Permeability and reservoir temperature correlation in Upper Jurassic carbonates in southern Bavaria and implications for diagenesis and exploration

Upper Jurassic Carbonates (“Malm”) are the most widely used hydrothermal reservoir in Germany. Located in the North Alpine Foreland Basin, reservoir temperature gradients vary with values below 24 K/km and up to 43 K/km. The lowest values are present within a large negative anomaly around the city of Wasserburg in eastern Bavaria – so far, a largely underexplored region.

The diagenetic history of the reservoir is complex, with indications of syn-sedimentary emergence, Cretaceous burial, subsequent local exhumation, and rapid burial during the alpine orogeny (Tertiary) to depth of up to 5.000 m. Widespread dolomitization occurred in several phases. Fluid inclusions of authigenic carbonates revealed temperatures far exceeding today’s values.

Until now, a correlation between permeability and reservoir pressure, approximately related to depth, was used to predict flow rates of geothermal projects. However, this was not viable for the region of the large negative temperature anomaly. Here, the correlation of permeability with reservoir temperature yields a significantly better fit. The trend deduced can also be used in adjoining areas, although the difference to the pressure correlation is less obvious here.

The relation of reservoir permeability to present-day temperatures indicates the formation of authigenic minerals during much higher palaeo-temperatures did not affect the reservoir properties significantly. Therefore, the current temperature field is likely to have been stable since a long time. Due to the new correlation, a higher reservoir permeability than previously can be predicted for the currently underexplored region, where higher flowrates may compensate for the relatively low temperatures.