Geothermal reservoir quality of the Middle Jurassic Haldager Sand Formation in Denmark

Petrographic investigations combined with porosity and permeability measurements of sands and sandstones from the Haldager Sand Formation are used to estimate the reservoir quality and the risk of reaction when exposed to injected water. The rather shallow burial of the formation combined with an initially mature mineralogy have resulted in limited diagenetic reactions so the amount of authigenic minerals is small and much of the primary porosity is preserved. The type and amount of diagenetic alterations seem to be related to the depositional environment. The decrease in porosity and permeability with increasing burial depth is primarily an expression of the degree of mechanical compaction. Kaolinite is the dominant detrital phase and is not fatal for the reservoir quality due to its patchy distribution, and from a geothermal perspective it is beneficial that most of the degradable minerals such as feldspars, rock fragments and some heavy mineral types have already been altered prior to water injection. The reservoir quality is in general good and the high mineralogical maturity implies that the reactivity of the sandstones will be small during water injection so scaling in the geothermal plant during injection is less likely to occur.