The well that drilled a prospect sealed by a thrust faultHavelange drill site. Photo: V. Stenmans.

The well that drilled a prospect sealed by a thrust fault

Belgium’s deepest borehole that was originally drilled as a gas exploration well is now being studied for geothermal energy potential.

The deepest borehole ever drilled in Belgium took five years to complete, from 1980 to 1985. Operated by the Geological Survey of Belgium, its aim was to test gas prospectivity below a thrust fault.

This is the first edition of a series of articles dedicated to the deepest boreholes drilled in the countries around the North Sea.

The location of the Havelange borehole was selected based on the appearance of a domal structure below a discontinuity surface mapped on seismic data. In other words, a potential closure. The discontinuity was part of a gradually southward dipping reflection that was interpreted as the Midi Thrust, the Variscan thrust fault that crops out along an east-west line in the north of Belgium (see diagram below). The strata above the Midi Thrust fault are part of the so-called Dinant syncline, mainly consisting of folded Lower Carboniferous and Devonian rocks. The Dinant syncline developed during the Carboniferous as part of the Variscan foredeep that formed as a consequence of Gondwana colliding with Laurussia in the north.

Diagram illustrating the position of the Havelange borehole in the setting of the foreland basin of the “Ardenne Allochton” in the north of Belgium. The Midi-Aachen thrust fault outcropping in the north forms the detachment drilled by Havelange at a depth of around 5 km. Source diagram.

At the time, geologists hoped to find relatively undisturbed and gas-prone Carboniferous rocks below the thrust fault. This may have been triggered by the success of gas exploration in the Southern North Sea further north, where the Carboniferous was not only known to be the main source rock for Rotliegend gas fields but also formed a reservoir in places.

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However, after the borehole had reached its final depth at 5648 m in 1985, the only stratigraphic interval that was drilled was a series of faulted Upper to Lower Devonian rocks (see diagram below). Two intervals of gas influx were noted; one around 4800 m and one at an approximate depth of 5039 m. Tests however concluded that the formations had very low permeability because a gradual lowering of the mud weight did not result in an increased influx of gas.

Strata penetrated by the Havelange well. The Midi Thrust fault (F. du Midi) is indicated near the bottom of the well, and the domal structure that was mapped on seismic in advance of drilling can also be seen. Source diagram.

Even though Carboniferous rocks were not found, it is still believed that the well did penetrate the Midi Thrust fault.

Geothermal potential

The Havelange borehole has now been “re-opened” to study its potential for geothermal heat extraction. “We restudy this borehole since it is a rare example of well-documented well (logs, cuttings, cores, seismic) in low-grade metamorphic rocks. The current target is to better understand the geothermal potential of fractured Lower Devonian quartzite units, as temperatures of up to 126°C were recorded during drilling,” as Yves van Brabant, geologist at the Geological Survey of Belgium explained in an email.

More information about the Havelange test site can be found here.