Subjective and objective bias in borehole image fracture interpretations

Mitigating bias in geological interpretations in the domain of structural geology and more specifically in seismic images and outcrop data is a well-known challenge. So far, the same issue in borehole image interpretation has rarely been addressed, even though these data are crucial for our knowledge of the subsurface. The Lower Cretaceous carbonate rocks of the Geneva Basin, Switzerland is a naturally fractured geothermal reservoir which has been penetrated by the GEo-01 well. Borehole images of this well were interpreted independently by five interpreters offering a unique database to compare and contrast. In this study we analyzed the impact of lithology and borehole image quality/complexity on the frequency and orientation of fracture picks acquired by the five interpreters. For the fracture frequency, we demonstrate that in GEo-01, the lithology sets the baseline for the level of agreement between the different interpreters, while image quality/complexity plays only a moderate role. On the other hand, the variability between interpretations of the fracture orientation appears mainly to be affected by the dip of strata, where a trend is observed between intervals with low bedding dip angles and more consistent feature orientation picking. As a recommendation for future exploration in the Geneva Basin and beyond, we suggest guidelines based on the geological context of the well in order to increase the value of borehole image interpretations.