Effect of tectonic inversion on supra-salt fault geometry and reactivation histories in the Southern North Sea

Abstract

High-resolution 3D seismic and borehole data in the Broad Fourteens Basin, Southern North Sea, are used to investigate the relationship between lithology and fault throw distribution, as well as to understand the reactivation and growth histories of faults developed due to tectonic inversion. Two (2) distinct tiers of faults are identified, and their geometry analysed in detail. Tier 1 faults comprise closely spaced sets of normal faults that resulted from the progressive buckling and stretching of Upper Mesozoic strata during Late Cretaceous to Paleogene tectonic inversion. They have been reactivated, but still show net normal throw separations, even though they were formed during a period of regional compression. Tier 2 faults comprise densely spaced sets of normal faults in Paleogene strata with a broad range of strikes, forming a polygonal pattern. These faults relate to early diagenesis but still record the effect of the Paleogene inversion episode. An important characteristic is that Tier 1 faults are highly segmented, and show differences in throw distribution between shale-rich and sandy intervals. These faults are more segmented with relatively small throw maxima of 14 ms (17.7 m) in shale-rich intervals, while sandy intervals are less segmented with larger throw maxima of 32 ms (40.3 m). Discrepancies in fault throw distribution and segmentation increase the chances of compartmentalisation or localised fluid flow through fault linkage zones, presenting at the same time significant risks when injecting CO2 in subsurface traps. Recognising the effect of tectonic inversion on supra-salt fault geometry, and reactivation histories, can be crucial to the characterisation of faulted hydrocarbon and carbon capture and storage (CCS) reservoirs in tectonically inverted basins such as the Southern North Sea.