Geological fluid flow systems at Nyegga of the Mid-Norwegian margin

Abstract

The Nyegga area is located at the north-eastern escarpment of the large Storegga Slide on the south of the Vøring plateau. The region has long been a natural laboratory for shallow fluid flow investigations, mainly due to the large number of fluid flow expressions found on the bathymetric and seismic data, and the possibility to investigate the relationship between fluid flow, gas hydrate formation and slope stability. The seafloor at Nyegga is marked by hundreds of small depressions and mounds. These are in turn the upper termination of semi-circular zones of low amplitude, upbended or downbended reflections called acoustic chimneys or pipes. These chimneys terminate also at different stratigraphic horizons below the seafloor and have been interpreted as episodic fluid flow expulsion features. Underlying Eocene-Oligocene polygonal fault systems are suspected of being a long term source of fluids to the shallow subsurface sediments, while Tertiary domes are suspected to be possible leakage areas for thermogenic fluids. The area is characterized by the presence of free gas trapped beneath a seal of gas hydrates as indicated by a bottom simulating reflector (BSR). In this study 287 pockmarks, 23 mounds and 441 acoustic chimneys were investigated using high resolution swath bathymetry and 3D seismic data. The data allowed mapping and quantification of several parameters of seabed and subseabed expressions of fluid flow. The results showed that the pockmarks and mounds at Nyegga are not directly related to the polygonal fault system, but instead are product of blowout events originating from two locally overpressured shallow reservoirs overlying the polygonal fault system. Indications of fluid migration starting from the base of the polygonal faults system were found, but these fluids are most likely being distributed by the shallow reservoirs. The blowout events are the result of hydraulic fracturing starting mainly from structural crests or updip pinchouts within free gas-accumulation zones beneath the base of the gas hydrate stability zone or from traps beneath glacigenic debris flow deposits. Quantification of the pockmarks, mounds and acoustic chimneys showed that the largest pockmarks and mounds are found overlying structural closures and traps with presumably highest overpressure within the free gas zones, indicating a relationship between their size and the degree of overpressure. Pockmarks, mounds and acoustic chimneys are mainly elongated features and in some areas their orientation was found to be parallel to the free gas trapping structures in the subsurface. This is an indication that the orientation of pockmarks and mounds in these areas is inherited from the acoustic chimneys and hence from the axis of hydraulic fracturing.

Nøkkelord: Nyegga, fluid flow, pockmarks, carbonate mounds, mud mounds, acoustic chimneys, pipes, gas hydrates, BSR, polygonal faults.