SUMMARY:Subsurface 3D geological models of aquifer and seal rock systems from two contrasting analogue sites have been created as the first step in an investigation into methodologies for geological storage of carbon dioxide in saline aquifers. Development of the models illustrates the utility of an integrated approach using digital techniques and expert geological knowledge to further geological understanding. The models visualize a faulted, gently dipping Permo-Triassic succession in Lincolnshire and a complex faulted and folded Devono-Carboniferous succession in eastern Scotland. The Permo-Triassic is present in the Lincolnshire model to depths of -2 km OD, and includes the aquifers of the Sherwood Sandstone and Rotliegendes groups. Model-derived thickness maps test and refine Permian palaeogeography, such as the location of a carbonate reef and its associated seaward slope, and the identification of aeolian dunes. Analysis of borehole core samples established average 2D porosity values for the Rotliegendes (16%) and Sherwood Sandstone (20%) groups, and the Zechstein (5%) and Mercia Mudstone (<10%) groups, which are favourable for aquifer and seal units respectively. Core sample analysis has revealed a complex but well understood diagenetic history. Re-interpretation of newly reprocessed seismic data in eastern Scotland has significantly reduced interpretative uncertainty of aquifer and seal units
Seismic and well data establish the three-dimensional geometry and structural evolution of the Northumberland–Solway Basin. The basin-controlling southerly bounding normal faults, the E–W-trending Maryport–Stublick–Ninety Fathom fault system, formed in early Carboniferous times by extensional reactivation of a major basement thrust zone. The faults are sub-planar with a present-day throw (at the base of the Carboniferous) of <4 km. Other important Dinantian syn-sedimentary normal faults trend roughly NNE-SSW and suffered dominantly oblique-slip displacements. Extension and syn-sedimentary faulting accompanied deposition of the Lower and Middle Border groups. Thereafter, until late Westphalian times, sedimentation was mainly in response to regional thermal relaxation subsidence. The maximum thickness of Carboniferous rocks ranges from around 5000 m in the Northumberland Trough to about 7000 m in the Solway Basin. At the end of Carboniferous times, the basin was partially inverted by Variscan transpression, with preferential reversal of the NNE-trending faults and the development of major monoclinal and anticlinal folds. Locally, there is evidence of embryonic syn-depositional folding in late Namurian times. In the Solway area, a Permo-Triassic to Jurassic succession, locally over 1.5 km thick, rests with marked angular unconformity on the Carboniferous strata. The argillaceous rocks of the basin commonly have high TOC values but are dominantly gas prone. The reservoir potential of the interbedded limestones and sandstones is poor. Most strata at outcrop are in the oil window, but in some boreholes, or where the rocks have been metamorphosed by the Whin Sill, overmature values have been recorded. Peak hydrocarbon generation was in late Westphalian times, prior to inversion, although some post-Variscan generation is possible in the Solway area, where there are oil and gas shows.
Groundwater flooding, which occurs when the groundwater table rises in response to exceptional recharge rates either to the ground surface or to a point where subsurface infrastructure is affected, has been recognized as a significant issue with real economic impacts.A methodology has been developed to produce maps of groundwater flooding susceptibility, using geological and hydrogeological data. While good geological map data are available in digital form for England and Wales, there are much less data on water levels. These levels are usually measured during the construction of water boreholes, and while there is a national groundwater level monitoring network for regulatory purposes, at a national level data are sparse. To assist in developing a comprehensive map of water levels, the British Geological Survey (BGS) has adopted a number of strategies for data interpolation for areas with limited water level data and a surface has been derived from a terrain model by considering interactions between groundwater and surface water in rivers and lakes. When comparing the calculated levels against the available field measurements, a high correlation was found to exist. However, it was considered that in areas where bedrock aquifers dominate, this interpolated surface was probably inaccurate, and so refinements were developed to improve the modelled water levels surfaces.The resulting groundwater levels have been used to develop maps of areas where shallow groundwater may pose a risk. With potential changes in groundwater recharge postulated as a result of global climate change, identifying areas prone to flooding from groundwater, or areas where groundwater is likely to increase the impact of surface water flooding, is increasingly important.
Ground source heat pump (GSHP) systems exchange heat with the subsurface to provide space heating or cooling. Groundwaterbased open-loop systems exchange heat directly with groundwater and can be more efficient than closed-loop systems owing to the water generally maintaining a constant temperature, whereas in closed-loop systems the ground is affected by heat extraction or injection. They could make a substantial contribution to meeting the UK's heating or cooling demands while reducing CO 2 emissions, but this depends on overcoming obstacles to GSHP uptake. Two of these obstacles are the lack of public awareness of GSHP technology (Enviros Consulting Limited 2008;Roy & Caird 2013) and the higher uncertainty (compared with conventional heating or cooling systems) regarding the economic viability of a planned scheme owing to unknown (hydro)geological conditions at the installation site.To address these issues, the British Geological Survey (BGS) (with support from the Environment Agency (EA) and advisors from the GSHP industry) is developing methods for identifying favourable (hydro)geological conditions for the installation of GSHP systems at the local administration or regional scale. Developed in a geographic information system (GIS), the results are made available as simple-to-use, web-based tools intended for use in first-pass assessments of the potential of a given locality for GSHP installation and/or for use in resource assessments. This paper presents the development of the open-loop GSHP screening tool for England and Wales, which maps hydrogeological and economic factors relevant for groundwater-based open-loop GSHP installations. Construction of thematic maps and data layers Data sourcesThe screening tool has been developed for England and Wales at a scale of 1:500000 and is freely available on the BGS website (http://www.bgs.ac.uk/research/energy/geothermal/ gshp.html). It is based on national datasets available from the collaborators in this study or sourced under an Open Government licence from Natural England and Natural Resources Wales. Some layers, such as the protected area map, were derived by combining existing maps and reattributing them to fit the purpose of this tool. The bedrock aquifer map and the underlying data layers have been specifically created as part of this project, based on the evaluation and mapping of aquifer productivity at the national scale. (The term 'bedrock' is used by BGS to refer to deposits of approximately Pliocene age and older. It includes unconsolidated sediments such as Palaeogene sands and the Crag, which is Pliocene to Pleistocene in age.) The data layers are briefly described below. A more detailed description of the tool and the underlying mapping method has been given by Abesser (2012). Simplifications and assumptionsThe tool was developed based on the following assumptions. Abstract: The UK Government expects that, by 2020, 12% of the UK's heat demand will come from renewable sources, and is providing incentives to help achieve this. Open-loop ground sour...
High quality seismic reflection data acquired during hydrocarbon exploration activities provide evidence for the subsurface structure and evolution of one of England's most well known structures at outcrop: the Isle of Wight Monocline. It is generally seen as a major northerly verging monoclinal structure linked to the Purbeck Monocline to the west. The Isle of Wight Monocline is the result of the interplay between two east-west trending, southerly dipping and overlapping down-to-the-south major syndepositional normal faults that were active during Triassic and Jurassic times: the Needles and Sandown faults. The area between the two faults tips forms an easterly-dipping relay ramp, down which sequences of all ages thicken. Both of these major normal faults were inverted during Cenozoic (Miocene: Alpine) compressional events, folding the overlying post-rift sequences of early Cretaceous to early Cenozoic (Palaeogene) age. Interpretation of the seismic reflection data suggest that a previously unknown high-angle, down-to-the-north reverse fault cuts the northern limb of both anticlines forming the composite monocline and was likely to come to crop in the steeply-dipping Chalk and/or the drift-covered Cenozoic sequences. Its identification marked a period of discussions and testing of the model by detailed field mapping. The existence and location of such a fault was proved through an iterative process with the result that a reverse fault zone is now mapped along the northern limb of the northern Sandown Anticline section of the Monocline. The main reverse faults on the Brighstone and Sandown anticlines result in circa 550 m of displacement at top Chalk level. It is thought that a series of smaller footwall short-cut faults affect the Cenozoic strata to the north of the main reverse fault, producing upfaulted sections of flatter-lying Cenozoic strata. Reverse displacements and the severity of folding on the inverted faults decreases on each fault segment in a complementary fashion in the area of the relay ramp as one fault takes up the movement at the expense of the other. The swing in strike of the Chalk in the area of shallowly dipping strata between Calbourne and Garstons is a result of deformation of the post-rift sequences across the relay ramp established between the overlapping fault tips of the Needles and Sandown faults and the interaction of the folds developed at the tips of the reverse faults.
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