulting, syn-faulting, and post-faulting joint sets. We find fault zone internal structure depends on whether the fault is self-juxtaposing or cuts multiple lithologies, the presence of shale layers that promote bed-rotation and fault-core lens formation, and the orientation of joints and coal cleats at the time of faulting. During ongoing deformation, cementation of fractures is concentrated where the fracture network is most connected. This leads to the counter-intuitive result that the highest-fracture-density part of the network often has the lowest open fracture connectivity. To evaluate the final bulk hydraulic properties of a deformed rock mass, it is crucial to appreciate the relative timing of deformation events, concurrent or subsequent cementation, and the interlinked effects on overall network connectivity

Core surprise : Characterising the internal structure of an ancient plate boundary fault in Scotland

Knowledge of the structure and rheology of large, earthquake-hosting plate boundary faults is lacking as they are normally poorly exposed or difficult to find on the surface. Recently, several drilling projects have been undertaken to explore the internal structure of active plate boundary fault zones at depth to understand how this constrains seismic slip behaviour. All of these projects highlight the presence and importance of clay-rich rocks within the fault core in controlling slip behaviour along these large faults. The Highland Boundary fault (HBF) in Scotland, provides a rare opportunity to study the internal fault architecture of a well-exposed along-strike section of an ancient plate boundary fault. The HBF extends for over 240 km, however, is only well-exposed along a 560 m section at Stonehaven. Here, serpentinite juxtaposes quartzofeldspathic crustal rocks, a common feature at many plate boundaries (e.g., sections of the San Andreas fault and Alpine fault, New Zealand). We collected six across-fault transects aiming to capture the internal structure of the HBF and its along-strike variability. Within the fault core we discover four mechanically and chemically distinct clay-rich units, which have sharp contacts. Despite evidence of internal strain within the clay-rich fault rocks, relatively intact clasts of wall rock and microfossils are preserved. From mineralogical observations it can be interpreted that the clay-rich rocks along this section of the HBF, formed through fluid-assisted, shear-enhanced chemical reactions between wall rocks of contrasting chemistry. Our field evidence also demonstrates that plate boundary faults can be structurally variable along strike at various scales. The total thickness of the fault core varies from 3 to 10.7 m over an along strike distance of 560 m. Not every unit is laterally continuous along strike, and each unit varies in thickness. We compare our observations with studies on other plate boundary systems. For example, the HBF has analogous thickness and mineralogy to drill core recovered from the San Andreas fault. Highly variable fault core structures and related properties such as mineralogy, may exert significant control on earthquake rupture and slip behaviour at large plate boundaries

Quantifying fault interpretation uncertainties and their impact on fault seal and seismic hazard analysis

We would like to thank DugInsight for the provision of an academic license for their software package. We would like to thank Emma Miche and two anomalous reviewers for constructive feedback on the original version of the manuscipt.Peer reviewe

I Wanna Be Loved

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Patience and fortitude, patience and fortitude, [first line]

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