A folded layer of impermeable shale has created a trap for oil and gas migrating upwards from even deeper shales. The sandstones in which the hydrocarbons have accumulated were probably deposited in river deltas. The shape of the field meant that it was originally called Papillon, or the Chinese butterfly.
Because it comprises sandstones covered with a domed cap of shale, the Frigg formation differs from the chalk reservoir on Ekofisk. Statfjord, on the other hand, is also a sandstone reservoir but has been sealed by faulting. So this field, Frigg and Ekofisk represent three different reservoir types.
Frigg has very good reservoir properties. The sandstone is clean with a good porosity of 25-32 per cent and high permeability of 0.9 to 3D.