A fracture rarely comes alone: associations of fractures and stylolites in analogue outcrops improve borehole image interpretations of fractured carbonate geothermal reservoirs

Abstract

In this study, we present a method that uses associations of discontinuity sets to demonstrate similarities between the outcrop and the subsurface. A discontinuity association comprises up to four discontinuity sets (fractures and stylolites) that can form coeval in a single stress field, a wellknown concept that is rarely applied for subsurface characterization of discontinuities. We use this concept to improve the interpretation of borehole image logs of naturally fractured geothermal reservoirs in the Geneva Basin, Switzerland. Here, the naturally fractured Lower Cretaceous pre-foredeep carbonate rocks are targeted for geothermal exploitation, and exposures of this formation are found in three mountain ranges that surround the basin. In these outcrops, the orientations of the discontinuity associations are used as paleostress indicators in order to map out principal stress trajectories of regional discontinuity-forming events that created the background discontinuity network. We document two multiscale discontinuity-forming events that formed prior to Alpine fold-and-thrusting and thus constitute the regionalscale background network. Given the regional character of these events, we predict that the target reservoir is impacted by them as well. This prediction is subsequently used to isolate the background-related discontinuities on image logs from two boreholes that penetrate the target reservoir in the Geneva Basin. This analysis reveals that ∼ 45 % of the observed discontinuities can be understood in the framework of the regional-scale background. In this way, we demonstrate that defining discontinuity associations in outcrops is a powerful tool to predict the geometry of natural discontinuity networks in the subsurface and subsequently can be used to develop geothermal exploitation strategies in naturally fractured reservoirs.