Estimation of the highest potential transmissivity of discrete shear fractures using the ductility index

Previous studies indicated that the highest potential transmissivities of fractures in fault zones might be well estimated by a mechanical indicator: the ductility index (DI). The DI is defined as the effective mean stress normalized to the tensile strength of the intact rock mass. The mechanism of formation and preservation of pore structures in fault-zone fractures can be explained by local shear-induced tensile stresses, and pore structures in discrete shear fractures may form in the same way. I investigated, therefore, whether the DI model can correctly predict the highest transmissivities of shear fractures recorded in fractured Neogene diatomaceous mudstone (Koetoi Formation), where fault zones and joints are rare but discrete shear fractures, without mineral fillings, are abundant. Analyses of the shear fractures reveal that the detected states of paleostress are nearly equal to the current stress state, implying the shear fractures have potentially been tectonically active. Thus, the highest transmissivities detected at fractures by flowing-fluids electric conductivity (FFEC) borehole logging may potentially be treated as the highest potential transmissivities in terms of the given DIs. Relationships between the transmissivities of flow anomalies and DIs fit the existing DI model well, which suggests the model is applicable to discrete shear fracture systems as well as fault zones.