Fracture characteristics of Lower Carboniferous carbonates in northern Belgium based on FMI log analyses

Abstract

Recently drilled geothermal boreholes in Mol, northern Belgium, provide new information on the Lower Carboniferous carbonates in the Campine–Brabant Basin. Because of low primary porosity, fractures in these limestones and dolostones are of major importance for reservoir permeability. The Fullbore Formation MicroImager (FMI) log of the MOL-GT-01 borehole enabled interpretation of bed boundaries and fractures in the subsurface. Relationships between the frequency of these fractures and bed thickness, lithology variations and the presence of faults were explored. The results show that thick beds contain relatively few fractures and thin beds relatively many. Except for lower values in shaly intervals, the fracture frequency (number per metre) is largely independent of lithology. Zones with substantial changes in the structural dip (called a cusp) and/or azimuth of bed boundaries were identified. The clearest cusp is present at a depth of 3284 m. Since the presence of a normal fault is most likely regarding the local and regional geology, this cusp likely resembles a fault-tip fold of a WSW-ward dipping normal fault with an inclination of at least 54°. It is uncertain whether the borehole crossed the fault itself or only a monocline on top of it. Fracture frequency is increased in the vicinity of the interpreted possible faults. Up to a vertical distance of c.45 m from the faults, the mean fracture frequency is higher than in a non-faulted zone with similar lithology. However, frequency differences between these faulted and non-faulted zones are mostly insignificant, so no clear damage zones are present.

Controls of dolomitization and bed thickness on fracture networks in Lower Carboniferous carbonates in southern Belgium

“…These results are similar to the subsurface Lower Carboniferous carbonates in Mol, northern Belgium. FMI log analyses of the MOL-GT-01 and -03 have shown that no clear correlation is present 27 between bed thickness and fracture frequency in these boreholes (Van der Voet et al, 2020a, 2020b.…”

“…Fracture characteristics of these carbonates were studied based on borehole cores (Van der Voet et al, 2020c) as well as from image logs ( Van der Voet et al, 2020b).…”