Geological Description of the Gullfaks Field

Tampen is considered one of the most oil-rich areas in the North Sea, with a number of significant discoveries on both the Norwegian and British continental shelves. Here, fault blocks lie in a row, many of them containing oil and gas.

Geological Development and Structures

The Gullfaks area has evolved over several hundred million years. Around 400 million years ago, Greenland (Laurentia) collided with the Baltic Shield. This led to the formation of the Caledonides, a massive mountain range comparable in scale to today’s Himalayas. After this collision, the rocks were compressed, deformed, and later worn down by erosion.

Fact: Laurentia is an ancient geological continent consisting of the craton, or geological core, of what is now North America and parts of Greenland. It is considered one of the oldest parts of the Earth’s crust, with some bedrock over 4 billion years old.

Fact: The Baltic Shield is one of the oldest and most stable parts of the Earth’s continental crust, comprising Norway, Sweden, Finland, parts of northwestern Russia, and the Baltic states.

Around 150 million years ago, the North Sea began to open due to stretching of the Earth’s crust. This created fault blocks and deep basins that were filled with sediments from land. Clay mixed with algae, plankton, and plants was deposited in deep, oxygen-poor marine environments. These sediments became, among other things, the Draupne Formation shale, or Kimmeridge Clay, one of the world’s most important petroleum source rocks. As the layers were buried deeper, temperature increased, and the organic material was transformed into oil and gas.

Oil and Gas Window

Oil forms at temperatures between 80–160°C, known as the oil window. Gas forms in the gas window at temperatures between 140–250°C. At temperatures above 250°C, all hydrocarbons break down into graphite, the mineral used in pencils.

Reservoirs and Rock Types

The Gullfaks reservoir covers an area of about 51 km² and lies within a fault block. The main reservoir consists of sandstone from the Brent Group (Middle Jurassic). Deeper down, oil is also found in the Cook, Statfjord, and Lunde formations. Above the main reservoir lie the Shetland Group and the Lista Formation, which contain oil in fractured chalk and shale layers.

The Gullfaks reservoirs consist mainly of sandstone with good porosity (20–34%) and high permeability (up to 4 darcies), providing excellent flow characteristics. The oil column – the “height” of oil in the reservoir from the top of the oil zone down to where oil gives way to water – is about 250 meters thick.

Production Challenges

Faults divide the reservoir into many small blocks, where pressure communication between them can be poor. This makes it difficult to manage oil production and predict how the oil will drain. Some of the deeper reservoirs, such as the Statfjord Formation in Gullfaks South, have poor flow properties and are strongly segmented – meaning the oil is not contained in one continuous “tank,” but divided into many smaller, separate compartments.

Sand production is also an issue, because the reservoir rocks are poorly cemented. Poorly cemented means the grains in the rock (for example, sand grains in a sandstone) are not well bound together by minerals acting as “glue” – typically quartz, calcite, or clay minerals. As a result, sand can be produced along with the oil, which wears down equipment and pipelines.

The satellite fields lie on separate fault blocks and have limited communication with the main field, further complicating oil recovery.

Technology and Strategies

To deal with the geological challenges, advanced technologies such as horizontal and extended-reach wells are used, along with special solutions to prevent sand production. Reservoir pressure is maintained mainly by water injection, but also through gas injection and WAG technology (water-alternating-gas injection) in selected areas.

4D seismic (time-lapse seismic) is used to monitor the reservoir over time. This makes it possible to adjust drilling and production strategies based on how the reservoir responds, which is especially important in segmented areas with varying pressures.

Future Opportunities

Above the main reservoir are tight chalk and shale reservoirs in the Shetland Group, which in some places contain oil but have very low recovery rates. Producing from these layers is technically challenging. In 2015, a revised plan for development and operation (PDO) was approved, aimed at recovering oil from these reservoirs using new technology. Significant challenges remain, particularly in improving drilling techniques and understanding how oil drains in these tight reservoirs.[REMOVE]Fotnote: NDLA. Anna Aabø. (29.06.2025) Reservoarformasjonenes porøsitet og permeabilitet https://ndla.no/subject:1:6951e039-c23e-483f-94bf-2194a1fb197d/topic:f731d600-98c4-4f56-a062-cf948cef34e4/resource:1:157664; Nordgulen, Ø., Solli, A., Nøttvedt, A., Bryhni, I., Ramberg, I. B., Norsk geologisk forening, & Norges geologiske undersøkelse. (2006). Landet blir til: Norges geologi. Norsk geologisk forening og Kristin Alne, Rune Jerstad, Håvard Larsen, Kjersti Bysting, Ole Martin Holmefjord. (2006. 9. mai). Bruk av multilaterale brønner på Gullfaks Sør Statfjord. Norges Teknisk-Naturvitenskapelige Universitet Institutt for Petroleumsteknologi og Anvendt Geofysikk

Timeline of Geological Development