Seismic Estimation of Gas Hydrate Concentrations in Deepwater Environments: Assumptions and Limitations

Bob A. Hardage and Diana Sava
Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

Several assumptions must be made to translate deep-water seismic attributes into estimates of hydrate concentrations distributed in near-seafloor sediments. It is important to understand the effects each assumption has on seismic-based predictions of hydrate concentration, so that these effects can be considered when estimates of deep-water gas hydrate resources are evaluated. Principal assumptions involve

• Internal morphology of clathrate/sediment grain mixtures,
• Mineralogy of the host sediment,
• Porosity of the host medium,
• Effective pressure confining the hydrate,
• Spatial resolutions of seismic images, and
• Depth registration accuracy of P-wave and S-wave seismic images spanning a hydrate interval.

Each of these assumptions will be discussed, and data examples will be presented that show how variations in assumed properties and principles affect end results. The data used in these examples come from deep-water areas across the northern shelf of the Gulf of Mexico. Seismic data involve high-resolution P-wave chirp-sonar data and P-wave and S-wave images constructed from four-component ocean-bottom-cable data. Geologic calibration data acquired in seafloor borings will be used to constrain seismic interpretations and assumptions.