Hydrates are typically methane gas molecules trapped in ice-like crystals of water. The low temperature(<15°C) and high pressure(>5 MPa) conditions of stability for naturally occurring hydrates commonly exists in deep water just beneath the sea floor. The bottom-simulating reflector (BSR) is a reflection event that is closely associated with identifying hydrates in seismic cross-section. Identifying and analyzing hydrates is important. Drilling through hydrates can be challenging, and can cause drilling to be hazardous and cost more. Hydrates are also a potential energy source with 200 years worth of energy from just %15 of the world’s reserves. Gas hydrates are also of interest environmentally due to the possible seeping of greenhouse gas into the atmosphere where the base of the hydrate-concentrated sediment meets the seafloor.
A Brief History of Hydrates
Naturally-occuring gas hydrates were first discovered when anomalous high pressures restricted flow in natural gas pipelines. A BSR (Bottom Simulating Reflector) is the first event from geophysical data that has been associated with identifying hydrates. This BSR exists on the Blake Outer Ridge along the eastern United States margin . Discoveries with hydrate identification in the Blake Outer Ridge followed. Drilling into the BSR proved the existence of high methane contents, and further verified the association between BSR’s and hydrates. Seismic velocities within the upper 600 meters were 2.0 to 2.2 km/s, and Stoll et al. (1971) suggested these anomalously high velocities to be a result of increased rigidity when methane is present in the sediment. Seismic data also returned high amplitude responses there. It was later suggested that the high amplitude responses of the Blake Outer Ridge were caused by an accumulation of gas occurring at the base of the hydrated zone. Tucholke et al. (1977) showed a strong correlation between the BSR and the pressure and temperature of the phase boundary in the methane + 3% NaCl + water system.
The bottom-simulating reflector(BSR) is the most common attribute used for gas hydrates. The BSR is an indication of a physical boundary between the upper gas-hydrate bearing sediments and the lower non-bearing sediments. the BSR mimics the ocean floor at a depth to which the hydrates maintain stability. The depth of stability depends on the temperature and the pressure. It is because of the relationship between stability and temperature and pressure that we see a general decrease in BSR depth landward. One behavior of the BSR is that it cuts through strata, which makes it more easily identifiable for seismic interpreters. Once a BSR candidate is found, it can become a stronger candidate if the depth decreases landward. Further investigation could involve a quantitative analysis of the depth of stability; if it correlates well to the temperature and pressure.
The BSR will have a polarity opposite to that of the ocean bottom reflector, and a hard reflection is likely to exist due to the sharp change in acoustic impedance. The sharp change in the acoustic impedance is from a dense concentration of hydrates at the base and fast velocities of about 2.0 to 2.2 km/s within the hydrate sediments with free gas beneath the BSR causing slow velocities. This hard reflection can also cause a polarity reversal. With a calculation of the reflection coefficient, one could determine the magnitude of the polarity reversal.
Unfortunately, there is uncertainty associated with the BSR. There have been BSR's found with no hydrates present after drilling, and there have been hydrates present with no BSR's in seismic cross section. Attributes beyond the scope of the BSR can be used in identifying hydrates.
Other Indications of Hydrates
Other than the BSR, there are other seismic attributes that correlate well with hydrates.
Sediments that underly the hydrate stability zone (HSZ) typically contain free gas. The free gas lowers the seismic velocities and have high reflection strength due to variations in gas saturation.
Presence of free gas is also indicated by shadows in the instantaneous frequency plot. High reflectivity and low frequency 'shadows' over a large time window indicate a thick sequence of strata altering between gas-rich and gas-poor beds.
Within the HSZ, gas hydrates cement the layers and homogenize to decrease the seismic amplitudes.
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