Contrasting influence of salt structures and faults on the geothermal potential of regional structural highs: The Cleaver Bank High, Southern North Sea

High-quality 3D seismic reflection data, complemented by 448 bottom-hole temperatures (BHTs) from 48 boreholes, are used to investigate the influence of salt structures and faults on the geothermal potential of the Cleaver Bank High, Southern North Sea. Developed salt structures include multiple salt diapirs, salt pillows and a salt wall, with their presence influencing local geothermal potential. Strata deposited above the Zechstein Group record geothermal gradients that are enhanced proportionally to the thickness of this salt unit. Conversely, strata buried below the Zechstein Group reveal a moderate decreasing trend in geothermal gradients as salt thickens. Large supra-salt faults can act as fluid paths to deep and hot fluid into shallow strata, resulting in the presence of high geothermal gradients in shallow strata. Importantly, geothermal gradients on the footwall of these faults are much higher than that on the corresponding hanging-wall, decreasing as one moves away from them. For example, average geothermal gradients on the footwall of the largest supra-salt fault (Fault A) are, relative to its immediate hanging-wall, 105 % higher in the North Sea Group, 26 % higher in the Chalk Group, and 41 % higher in the Rijnland, Upper and Lower Germanic Trias Groups. Additionally, sub-salt faults influence the geothermal gradient of supra-salt strata in parts of the study area where there is very thin, or even absent, salt (<100 ms; or similar to 230 m), forming distinct low-amplitude trails of fluid above these same faults. They also indirectly influence geothermal gradient by controlling the position, geometry and distribution pattern of salt structures. As a corollary, three potential geothermal exploration targets are suggested on the Cleaver Bank High, one located on the footwall of a large supra-salt fault, one above thick salt, and a third target above very thin Zechstein strata where low-amplitude fluid chimneys are found. The results in this work can be applied to similar salt-bearing structural highs in Northern Europe and worldwide.

Automated summary

This study investigates the influence of salt structures and faults on the geothermal potential in the Cleaver Bank High, Southern North Sea, using high-quality 3D seismic reflection data and bottom-hole temperature data. The results show that salt structures and faults play a significant role in the geothermal gradient, with higher gradients found on the footwalls of faults. Three potential geothermal exploration targets are identified based on the findings.