Fracture and Fault Analyses Based on Formation MicroImager Logs in the Lower Carboniferous of Two Geothermal Boreholes in Northern Belgium

Abstract

Since the primary porosity of the Lower Carboniferous carbonates in the Campine-Brabant Basin is generally very low, fractures present in these rocks are of major importance for the permeability of this geothermal reservoir. A Formation MicroImager (FMI) geophysical well log is available from the first and third geothermal MOL-GT borehole drilled in Mol, northern Belgium. The FMI logs were used to identify different features based on resistivity contrasts, among which were bed boundaries and conductive fractures. These quantitative results enabled analysis of which factors control fracture characteristics and in which way. In one of the boreholes, thicker beds show larger mean fracture frequencies than thinner beds, while no such trend was found in the other borehole. Besides fractures, also possible fault intersections were interpreted from the FMI log of MOL-GT-03, based on changes in bedding dip and azimuth.