A comparison of structural core analyses from the Gullfaks and Gullfaks Sor fields, northern North Sea, shows that there are important differences between the two fields that have serious consequences for field development plans. Although the general deformation characteristics are the same, the temperatures within the reservoirs on Gullfaks Sor have exceeded the critical temperatures for accelerated quartz dissolution and precipitation. This has led to reduced porosity and permeability within the reservoir and the many shear bands found within damage zones around the many larger scale faults. This has a larger effect on two-phase flow properties than accounted for in standard reservoir models and has a detrimental effect on the production from compartmentalized high-temperature reservoirs. Prior to the detailed core analyses carried out on Gullfaks Sor, it was assumed that faults in Gullfaks Sor and Gullfaks had similar effects on hydrocarbon flow. However, the important difference became obvious after the drilling of two producers within the Statfjord Formation reservoir in Gullfaks Sor. Due to rapid drop in pressure, the actual production rates are less than 15% of those planned. The drop in pressure likely is caused mainly by quartz cementation of the shear bands, which transforms the bands into low-porosity microporous rock in which the oil capillary entry pressure becomes so high that the oil phase is discontinuous in the field. Hence, during production, oil does not flow easily, resulting in a rapid drop in production.