Fracture properties, structural heterogeneity, and permeability in the Þeistareykir geothermal system, NE Iceland

• Fractures are characterised in terms of their occurrence, orientation and relative distribution. • Fracture strikes range from NNW-SSE, N-S, NNE-SSW to NE-SW, with low to high dips. • Natural fracture characteristics vary within different lithologies and depth. • Structural nature of the permeable fluid-flow channels is inferred in the Þeistareykir geothermal system. Permeability in the Þeistareykir geothermal system of Iceland is structurally controlled. Natural fracture networks are abundant in Þeistareykir and contribute significantly to fluid flow. Understanding which features enhance permeability and hydraulic conductivity, and how their properties interact with lithology and reservoir structure, is key to predicting reservoir behaviour. To address this, we utilise a range of borehole data to characterise natural fractures in terms of their occurrence, orientation, relative distribution, their relationship with the major lithological units and permeable flow zones in the subsurface. Results show systematic variations in fracture density, thickness, and distribution pattern across different lithologies and depths, with orientations ranging from NNW-SSE, N-S, NNE-SSW to NE-SW. Fractures exhibit the highest intensity in the deeper acidic intrusive units or coarser grained basalt with a predominant N-S-trend and bimodal dip distribution. However, permeability is controlled by a complex interplay of fracture geometry, openness and connectivity rather than simply high fracture abundance or a preferential set of fractures. Permeable feed zones show diverse structural expressions, ranging from high-density fracture clusters and large-aperture fractures to intensely fractured damage zones and multiple intersecting fracture sets. These findings demonstrate that the structural character of the potential fluid-flow channels is highly variable in Þeistareykir. The results of this study can be incorporated into fracture and flow models to enhance our understanding of the permeability distribution and fluid pathways in the Þeistareykir geothermal system.