3D geometric modelling is a powerful tool to better understand geology. It allows to
check and validate the consistency of the separate 1D or 2D data interpretations.
Building a 3D model is also a way to share and communicate a geological view.
Furthermore, a consistent 3D geometric model is essential for post-process
computations that need an accurate and coherent geometry of geological bodies.

By constructing a 3D geological model, geologists in charge of defining and exploring
a geothermal reservoir can test different geological hypotheses using the data they
have collected. Then the resulting 3D geometrical model can be used to calculate
geological volumes, heat flow and related computations depending on the geometry of
the reservoir.

An original methodology has been developed in BRGM (French Geological Survey) to
interpolate at the same time geological contacts locations and dips of the
formations. The model is calculated by co-kriging these 2 types of data to obtain a
3D potential field. A geological pile allows automatic computation of intersections
and volume reconstruction using the geological history of the area and the
relationship between geological units. The 3DGeoModeller software has been developed
for geologists to apply this methodology to their data.

The 3DGeoModeller was used in the Clermont-Ferrand basin, a part of the Limagne
graben system, where a new hospital is planed to be built. The Limagne area is
characterized by a geothermal anomaly with clastic reservoirs showing 100°C at 1.5km
depth. A 35km x 30km x 3km 3D model was set in order to study the feasibility of
heating the new building by geothermal power. Field, drill holes and seismic data
collected over the last 25 years were input in the software. The inconsistencies in
their respective interpretations were checked and turned into a coherent 3D
interpretation of the whole area. The 3D model was then used to compute volume of the
geological formations of the reservoir. The next step will be to calculate the heat
flow by meshing the geometry of the 3D model.

This work is supported by ADEME (French Agency for Energy and Environment).

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