Chloride waters of Great Britain revisited: from subsea formation waters to onshore geothermal fluids

Paul L Younger,Adrian J Boyce,Andrew J Waring

doi:10.1016/j.pgeola.2015.04.001

Abstract

It has long been known that chloride-dominated saline ground waters occur at depth in the UK, not only beneath the sea but also onshore at depths of a few hundred metres. In a few places in northern England, these saline waters discharge naturally at surface in the form of springs. In recent years, however, these saline ground waters have come to be regarded as resources: as potential geothermal fluids intercepted in deep boreholes. Comparisons of the major ions and stable isotopes (δ2H, δ18O and δ34S) of these saline ground waters with North Sea oilfield formation waters, and with brines encountered in former subsea workings of coastal collieries, reveal that they are quite distinct from those found in North Sea oilfields, in that their as δ2H/δ18O signatures are distinctly “meteoric”. δ34S data preclude a significant input from evaporite dissolution – another contrast with many North Sea brines and some colliery waters. Yet, enigmatically, their total dissolved solids contents are far higher than typical meteoric waters. It is tentatively suggested that these paradoxical hydrogeochemical properties might be explained by recharge during Cenozoic uplift episodes, with high concentrations of solutes being derived by a combination of high-temperature rock–water interaction in the radiothermal granites and/or ‘freeze out’ from overlying permafrost that surely formed in this region during cold periods. Geothermometric calculations suggest these saline waters may well be representative of potentially valuable geothermal reservoirs.

Highlights

Saline ground waters have long been an object of fascination in the UK
This paper reports new analyses for saline waters obtained from boreholes in Gateshead and Newcastle, as well as new isotope data for the saline waters accessed by the Eastgate No 1 geothermal borehole in Weardale (County Durham)
In the last few decades it has become increasingly apparent that these saline waters are widespread in the region, often occurring at depths of just a few hundred metres, and still giving rise to surface springs in rural Cumbria where they have been largely unaffected by the hydrogeological disruption caused by large-scale mining (Manesty being the exception to this)

Summary

Introduction

Saline ground waters have long been an object of fascination in the UK. Some saline springs were venerated in antiquity (Robins and Smedley, 2013), but they only entered the era of scientific research in the mid-19th Century when issues arose with such waters in coal mines (e.g. Armstrong, 1856; Dunn, 1877; Clowes, 1889). Armstrong, 1856; Dunn, 1877; Clowes, 1889). Clowes, 1889). There were insufficient data available at the time to support a comparative assessment of the affinities of the different saline ground waters, and isotope hydrogeochemistry had not been pioneered. All of this began to change as deep mining proceeded beneath the North Sea, encountering prolific brines, and as the North Sea oil and gas fields were opened up

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Discussion

Conclusion