Geophysical Studies of Marine Gas Hydrate in Northern Cascadia

Abstract

This paper reviews the extensive geophysical studies and Ocean Drilling Project (ODP) results that have provided constraints on the occurrence, distribution, and concentration of gas hydrate and underlying free gas beneath the continental slope of the northern Cascadia subduction zone. On this margin there is a large clastic accretionary sedimentary prism, the most common environment for high concentrations of marine gas hydrates. Most information on the gas hydrate has come from a wide range of seismic surveys, including mapping the area of the characteristic bottom-simulating reflector (BSR), determining the depth distribution of hydrate and underlying free gas, and the geological controls of hydrate formation. BSRs are evident beneath about half of the mid-continental slope. Special seismic studies include BSR reflection coefficients, the frequency dependence of the BSR amplitude, BSR amplitude-versus-offset (AVO), and full waveform inversions. Additional information on hydrate concentration is provided by electrical resistivity profiling, and measurements of seafloor compliance. Detailed heat flow surveys have mapped the thermal regime that controls the depth to which gas hydrate is stable. Ocean Drilling Program Leg 146 drilled through the gas hydrate on the midcontinental slope and a reference hole in the adjacent deep sea Cascadia basin. Downhole geophysical logs and core analyses have constrained the concentrations of hydrate and free gas at the drill sites, and provided calibration of the regional geophysical data. In the region of the ODP site, the estimated concentration of hydrate from both geophysical and borehole data is 20-30% of the pore space (10-15% of total volume) over 100 m above the BSR, with an underlying 10-20 m layer of less than 1% free gas. this region, the Juan de Fuca plate is underthrusting the margin approximately orthogonally at about 40 mm/yr. The Juan de Fuca spreading center is located only several 100 km offshore and the oceanic plate is very young, 6-8 Ma, at the margin. The buoyancy of the young hot oceanic plate and the thick incoming sediment section result in a very subdued and shallow trench with a depth of about 2,500 m. Most of the 2-3 km thickness of incoming hemipelagic and turbidite

Review of exploration and production technology of natural gas hydrate

“…Previous studies suggest that most of the BSR amplitude is due to the velocity reduction of the underlying free gas (Singh et al, 1993;MacKay et al, 1994;Hyndman et al, 2001;Pecher et al, 2001). The presence of a free-gas zone (FGZ) is an important part of the gas hydrate system, and is particularly important if the presence of gas hydrate is to be…”

Geological Hazards

“…The upper bound of the gray area corresponds to values of α = 0.77 (zero hydrate at the BSR) and υ = 0.45; the lower bound corresponds to low end values of α = 0.67 (25% bulk hydrate) and υ = 0.41. Point locations: 1, Barents Sea (Andreassen et al, 1990); 2, the norwegian margin (Mienert et al, 2000); 3, Bering Sea basin (Scholl and Hart, 1993); 4, 5, Blake Ridge (Holbrook et al, 1996); 6, Beaufort Sea (Andreassen et al, 1995); 7-9, basin in Makran accretionary prism that has experienced rapid sedimentation (Sain et al, 2000;Grevemeyer et al, 2000); 10, offshore Cascadia (Hyndman et al, 2001); 11, offshore Chile (Bangs et al, 1993;Miller, et al, 1996), niger delta (Hovland et al, 1997); 13, 15, offshore Colombia (Minshull et al, 1994); 14, Gulf of Oman anticline (Minshull and White, 1989); 16, offshore Peru (Pecher et al, 1996); 17, South Shetland islands (Tinivella et al, 1998); 18, 19, Oman accretionary toe (White, 1979). Top inset, model of the hydrate stability zone and critically thick FGZ below for a bottom water temperature of 0°C.…”

Analytical investigation of high temperature 1 kW solid oxide fuel cell system feasibility in methane hydrate recovery and deep ocean power generation

“…Numerical calculations of the dissociation pressure in the temperature range of 273-293 (K) and water salinity in the range of 0-70, have been accomplished by the research group of Duan et al [10][11][12]. Hydrate saturation values for the Malik 2L-38 site at the arctic Mackenzie Delta have been reported to be 35-40% and at Northern Cascadia Margin confirmed to be 20-25% [13]. In one report by Colett et al, hydrate saturation in some cases exceeds 80% [14].…”