Characterization Of Sub Seabed Fluid Flow And Hydrate Systems At Nyegga Offshore Mid Norway Integration Of Seismic Imaging And Velocity Modeling PDF Download

The study focuses mainly on two gas hydrate-related targets, located on the Northern Cascadia Margin, offshore Vancouver Island: (1) a recently identified 70-80-m high carbonate mound, Cucumber Ridge, located ~3.5-km west of Ocean Drilling Program (ODP) Site 889 and Integrated Ocean Drilling Program (IODP) Site U1327, and (2) a large cold vent, Bullseye vent, which is up to ~500 m in diameter and was drilled by IODP at Site U1328. The objective of this thesis is to analyze seismic data that provide indicators of locally focused fluid flow and characteristics of the gas hydrate occurrence associated with these two features. A grid of closely-spaced single channel seismic (SCS) data was collected at Cucumber Ridge in July/August 2001, and deep-towed multichannel seismic (MCS) lines were collected using Deep-towed Acoustics and Geophysics System (DTAGS) at the Bullseye vent area and at Cucumber Ridge in October 2002. The high-resolution SCS data, with a frequency bandpass of 40-150 Hz, recorded coherent reflectivity down to about 400 m beneath the seafloor, and provide excellent images of the subseafloor structure of Cucumber Ridge and of the gas hydrate bottom-simulating reflector (BSR) beneath it. Cucumber Ridge is interpreted to have developed as a structural topographic high in the hanging wall of a large reverse fault formed at the base of the current seaward slope. The fault zone provides pathways for fluids including gas to migrate to the seafloor where diagenetic carbonate forms and cements the near-surface sediments. Over the seismic grid, heat flow was derived from the depth of the BSR. A simple 2-D analytical correction for theoretical heat flow variations due to topography is applied to the data. Across the mound, most of the variability in heat flow is explained by topographic effects, including a local 6 mW/m2 negative anomaly over the central mound and a large 20 mW/m2 positive anomaly over the mound steep side slope. However, just south of the mound, th.