The low acoustic impedance of fault zones in the Wisting field, Barents Sea: Gas saturation or damage zone?

Abstract

The presence of faults in reservoirs can directly affect reservoir energy and feasibility in hydrocarbon bearing formations as well as repository potential for subsurface storage. Subsurface fault characterisation is an important but challenging workflow because a geoscientist is limited to the resolution of seismic data and sparse well data. The Barents Sea is renowned for exhumation generated gas seepage allowing preferential retainment of oil. In this study, observations in the fault zones of the Wisting field show reductions of acoustic impedance (A_imp) on seismic images. We characterise five intersecting E–W and N–S normal faults in the Wisting Central blocks using a novel approach, whereby low A_imp is mapped onto modelled fault surfaces and compared to parameters such as fault throw and shale gouge ratio. Low A_imp dominates E–W faults which exhibit more brittle response when compared to two N–S faults, where low A_imp values are sparse and the faults tip-out below a monocline. Two potential mechanisms causing the reduction of A_imp are proposed: 1) gas saturation, which can be as little as 2% to significantly reduce A_imp; 2) fault damage, which can decrease wave velocities and A_imp. The E–W faults in Wisting Central show higher reductions of A_imp than their N–S counterparts, indicating that they are either saturated with gas or have undergone higher damage.