Hydrates

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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.[1][2]

A Brief History of Hydrates

Naturally-occuring gas hydrates were first discovered when anomalous high pressures restricted flow in natural gas pipelines[3][4]. 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 [5]. 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[6]. 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.[7] 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[8]. 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.[9]

Hydrate Stability

The bottom simulating reflector (BSR) is pointed out at the base of the hydrates. CREDIT: (Shipley et al. 1979)[10]

BSR

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[1]. 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.[1]

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 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. 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.[10]

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.

Reflection Strength

Reflections in a free-gas saturated media will be stronger.

Instantaneous Frequency

The Instantaneous Frequency will ...

Seismic 'blanking'

When hydrates saturate a layer, the

References

  1. 1.0 1.1 1.2 Ojha, M., & Sain, K. 2009. Seismic attributes for identifying gas-hydrates and free-gas zones: application to the Makran accretionary prism. 32, 264-270.
  2. Kroeger, K. F., Crutchley, G. J., Hill, M. G., Pecher, I. A., 2017. Potential for gas hydrate formation at the northwest New Zealand shelf margin – New insights from seismic reflection data and petroleum systems modeling. Marine and Petroleum Geology, 83, 215-230. 
  3. Deaton, W. M., and E. M.Frost, Jr., 1946, Gas hydrates and their relation to the operation of natural gas pipe lines: U.S. Bur. Mines Mon. 8, 101
  4. Hammerschmidt, E. G., 1940, Elimination of hydrate troubles: Oil and Gas Journal, 39, 61-68.
  5. Markl, R. G., G. M. Bryan, and J. I. Ewing, 1970, Structure of the Blake-Bahama Outer Ridge: Jour. Geophys. Research, 75, 4539-4555.    
  6. Hollister, C. D., et al, 1972, Initial reports of the Deep Sea Drilling Project, v. II: Washington, D.C., U.S. Govt. Printing Office, 1077
  7. Stoll, R. D., 1974, Effects of gas hydrates in sediments, in Natural gases in marine sediments: Marine Sci., 3, 235-248.
  8. Bryan, G. M., 1974, In situ indications of gas hydrate, in Natural gases in marine sediments: Marine Sci., 3, 299-308. 
  9. Tucholke, B. E., G. M. Bryan, and J. I. Ewing, 1977, Gas-hydrate horizons detected in seismic-profiler data from the western North Atlantic: AAPG Bull., 61, 698-707. 
  10. 10.0 10.1 Shipley, T. H., Houston, M. H., Buffler, R. T., Shaub, F. J., McMillen, K. J., Ladd, J. W., & Worzel, J. L., 1979. Seismic evidence for widespread possible gas hydrate horizons on continental slopes and rises. AAPG bulletin, 12, 2204-2213.