The North Sea Rift Basin ranks as one of the world’s most famous and indefatigable super basin areas. Like a pugnacious prize fighter, it’s been counted out again and again, only to rebound and punch back into contention.
While some explorers consider the North Sea province highly mature, and even late life, it continues to produce plenty of oil and gas.
“The demise of the North Sea has been predicted by many, but there are many reasons to believe that the basin has a future for decades to come, not only as an oil and gas producer but also with a crucial role in the energy transition,” said John Underhill.
Underhill is academic lead and director of the UK Centre for Doctoral Training and professor of exploration geoscience in the Institute of GeoEnergy Engineering at Heriot-Watt University in Edinburgh.
An AAPG member since 1982, he has received the Association’s George C. Matson, Grover E. Murray Distinguished Educator and Ziad Beydoun Memorial awards.
Underhill said advances in exploration, development, engineering and drilling technology have all led to more North Sea production than first thought possible.
“It easily qualifies as a global super basin because over 95 billion barrels of oil equivalent have been produced from maritime waters of the UK, Norway and Denmark over the past five decades,” since production first came onshore from the Argyll Field, Underhill noted.
“Current estimates suggest that between 10-20 billion barrels (equivalent) remain to be produced or have yet to be discovered,” he added.
Thanks to 3-D and 4-D time-lapse seismic data, North Sea operators have been able to identify missed and unswept pay, to target subtle stratigraphic traps and to address structural complexities, Underhill explained.
“The advent of deviated and horizontal drilling and innovative casing designs have enabled wells to extend their reach and to handle the high pressures and high temperature that characterize deep plays in parts of the basin,” he noted.
Several petroleum-system areas occupy a space between the United Kingdom to the west, Norway and Denmark to the east and offshore and onshore northern Europe to the south. The North Sea Rift super basin, a trilete extensional rift system, consists of the Viking Graben, Central Graben and Moray Firth, Underhill said.
Rifting took place in the Upper Jurassic after a period of thermal doming and was followed by an extended period of Cretaceous-to-Recent, post-rift thermal subsidence, he noted.
“The North Sea Rift super basin’s main source rock is Upper Jurassic in age and referred to as the Kimmeridge and Heather formations, which form component parts of the Humber or Viking Group, the deposition of which took place in marine conditions in half-graben depocenters that developed during the rift episode,” he said.
“Maturation occurred during the Early Cenozoic, and the petroleum system is at its maximum geographical extent at the present day in central and eastern parts of the basin,” he added.
Natural Gas Production
Famous for its oil resources, this super basin also boasts abundant natural gas production. A single field, the huge Troll Field offshore Norway in the eastern margin of the Viking Graben, holds about 40 percent of total gas reserves on the Norwegian continental shelf.
“While the Kimmeridge Clay Formation is a prolific Type-2, oil-prone, marine source rock, deep burial has led to gas also being expelled, and many fields either have a gas cap, e.g., Brent Field, or else are exclusively gas filled, sometimes with a biodegraded oil precursor,” like Troll, Underhill said.
The combined effects of Iceland Plume- and Atlantic Ocean-opening during the Paleogene caused the uplift of western parts of the Moray Firth rift arm, where maturation of the Kimmeridge Formation was arrested and some traps were breached, he said.
“Despite the deformation, petroleum potential still exists in this area, but is limited to areas where Middle Devonian lacustrine, waxy crude source rocks locally charged extant traps, for example, the Beatrice Field,” Underhill explained.
“Late Cenozoic uplift also characterizes inboard areas on the Norwegian margin, too, leading to gas expansion and oil being forced out of some palaeo-structures,” he said.
While the Upper Jurassic Kimmeridge Clay is stratigraphically younger than many of the reservoir-seal pairs, the North Sea’s tectonic history led to it being structurally deeper, he noted.
“Middle and Lower Jurassic, Triassic, Permian and older pre-rift plays housed in traps within and on the flanks of the main grabens have been charged by a fill-and-spill mechanism, whereby those deeper ones located closer to the graben axis were charged first and more marginal ones filled later,” Underhill observed.
“Syn-rift plays like the Brae Trend and Buzzard Field are effectively self-sourcing, self-sealing systems, and Lower Cretaceous/Upper Cretaceous chalk and Cenozoic post-rift plays rely on vertical migration, often aided by overpressure,” he said.
Southern Permian Basin
This combined North Sea super basin province borders another, separate super basin to the south. Covering parts of onshore and offshore northern continental Europe, and home to the giant Groningen gas field, the area is known as the Northern European Basin, the Southern Permian Basin or simply the Rotliegend.
“The North Sea Rift super basin is distinct from its Anglo-Polish super basin counterpart, which lies to the south. As the name implies, it stretches from eastern England to Poland and encompasses parts of Holland, Germany and Denmark,” Underhill said.
“Also known as the Southern Permian Basin, charge is from Upper Pennsylvanian and Lower Mississippian Carboniferous source rocks, with the prospective plays in the Carboniferous, Permian Rotliegend and Zechstein Groups, and Triassic Bunter intervals,” he said.
Keeping Eyes on the North Sea
In recent years – in recent decades – the North Sea at times has been all but ignored as an ongoing exploration province. When oil prices spike, interest and discoveries return. Then the cycle starts over again. Underhill said government action has helped the North Sea remain viable in the industry’s eyes.
“The UK and Norwegian regulators, the Oil & Gas Authority and Norwegian Petroleum Directorate, are acutely aware of the need to ensure infrastructure remains active and have been very proactive,” he noted.
“They have sought to maximize the economic recovery from the basin by commissioning and public release of seismic and well data, as well as innovative initiatives that promote licensing and drilling in the current cost-constrained environment,” he said.
In recent years, North Sea operators have trended away from international oil companies toward smaller independents, companies backed by private equity and late-life production specialists in managing mature fields, Underhill said.
“There continues to be interest in licensing offshore areas of the North Sea Rift super basin, as testified by the number of bids and awards in the latest 32nd UK licensing round announced in September 2020,” he said.
Underhill noted that in the North Sea, as in the rest of Europe, there is “a tension between continued oil and gas production and the need to decarbonize.” Scotland has committed to achieving net-zero greenhouse gas emissions no later than 2045.
“Major efforts are being made to re-purpose old platforms for wind energy and produce hydrogen via electrolysis, to use depleted fields for carbon storage and to utilize any electrical and geothermal resources generated to heat platform,” he said.
“Challenges certainly lie ahead, and the competition for the seabed and subsurface below – between wind farm operators, oil and gas companies and carbon storage opportunities and their interdependencies – needs to be recognized and addressed,” he added.
Fueling the world’s economy in a safe and sustainable way will require retaining the industry’s existing expertise and giving the next generation of geoscientists the skill sets and technical expertise needed to understand, evaluate and manage the subsurface of the North Sea, Underhill said.
In that context, Heriot Watt together with nine industry partners has established two centers of doctoral training in oil and gas, the Natural Environment Research Council CDT and the new GeoNetZero CDT, he noted.
“These uniquely combine PhD research with a 20-week training program that tackles geoscience and the role it plays in the petroleum industry, the low-carbon energy transition and the challenge of net-zero emissions that European governments have committed to,” he said.