During the initial development phase of the Mungo field in the central North Sea, wellbore instability and mud losses caused serious drilling problems. Geomechanical modeling around the salt diapir led to refinement of some drilling procedures but the direct cause of the drilling problems still remained poorly understood. A detailed fault-mapping study was carried out. Wellbore-instability and mud-loss zones were integrated with a 3D fault model. It was found that the intersection angle between the wellbore and the fault surface determines whether drilling through the fault will induce instability. The Mungo data indicated that small intersection angles (usually between 0 and 45 degrees) resulted in drilling problems while larger intersection angles (usually between 45 and 90 degrees) were not associated with drilling problems.