Toward fracture porosity assessment by gravity forward modeling for geothermal exploration (Sankt Gallen, Switzerland)

• Stripping gravity effect of irrelevant structures allows fracture porosity assessment.
• Synthetic cases reveal that fault geometry must be well constrained for stripping.
• The validity of the approach was tested down to about 4500 m.
• Density variation suggests about 4–8% fracture porosity in the Sankt Gallen fault zone.
• High fracture porosity has been confirmed qualitatively by the first well GT-1.

Fracture porosity is a crucial parameter in hydrocarbon and geothermal reservoir exploration and a major challenge in the absence of nearby exploration wells. The Sankt Gallen geothermal project targets a fault zone that affects Mesozoic sediments at a depth of about 4500 m. Spatial extension of these sediments, a major fault zone and indication for graben structures in the crystalline basement are observed in 3D seismic. Both the graben and the fault zone coincide with negative gravity anomalies acquired and analyzed during this study. Forward modeling of gravity anomalies based on a 3D seismic survey is used to estimate possible fracture porosity. After stripping gravity effects of geothermally irrelevant geological units from the residual anomaly, most likely only local structures related to the fault zone account for remaining anomalies. Synthetic case study on the effect of density variation and considerable gas content in the well support possible fracture porosity between about 4% and 8%.