Scientists have discovered hundreds of gigantic sand bodies beneath the North Sea that appear to challenge fundamental geological principles and may have important implications for energy and carbon storage.
Using high-resolution 3D seismic imaging (sound waves), combined with data and rock samples from hundreds of wells, researchers at the University of Manchester, in collaboration with industry, identified vast mounds of sand — some several kilometres wide — that appear to have subsided, displacing older, lighter and softer materials that lay beneath them.
The result is a stratigraphic inversion — a reversal of the normal geological order, in which younger rocks are normally deposited on top of the older ones, on a scale never before seen.
Although stratigraphic inversion has previously been observed on a small scale, the structures uncovered by the Manchester team are the largest example of the phenomenon documented to date.
The discovery, published in Communications Earth & Environment, challenges scientists’ understanding of the subsurface and may have implications for carbon storage.
The lead author, Professor Mads Huuse, of the University of Manchester, states that “this discovery reveals a geological process we had never seen before at this scale. What we have found are structures in which dense sand sank into lighter sediments that floated on the surface of the sand, effectively reversing the conventional layers we expected to see and creating enormous mounds beneath the sea”.
It is believed these structures formed millions of years ago, during the Late Miocene to Pliocene periods, when earthquakes or sudden changes in underground pressure may have caused the liquefaction of the sand and its sinking through natural fractures in the seabed. This displaced the underlying mud rafts, more porous but rigid, composed mainly of microscopic marine fossils, bound by contraction cracks, causing them to rise upward.
The discovery could help scientists better predict where oil and gas may be trapped and where it is safe to store carbon dioxide underground.
Professor Huuse stresses that “this investigation shows how fluids and sediments can move through the Earth’s crust in unexpected ways. Understanding how these structures formed could significantly alter how we assess subterranean reservoirs, sealing, and fluid migration — all vital factors for carbon capture and storage”.
Now the team is busy documenting other examples of this process and evaluating exactly how it affects our understanding of subsurface reservoirs and sealing intervals.
Huuse concludes: “As with many scientific discoveries, there are many skeptical voices, but also many who express their support for the new model. Time and further investigation will tell to what extent the model is broadly applicable”.
