The Cambrian to Lower Silurian succession in NW Europe is dominated by organic rich black shales. Shale gas exploration in the area has focused on the Middle Cambrian to Lower Ordovician Alum Shale, Upper Ordovician (Katian) Dicellograptus/Fjäcka shale and the Lower Silurian (Llandovery) Rastrites shales (Figure 1). Licences have been acquired in Denmark, Sweden and Poland. Thermal maturity of the Lower Palaeozoic shales in NW Europe has traditionally been assessed based on optical properties of vitrinite-like particles in the Alum Shale that supposedly behave in a similar manner as suppressed vitrinite in younger shales with respect to maturation. This study presents a multi-proxy thermal study on a series of naturally matured shale and carbonate samples. It is intended to provide a full optical description of the maceral types present in the shales and to calibrate measured reflectances to other commonly used proxies such as the conodont colour index (CAI), the thermal alteration index of the organic microfossils (TAI) and geochemical changes in the kerogen.