The nature of the gas-hydrate/free gas transition zone at the base of the hydrate-stability zone from high-resolution 3D seismic data

Abstract

Geophysical evidence of a prominent BSR at Vestnesa Ridge and north to the northern flank of the Storegga slide escarpment infer that gas hydrates are ubiquitous within these regions. A wide range of fluid flow structures have previously been discovered from these regions, as well as gas seepage activity from pockmarks on the eastern segment of Vestnesa Ridge. This paper focuses on high-resolution 3D seismic interpretation and attribute analysis of amplitude anomalies in relation to the BSR and underlying free gas zone (FGZ), as well as BSR characteristics in relation to fluid flow structures. The study is carried out for three high-resolution 3D seismic datasets, two from Vestnesa Ridge and one located north to the northern flank of the Storegga Slide area on the mid-Norwegian margin. Pull-up and push-down effects of the BSR are frequently associated with chimney structures and are interpreted to reflect the occurrence of gas hydrate and free gas, respectively. However, these features are most likely a combination of several factors i.e. deformational processes, warm fluids ascending from larger depths. The extent and distribution of these effects and features show remarkable differences when comparing each dataset. As the BSR is formed by the termination of individual gas-charged horizons, the alternating high and low amplitude values are interpreted to result from varying free gas saturations trapped in beds of alternating sediment properties. As each investigated horizon show highest amplitude values against the BSR boundary, it is suggested to be partly affected by interference from thin gas-charged beds that pinch out against the GHSZ. Conspicuously lateral amplitude patterns observed along enhanced reflections are as well addressed to constructive and destructive interference between layers of varying gas saturations. The seismic characterization of the FGZ below the BGHSZ suggest that gas migration and accumulation are morphologically controlled by the anticlinal shape of the Vestnesa Ridge. The more complex FGZ geometry at the slope setting north to the northern flank of the Storegga Slide area, suggests that gas migration and accumulation are restricted to preferable strata and controlled by local geology.