Analysis of a basement fault zone with geothermal potential in the Southern North Sea

Abstract

Extraordinary 3D seismic data from the Central Offshore Platform (Southern North Sea), complemented by information from 38 boreholes, reveal a 10 km-wide basement fault zone above which fluid anomalies emanate from sub-salt reservoirs to terminate in lower Cretaceous strata. Fluid blow-out pipes, chimneys and low-amplitude trails were mostly sourced from the region where NW-striking syn-rift faults intersect the N-striking basement fault zone. As a result, 73% of the mapped fluid-flow anomalies (94 out of 129) occur within the basement fault zone of interest or follow a N-S strike along its shoulders. We postulate a strong control of the basement fault zone on past fluid and heat flow, as basin models confirm that fluid and heat were mostly produced during the Cretaceous. Bottom-hole data record temperatures of ∼140 °C at present, highlighting the geothermal potential of the study area. These temperatures nevertheless contrast with the relatively constant gradient of ∼ 32 °C/km occurring both in and outside the basement fault zone. This work is important as its shows that past fluid and heat flow over a basement fault zone does not necessarily correlate with the existence of an enhanced hydrothermal system at present. However, as bottom-hole temperatures are within the benchmark values considered across Europe, it also stresses the importance of basement fault zones as key structures to find, and assess, as potential geothermal sites.