Identification of fracture zones in geothermal reservoirs in sedimentary basins: A radionuclide-based approach

•Spectrometric measurements of drill cuttings reveals 226Ra-accumulation and thus, a disruption of the secular equilibrium in the uranium series.•Radium removal is most likely controlled by co-precipitation in sulphate minerals such as barite.•Ra/K and Th/Ra activity ratios in rocks indicates fractured zones in the geothermal reservoir. A methodology for fracture zone identification in geothermal reservoirs was tested at the Bruchsal geothermal site, located close to the eastern main boundary fault of the Upper Rhine Graben. The site is explored and developed by a well doublet (GB1 and GB2). Fracture zones provide substantial contribution to the effective hydraulic conductivity and are the focus of geothermal fluid circulation. In this context, water-rock alteration processes and the resulting changes in the composition of fluid and rock provide information on the type and the degree of fluid flow in the region of the fractured material. Thus, drill cuttings of the production well GB2 were analyzed with respect to their radionuclide signatures allowing conclusions on mechanisms and time-scale of the geochemical processes. The reservoir section at Bruchsal is defined by a radium anomaly (especially 226Ra) in deeper borehole sections corresponding to the Lower Buntsandstein and Permian Zechstein. The Bruchsal fluid, as well as other deep geothermal fluids in the Upper Rhine Valley, are characterized by radioactive disequilibria of radium isotopes (226Ra, 228Ra, 224Ra) whose concentrations greatly exceed those of their thorium progenitors. It appears that radium is adsorbed onto hydrothermally altered minerals such as clay minerals and Fe-hydroxides or is co-precipitating with sulphate minerals such as barite. This leads to an average 228Ra/226Ra signature of 0.54 in the drill cuttings, in contrast to an expected value of 1.2 based on the average crustal Th/U ratio of 3.8. For the Lower Buntsandstein and the Zechstein group, 234U/238U and 230Th/234U ratios are equal to unity, while 226Ra/230Th ratios range between 1.7 and 3.3. Radioactive disequilibrium was not observed for the thorium decay series. Using depth-related Th/Ra and Ra/K ratios, five fractured zones were identified within the Buntsandstein / Zechstein rock series, four of which were confirmed by conventional geophysical borehole logs. However, some issues remain unresolved such as the differentiation between active flow pathways and tight fractures, following mineral precipitation processes.