Abstract It has long been recognized that the faults in the Hassi Messaoud field are necessary for the successful production from the Cambrian sandstone reservoirs. What is less understood is the dual nature of these faults in that they can both control transmissibility through permeability enhancing fractures, and control trapping (sealing) through permeability reducing diagenesis. The key to optimally exploiting this or any fractured reservoir is the ability to predict the existence and properties of both. Without these faults and fractures, the Cambrian reservoirs would be classified as being tight with matrix permeabilities that are less than 40 mD, usually less than 3 mD, and porosities of between 3-8 %. In addition, the production history of the wells indicate variability in matrix versus fracture contribution, which suggests that the field was composed of more than one type of storativity-based, fractured reservoir. An integrated reservoir characterization study was conducted in Zones 1A and 1C to confirm the above premises. Reservoir quality and compartmentalization of the Cambro-Ordovician Ra and Ri reservoirs are believed to be related to stratigraphic, diagenetic, and structural variability. There is much evidence for reservoir heterogeneity especially around faults i.e. variation in reservoir thickness, rapid changes in porosity and permeability, variable production performance, variable water production. There is also evidence that fracture intensity varied from north to south and that this affected well performance. A number of wells were picked for detailed evaluation based on their production history and proximity to faults.