Petroleum geology and exploration on the Norwegian shelf

How oil and gas form

Oil and gas are the remains of organic material, mostly microscopic phytoplankton, that sank to the seabed and were buried under new layers. Where the bottom water had little oxygen, the remains were not fully broken down, and kerogen formed: organic matter locked in the rock. A rock rich in kerogen is called a source rock, and it is usually a dark shale or mudstone.

The deeper the layers are buried, the warmer they get. On the Norwegian shelf the temperature rises by about 25 °C per kilometre. When the kerogen reaches 60–120 °C oil forms, and at higher temperatures mainly gas (norskpetroleum.no, read 5 Oct 2026). The process is called maturation and takes millions of years.

Oil and gas are lighter than water. They are expelled from the source rock and move upwards through porous layers. This migration can take thousands of years and extend over tens of kilometres, until the oil and gas are stopped by tight layers or leak into the sea. Producible oil and gas are only found where a source rock, a reservoir, a cap rock and a trap are all in place.

Reservoir, seal and trap

A reservoir is a rock with small spaces, pores, between the grains, usually sandstone or limestone. The pores are always filled with water, oil and gas in varying mixtures. Two properties decide how good a reservoir is:

• Porosity is the share of the rock that is pore space, in other words how much it can hold.
• Permeability is how easily oil, gas and water flow through the rock, in other words how fast the wells can produce.

The two do not always go together. Ekofisk produces from naturally fractured chalk with high porosity but low permeability (norskpetroleum.no).

A cap rock, or seal, is a tight layer above the reservoir, such as mudstone, that stops oil and gas on their way up. A trap is the shape that makes them collect beneath the seal. Traps fall into two main groups:

• Structural traps form when layers are folded or displaced along faults, such as a dome (anticline) or a tilted fault block. Troll consists of three relatively large, rotated fault blocks (norskpetroleum.no).
• Stratigraphic traps result from how the layers were deposited, for example a sand layer that thins and disappears against tight mudstone, or layers that were eroded and then covered by a tight rock. They often lack a clear dome shape and can be harder to map.

The geology of the Norwegian shelf

Most of the oil and gas deposits on the Norwegian shelf come from a thick layer of black clay deposited around 150 million years ago, in the Late Jurassic, at the bottom of a sea that covered much of north-west Europe (norskpetroleum.no, read 5 Oct 2026). In the northern North Sea the layer is called the Draupne Formation, which the Norwegian Offshore Directorate describes as dark, organic-rich claystone deposited in bottom water that was often without oxygen, and as a prolific source of oil and gas. Layers of roughly the same age are called the Spekk Formation in the Norwegian Sea and the Hekkingen Formation in the Hammerfest Basin in the Barents Sea (FactPages).

The reservoirs are more varied (norskpetroleum.no):

• North Sea: the main reservoir of fields such as Gullfaks, Oseberg and Statfjord is sandstone from the large Brent delta of the Jurassic. Troll has sand deposited in a shallow sea in the Late Jurassic, and Balder sand from submarine fans in the Palaeogene. In the southern North Sea, thick layers of chalk are an important reservoir rock, as at Ekofisk.
• Norwegian Sea: Heidrun produces from Lower and Middle Jurassic sandstone in a heavily faulted reservoir, and Ormen Lange produces very dry gas from Palaeocene sandstone.
• Barents Sea: Snøhvit produces gas and some condensate from Lower and Middle Jurassic sandstone.

At the turn of 2025/2026, 69 fields were producing in the North Sea, 25 in the Norwegian Sea and three in the Barents Sea. The Barents Sea has the largest estimate of undiscovered resources of the three, and 54 % of the resources there are in areas not opened for petroleum activities (norskpetroleum.no, as at 31 Dec 2025, read 5 Oct 2026). See undiscovered resources by sea area and the structural elements layer on the globe, which shows basins, highs and platforms.

Seismic: a picture of the subsurface

Exploration starts with seismic. A vessel sends sound pulses into the subsurface, and the sound is reflected where the properties of the rock change. Sensors towed in long cables behind the vessel or placed on the seabed record the reflections (norskpetroleum.no). After processing, the data show the layers, the faults and the shape of possible traps.

• 2D seismic is acquired with a single streamer and gives a coarse picture, used to map new exploration areas.
• 3D seismic is acquired with several parallel streamers and gives a more detailed, three-dimensional picture, in exploration and when a discovery is appraised.
• 4D seismic is 3D seismic of the same area repeated over time. It shows how the reservoir changes as it is produced or as water or gas is injected.

Seismic measures how long the sound takes, not depth. Converting time to depth requires the sound velocity in the layers, which is calibrated against wells. Seismic shows the shape of the layers but rarely what the pores contain. Gas can cause strong reflections, but it takes a well to know.

Acquisition requires a production licence or an exploration licence, and surveys must be reported to the Norwegian Offshore Directorate at least five weeks before they start. The directorate has itself acquired seismic since 1969 in areas not opened for petroleum activities (norskpetroleum.no, read 5 Oct 2026). Seismic companies also acquire data at their own risk and sell licences to several oil companies (multi-client data). The seismic surveys layer on the globe shows surveys that are ongoing, planned or recently completed.

From prospect to well

A possible trap mapped well enough to drill is called a prospect. If it is less well defined, it is called a lead. A play is an area where several prospects with the same geology may be found, for example the same reservoir and seal.

For each prospect the company estimates how much it might hold, as a range, and the chance of success: the probability that a well proves mobile oil or gas. The range shows the uncertainty about size, while the chance of success shows the risk that there is no discovery at all. Each element is often assessed separately and the probabilities multiplied.

The first well on a prospect is a wildcat. If it does not prove mobile petroleum, it is dry. After a discovery, appraisal wells are often drilled to find out how large it is. From 2010 to 2024, 468 wildcat wells were completed on the Norwegian shelf and 229 of them made discoveries, about half. In 2025, 49 exploration wells were completed, 40 wildcats and 9 appraisal wells, and 21 discoveries were made (norskpetroleum.no, read 5 Oct 2026). Discoveries that are not commercial also count as discoveries. See the discovery rate by sea area and the guide on how to assess a discovery.

Well data: logs, cores and formation tops

Wells provide the only direct samples of the subsurface. The data decide whether there is a discovery, and they are used to calibrate the seismic:

• Cuttings are crushed rock carried up by the drilling fluid. They show roughly which rocks the well passes through.
• Logs are recorded with instruments lowered into the well on a cable (wireline), or with sensors in the drill string while drilling (MWD). The gamma ray log measures natural radioactivity, which is high in mudstone and shale and low in clean sandstone; the Draupne and Spekk formations give high readings (FactPages). The resistivity log shows how well the rock conducts electricity: salt water conducts well, oil and gas poorly. The density and neutron logs are used to calculate porosity.
• Cores are whole cylinders of rock and show porosity, permeability and the depositional environment directly. Operators submit cores and cuttings to the Norwegian Offshore Directorate, which publishes core photographs.
• Drill stem tests (DST) let the reservoir flow for a period so that pressure and flow capacity can be measured.

Formation tops are the depths at which the well enters a new layer, named according to the official division into groups, formations and members. In FactPages they are the directorate's interpretation and may change. The directorate also gives the content of each exploration well (dry, shows of hydrocarbons, gas, gas/condensate, oil or oil/gas) and the formation in which hydrocarbons were proven. Click a well on the globe on the geology page and the well card shows the layers it passed through, with top and base, and links to the core photographs where they exist.

Where to find the data

• FactPages from the Norwegian Offshore Directorate has a page for every wellbore with its history, formation tops, cores, core photographs, logs and documents. Seismic surveys are under Survey, and descriptions of the formations under Stratigraphy. The data is open under the Norwegian Licence for Open Government Data (NLOD).
• FactMaps shows wells, licences and surveys on a map.
• norskpetroleum.no explains seismic and exploration policy and has the exploration results year by year.
• At OESA, the geology page has the globe with layers for wells, seismic surveys and structural elements, and overviews of new wells and discoveries, the discovery rate, undiscovered resources and who is exploring. Well names and resource classes are explained in the guide to the Norwegian shelf.

Sources

• How is petroleum formed? – norskpetroleum.no – source rock, temperatures, migration and reservoir examples, read 5 Oct 2026
• Seismic surveys – norskpetroleum.no – 2D, 3D and 4D, licences and notification deadline
• Exploration activity – norskpetroleum.no – exploration wells and discoveries in 2025 and success rate 2010–2024, read 5 Oct 2026
• Resources per sea area – norskpetroleum.no – producing fields and undiscovered resources by sea area, as at 31 Dec 2025
• Fields – norskpetroleum.no – reservoir descriptions for Ekofisk, Troll, Heidrun, Ormen Lange and Snøhvit
• FactPages – Norwegian Offshore Directorate – the Draupne, Spekk and Hekkingen formations under Stratigraphy, well data under Wellbore, read 5 Oct 2026

The guides are written by OESA students to explain concepts and how things fit together. They are not investment advice. Figures that are not definitions carry a source and a date; if you find an error, let us know at philipstave@oesa-global.com.