This work presents a proof of concept for the application of smouldering combustion to valorize Uruguayan oil shales as a solid fuel resource. Due to their low calorific value oil shales mixed with biomass waste were tested under controlled conditions to assess their suitability for energy recovery. Experiments were conducted in a fixed-bed reactor to investigate the influence of air Darcy flux and particle size on the smouldering gas composition and its calorific value. Results show that a minimum air Darcy flux of 3 cm/s is required for self-sustained smouldering when using oil shales particles with 1.2–1.6 mm in size. The addition of 50 wt.% of biomass waste, as a supplementary fuel, increased the gas calorific value from 3.0 to 4.4 MJ/Nm3. The net energy balance analysis showed a shift from negative to positive net energy gain with increasing biomass, reaching up to 25% relative to the energy required for aeration in the scale tested. Analysis by X-ray diffraction and fluorescence of the residual ash identified crystalline phases and metals, indicating potential for valorisation due to presence of Fe, quartz, muscovite and zeolites.