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The permeability of a sand shale mixture is analyzed as a function of shale fraction and the permeability of the two end‐members, i.e., the permeability of a clay‐free sand and the permeability of a pure shale. First, we develop a model for the permeability of a clay‐free sand as a function of the grain diameter, the porosity, and the electrical cementation exponent. We show that the Kozeny‐Carman‐type relation can be improved by using electrical parameters which separate pore throat from total porosity and effective from total hydraulic radius. The permeability of a pure shale is derived in a similar way but is strongly dependent on clay mineralogy. For the same porosity, there are 5 orders of magnitude of difference between the permeability of pure kaolinite and the permeability of pure smectite. The separate end‐members' permeability models are combined by filling the sand pores progressively with shale and then …