November 12, 2019July 24, 2020 by Kristin Øye Gjerde

Geology

Norwegian Petroleum Museum

The Frigg area lies roughly in the centre of what is today the North Sea. It is located in the Viking graben, a north-south system of underwater rift valleys formed by movements in the Earth's crust. About 50 million years ago, in the early Eocene, large quantities of sand were deposited in this area.

— Geological department of Elf. Photo: Elf Aquitaine Norge A/S//Norwegian Petroleum Museum

This sediment was carried by big river systems which snaked eastwards over what is now known as the Shetland platform. On reaching the coast, the rivers built up huge deltas which later broke down partially to create big underwater fans of sand.

Compacted to sandstone, these deposits later served as reservoirs for the Frigg oil and gas. Impermeable layers of shale and clay covered the sandstone formations to seal in the hydrocarbons. The source of the oil and gas can be found in deeper layers of sediment, a dark shale deposited in the Lower and Middle Jurassic about 160 million years ago.

A special feature of the Frigg area is that oil and gas have migrated through fractures in the rocks, creating gas “chimneys” which can be detected on seismic maps of the area.