In the depths of the North Sea lies one of the most spectacular records of turbidity currents: The Claymore Sandstone. During the Upper Jurassic, these rocks were born from a geological phenomenon as violent as it was fascinating.
Imagine sediment-laden river floods cascading down the fault scarps of the Halibut Horst toward the North Sea basin floor 150 million years ago.
These gravitational flows, heavy with sediments, descended at extraordinary velocities across the seafloor. As they lost energy in deeper zones, they deposited their load in characteristic sequences: First the coarsest sands, then the finer ones, as can be seen in the first meters of core.
These sands, interpreted as clustered lobes at the base, record high-density flows with excellent connectivity. Higher up, the zebra facies, with their alternating rhythm of sands and shales, were formed by more diluted flows at the margins. At the top, channel-levee complexes acted as true sedimentary highways and internal barriers.
Each new discharge pulse built a new floor in the future reservoir. Layer upon layer, over millions of years, up to 152 m of reservoirs with excellent porosity and permeability accumulated.
But when the well was drilled in this location, it was never planned to core the Claymore sands. The reason? Geologists hadn’t mapped them when they mapped the prospect and planned the well. The primary target was deeper down, the Upper Jurassic Piper sands. So, when sands hit the shakers at the expected depth, people thought the targeted Piper reservoir had been reached, and coring started.
The Claymore Sandstones perfectly illustrate how turbidity currents can generate world-class reservoirs, also when it is a serendipitous discovery.
