Mapping High Dis-Equilibrium Oil Saturations in the Chalk of the South Arne Field, Danish North Sea

Free water level (FWL) dips in excess of 200 m/km are encountered in the South Arne Field in spite of relatively normal mobility of the oil in main parts of this Upper Cretaceous - Paleocene chalk field. These dips are convincingly explained by filling simulation studies to be caused by variation in water phase mobility in combination with relatively late charging pulses. The low water mobility is controlled by the presence of a bottom seal Externally imposed pressure gradients in the water phase only plays a quite insignificant role. Pressure gradients in the oil phase due to oil migration into the structure are generally of minor importance. However, on the south-east flank natural imbibition is of some importance. In that part of the field, high FWL dips can also be sustained by reduced oil mobility. The dynamic nature of the field has imposed partial states of imbibition prior to production start. By accounting for the degree of imbibition, spatial variations in oil mobility and irreducible oil saturation can be predicted.