Abstract This study contributes to geothermal exploration in 1660–1520 Ma old, reworked bedrock in Sweden. Our primary objectives are to constrain the orientation of horizontal stresses, and to discuss implications for geothermal exploration. High-resolution acoustic televiewer image data reveal the downhole distribution of stress indicators (borehole breakouts, drilling-induced fractures and petal centreline fractures) and pre-existing structures (natural fractures, foliation). About 135 m of stress indicators are measured from 0.2–1.0 km. The results suggest a uniform NNW–SSE mean maximum horizontal stress orientation. A total of 1525 pre-existing structures (natural fractures, foliation) are mapped in borehole GE-1. The prevailing stress regime controls whether natural fractures and foliation are well-oriented for stimulation. For strike-slip and normal faulting stress regimes, well-oriented fractures steeply dip towards the WSW. For a reverse faulting stress regime, shallow dipping fractures are well-oriented for stimulation. The downhole distribution of stress indicators and other stress measurements in the region and other parts of Fennoscandia tentatively suggest a strike-slip stress regime, but additional studies are needed to constrain the complete stress field at study depth and towards engineered geothermal systems reservoir target depths. Our secondary objective is to highlight that interpretation of high-resolution acoustic data, particularly in metamorphic crystalline rocks, is subjective and that more guidelines for data interpretation are needed. The interactive interpretation of the images is based on visual analyses of complex pre-existing structures and stress indicators with highly variable shapes. The application of three methods for data analyses in the GE-1 borehole proposes that drilling-induced fractures are little influenced by the method applied. Interpretations on individual borehole breakout azimuths may, however, result in over 10° differences in orientation.

Abstract During the current trend of the energy transition, many stratigraphic intervals that previously were thought as uneconomical from an oil and gas business perspective are being re-evaluated for low-carbon energy resource potential such as geothermal. This study presents a case study from the Vienna Basin, Austria, where the Early Badenian Rothneusiedl Formation has been targeted for a re-evaluation, utilizing existing sparse subsurface datasets of vintage quality. The historical cores were subjected to diverse analytical techniques covering sedimentological, petrophysical and thermophysical aspects. These analysis techniques provided the fundamental lithological characteristics and led to the creation of a synthetic conceptual sedimentological log, resultant selection of samples for further analyses, as well as the creation of gross depositional environment maps. The reservoir and thermal properties guided by the core-based lithofacies were required for evaluating the feasibility of the target strata. Overall, the limited and historical cores provided significant and robust data that proved crucial for assessing uncertainties and identifying sub-areas of the Vienna Basin for harnessing geothermal energy. This study demonstrates an important case example of the utilization of historical cores to their maximum potential and reiterates the value of core-derived data amongst digital technologies of the twenty-first century.