Characterising brines in deep Mesozoic sandstone reservoirs, Denmark


Hanne D. Holmslykke*1, Niels H. Schovsbo1, Lars Kristensen1, Rikke Weibel1 & Lars Henrik Nielsen1


GEUS Bulletin Vol 43 | e2019430104 | Published online: 17 July 2019

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The Danish subsurface contains several sandstone units, which represent a large geothermal resource (Vosgerau et al. 2016). Currently, only three geothermal plants are operating in Denmark, but several exploration licences are expected to be awarded in 2019. Geothermal energy is exploited from deeply buried porous sandstones by bringing warm form­ation water (brine) to the surface, extracting the heat and returning the cooled water to the same sandstones. The reduced temperature of the brine during this process implies a risk of scaling, which may reduce reservoir permeability and hence injectivity. Predicting the chemical composition of formation waters, however, could help to reduce the risk associated with scaling in planned geothermal facilities.

Here, we present a regional overview of the geochem­istry of brines from deep Mesozoic sandstones in the Danish Basin and North German Basin that supplements previous studies, notably by Laier (2002, 2008). The brine composition at shallow burial typically reflects the original (connate) formation water chemistry, which is determined by the original depositional environment of the sandstone, for example fluvial or marine. However, the mineralogical composition of the sandstone changes during burial, whereby some minerals may dissolve or precipitate when exposed to higher temperatures. These mineral changes are reflected in the brine composition, which typically becomes more saline with increased burial (e.g. Laier 2008; Kharaka & Hanor 2003). 

The brine chemistry reported here shows a distinct depth trend, which reflects original connate formation waters that are modified through burial diagenesis. We have classified the brines into brine types, which are shown to be related to their depositional environment, depth, geological formation and geographical domains. 


*Corresponding Author: Hanne D. Holmslykke | Email:

1 Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350, Copenhagen K, Denmark


This article is published in the Geological Survey of Denmark and Greenland Bulletin – an international, peer-reviewed and open access journal published by the Geological Survey of Denmark and Greenland (GEUS).


Holmslykke, H.D., Schovsbo, N.H., Kristensen, L., Weibel, R. & Nielsen, L.H. 2019: Characterising brines in deep Mesozoic sandstone reservoirs, Denmark. Geological Survey of Denmark and Greenland Bulletin 43, e2019430104.


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Date received: 20/02/2019

Date accepted: 15/05/2019

First published online: 17/07/2019


Denmark . Sandstone . Mesozoic . Geothermal energy . Brine chemistry

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