The reservoir potential of the Upper Palaeozoic carbonates in the Barents Sea area is primarily controlled by early diagenetic processes. Upper Bashkirian to Asselian shallow platform carbonates deposited in warm, arid to semi-arid climates were dominated by aragonitic organisms and mineralogically unstable aragonite and high-Mg calcite cements and mud. A reservoir model for these carbonates involves extensive dolomitization and dissolution of metastable carbonate during repeated subaerial exposure. The reservoir model is confirmed by drilling and is accordingly regarded as low risk. Artinskian and Upper Permian shallow water carbonates deposited in a cold temperate climate were dominated by calcitic organisms and silica sponges, and associated with calcite cements and mud and chert. A reservoir model for these carbonates involves either preservation of primary porosity in carbonate build-ups or extensive dissolution of build-up marine cement during prolonged subaerial exposure. This model is not confirmed by drilling and is regarded as high risk.