Abstract

Vertical fault leakage is a significant risk factor in petroleum exploration and has been suggested to be responsible for the underfilling of structural traps as well as for most of the exploration failures in the Hammerfest Basin. A previous investigation of 15 structures in this basin concluded that most of these structures leaked vertically (Hermanrud et al., 2014). The location where leakage took place was identified in most of these structures, as the hydrocarbon column tapers off to zero where the top reservoir surface meets the (presumed) leaky fault intersections or relay ramps. We here report a study where we analyzed amplitude variations in overburden rocks above these structures to see if differences between (presumed) vertically leaky and vertically sealing fault intersections or relay ramps could be identified from the seismic data.

Our analyses supported the previous conclusion that the position of the gas–water contacts is controlled by vertical leakage above one specific fault intersection or relay ramp. The seismic amplitude anomalies above the leakage positions are characterized by (1) major gas chimneys with associated brights, bottom-simulating reflectors, and seismic pushdowns that overlie the leakage location or (2) subvertical fluid flow pathways along fault intersections or relay ramps that connect the top reservoir surfaces to localized and bright seismic amplitudes in overlying formations (the Kolmule, Kviting, and Kveite formations). We suggest that these observations could have been used to improve the predictions of the hydrocarbon–water contacts in most of these structures if the data had been available before drilling. We also suggest that the seismic expressions of leaking hydrocarbons depend significantly on the permeability of the overburden rocks. Therefore, relationships between fluid contacts in reservoirs and overburden amplitude variations need local validation before they are applied to exploration in new areas.