Outcrop studies of fractured rocks are commonly made in order to use the results for the three-dimensional modeling of fractured reservoirs that are thought to be analogous in nature. A crucial yet rarely addressed question is to what extent the fracture system observed on outcrops is representative of the deeper subsurface. In order to investigate this, we studied fracturing both at the surface as well as in boreholes drilled through the same rocks. For the characterization at the surface we used multi-scale imaging that includes aerial photography and digital photography of outcrops with a GIS-linked positioning system. For the study in the boreholes we used electrical, optical and ultrasonic borehole images as well as continuous cores.
Our study area is located in the Tanqua-Karoo basin, where excellent outcrops offer three-dimensional views of four siliclastic basin floor fans and their intercalated shales. The entire sequence is over 300 meters thick and covers an area of about 800 km2. Boreholes drilled on the plateau behind the outcrops traversed most of the fans to a maximum depth of 270 meters. Extensive fracturing can be observed on the outcrops as well as on the cores and borehole image logs, although to various degrees due to the different nature of the techniques. Thus, for example, cemented fractures are well visible on optical borehole images while open fractures are more easily seen on ultrasonic borehole images. The electrical borehole images proved to be superior because it was equally sensitive to both types of fractures.
Our analysis showed that a prominent NNW-SSE striking fracture system is present at the surfaces as well as in the subsurface. We attribute this system to a strike-slip movement during a N-S compression associated with the Cape Fold Belt. Equally associated with this tectonic phase is a system of E-W striking faults and associated fracturing patterns, which we also observed both at the outcrops as well as in the subsurface. Interestingly, however, we observed that outcrops show an increase in fracture density in thin-bedded sandstones, whereas the borehole image logs indicate that fractures are present mainly in the massive sandstone layers.