Seismic and thermal investigations of the Blake Ridge gas hydrate area: a synthesis

Abstract

Analysis of seismic and thermal data acquired on Leg 164 has led to significant advances in our understanding of the distribution of free gas and gas hydrate on the Blake Ridge and of the geological processes responsible for this distribution. Zerooffset and walk-away vertical seismic profiling and in situ temperature measurements to hundreds of meters below the seafloor have provided constraints on physical parameters at a resolution not usually available in deep-water gas hydrate provinces. The most significant observations to emerge from the Leg 164 geophysical program are as follows. (1) A thick zone of gas-charged sediments characterized by high reflectivity and low velocity lies directly below the gas hydrate zone and the bottom-simulating reflector (BSR) at Sites 995 and 997. At Site 994, a BSR continuous with that at Sites 995 and 997 is not observed, and the top of the free-gas zone, which lies somewhat deeper (~550 mbsf), is not coincident with the base of gas hydrate occurrence. This result may be explained by low methane supply rates. (2) The quantities of gas hydrate present on the Blake Ridge crest (2%–3% of total volume) do not affect the P-wave velocity or attenuation enough to be resolved with seismic techniques on the scale of tens to hundreds of meters. The P-wave velocity (1700–1800 m/s) and attenuation (Q = 100–400) are consistent with values found in similar fine-grained sediments that do not contain gas hydrate. (3) The temperature at the BSR at Sites 995 and 997 may be as much as 0.5°–2.9°C lower than the accepted phase equilibria curves that constrain the temperature of gas hydrate dissociation at the appropriate in situ pressures. Effects of chemical impurities or capillary forces may contribute to the inhibition of gas hydrate in this setting, but uncertainty in the stability curves for the methane hydrate system at these pressures renders a precise interpretation of the temperature disparity impossible. (4) The only massive gas hydrate found on the ridge crest appears to be associated with a high-angle fault that acts as a conduit for fluid advection, and the pervasive nature of finescale, high-angle normal faulting within the sedimentary section leads to the inference that much of the gas hydrate on the Blake Ridge may be concentrated within fault zones. Theoretical studies have recently confirmed the role of vigorous advection in producing significant concentrations of gas hydrate over short time periods in natural systems.