Early Carboniferous limestones of southern and central Britain: characterisation and preliminary assessment of deep geothermal prospectivity

Pharaoh, Tim; Jones, Darren; Kearsey, Tim; Newell, Andrew; Abesser, Corinna; Randles, Tom; Patton, Ashley; Kendall, Rhian. 2021 Early Carboniferous limestones of southern and central Britain: characterisation and preliminary assessment of deep geothermal prospectivity. Zeitschrift der Deutschen Gesellschaft für Geowissenschaften, 172 (3). 227-249.

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In Britain, thick limestones of early Carboniferous (Mississippian: 359–323 Ma) age are present in two provinces, respectively south and north of the Wales-Anglo-Brabant landmass. In the southern province, early Carboniferous limestones were deposited upon a southward-deepening shelf, laterally continuous from Ireland to the Rhineland. They now occupy a number of discrete minibasins as a consequence of Variscan orogenic thrusting and significant post-Carboniferous erosion. In the northern province, local tectonic controls led to the development of a mosaic of deepwater basins, ramps and platforms in response to Mississippian extensional stress. The interaction with glacioeustatic sea-level change led to the development of complex carbonate system tracts on these ramps and platforms. Given favourable conditions of palaeokarst development and fracturing, hydraulic transmissivity could be sufficient to allow development as a geothermal resource. Deep geothermal prospectivity is controlled by a hierarchy of factors, operating on scales ranging from provincial (1000–100 km) down to outcrop (1000–100 m), reflecting processes operating on the lithospheric down to sub-basinal scale respectively. On the scale of the individual prospect, these factors include the mode of carbonate deposition, particularly depth of water and angle of depositional slope, which are tectonically controlled; the history of synsedimentary exposure, erosion and karstification, strongly influenced by sea-level change; by the diagenetic history and subsequent basin evolution; by deformation and fracturing during Variscan basin inversion; and by the post-Carboniferous history of subsidence, uplift and karstification. The contrasting impact of these various processes upon hydraulic transmissivity in the two provinces is reviewed, and a preliminary assessment of the geothermal prospectivity of each is presented. The most prospective areas for deep geothermal exploitation are considered to be basins, shelves and platforms lying at depths of 2 to 5 km below sea level. Deepwater basins are considered less prospective because of the lack of thick limestones, except in the hanging wall at fault-bounded margins, where Waulsortian mud-mounds with good residual porosity and fault-zones with polyphase history are likely present. Granite underpinned highs in N England, where Carboniferous limestones are typically at crop, and shallow basins of Carboniferous age lying on the Wales-Anglo-Brabant Massif, are considered less prospective in the deep geothermal context.

Item Type: Publication - Article
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ISSN: 1860-1804
Date made live: 15 Dec 2021 17:05 +0 (UTC)

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