A geological model for the Ormen Lange hydrocarbon reservoir

The Paleocene Egga Reservoir Unit of the Ormen Lange Field was deposited by high-density turbidity currents in a N-S elongated, structurally controlled sub-basin. A weak saddle area, separating two depressions, characterized the sea floor topography of this Ormen Lange Basin. The basin probably narrowed considerably towards the North, and the basin floor was tilted slightly towards the East. Deposition was confined by topography, preferentially preserving the coarsest grained deposits of the most powerful suspension currents. Basin topography was continuously or sporadically rejuvenated by differential subsidence along propagating polygonal faults due to the differential compaction of underlying Cretaceous shales. A high quality reservoir was then generated all the way up to the northern field boundary. Fault planes are frequent and are characterized by strongly varying throws. They generally juxtapose sand-rich reservoirs in the upper part and more clay-rich rocks towards the base. Combined with their early origin these fault characteristics may explain the observed static communication within the gas zone and pressure compartmentalization of the water zone. As the Egga reservoir unit is over- and underlain by scaling shales over large parts of the structure, it locally retains basal "water pockets" up to move than 100 m above the gas-water/oil-water contact.