The geometry of a geothermal reservoir constrains the exploitable resource volume and the sustainable power capacity. That geometry is typically illustrated by a conceptual model and is primarily defined using a pattern of natural state isotherms. Prior to drilling, surface exploration methods set out to constrain the reservoir geometry even though the shape is highly uncertain. Subsequently, it is useful to develop a range of conceptual models that could fit the available data and to use well-defined reservoirs as analogues. The uncertainty associated with the reservoir geometry will, however, remain significant until it is extensively drilled, so considering alternative geometries is an aspect of ongoing resource capacity assessment and makeup well targeting. We present initial results of research that uses case histories to define the geometry of reservoir permeability in relationship to the geologic architecture. The results are presented as a continuum that may be used as a practical tool for developing alternative conceptual models or selecting of analogue reservoirs. Development of this continuum has highlighted causal relationships between the geologic architecture and reservoir permeability, and we discuss two examples in detail: structures (faults & fractures) and intrusive rocks. Establishing these causal relationships is expected to yield a reduction in the uncertainty associated with estimating resource volume to the degree that the geologic architecture itself is predictable.