Reservoir prognosis of the Gassum Formation and the Karlebo Member within two areas of interest in northern Copenhagen. The EUDP project "Geothermal pilot well, phase 1b"

The present study provides a prognosis of the geothermal potential for the Gassum Formation and the Karlebo Member in two areas of interest within northern Copenhagen. The two areas were selected as being relevant for geothermal heat exploration in Phase 1a of the project based on infrastructure considerations (e.g. positions of existing district heating plants, current and future district heat networks and coupling points) as well as the composition of the subsurface in the larger Copenhagen area.

The Gassum Formation and the Karlebo Member were treated as a composite unit in Phase 1a of the project as the boundary between them is difficult to define in existing seismic data. However, the recognition of a boundary between the two units has been possible in the present phase of the project, based on a detailed and comprehensive interpretation of the seismic lines closest to the two areas of interest, guided by a sequence stratigraphic interpretation of the composite succession. As a consequence of this, more accurate estimates of depth, thickness and reservoir properties are given for both the Gassum Formation and the Karlebo Member in the present report. The Karlebo Member consists of sandstone beds that are petrographically very similar to those in the Gassum Formation, but are in general separated by thicker mudstone intervals than the sandstones are in the Gassum Formation. The Karlebo Member forms the lower part of the Fjerritslev Formation in northern and eastern Zealand. The remaining upper part of the Fjerritslev Formation in Zealand, as well as the entire Fjerritslev Formation in Jutland, consists almost entirely of mudstones and claystones. Petrophysical log data from deep wells shows that the Gassum Formation in general contains the largest amount of reservoir sandstones and is thus considered to be the main target for geothermal recovery in the two areas of interest.

A secondary scope of WP2 is to evaluate if the clay mineralogy in mudstone intervals of the Gassum Formation, the Karlebo Member, and especially in the thick mudstone-dominated part of the Fjerritslev Formation (i.e. the overburden of the potential sandstone reservoirs) may cause a particular drilling risk, e.g. due to a high content of reactive clay minerals. Finally, the geological data form input for considerations about development of drilling equipment, selection of well completion techniques and logging tools and for financial assessment of the profitability of incorporating a geothermal plant in the district heating system; all subjects which are reported in other work packages and/or are to be dealt with in Phase 2 of the project.