Determine the linkage between hydrate concentration and seismic characteristics in subsea sediments.
University of Wyoming, Dept. of Geology and Geophysics – project management and research products
Laramie, Wyoming 82071
This project built on the wealth of information from Ocean Drilling Program (ODP) sites, drilled during Leg 164 (1995) over a larger part of Blake Ridge, by collecting and analyzing three-dimensional, multi-channel seismic (MCS) data and three-component, ocean-bottom seismometer (OBS) data. These data elements were combined with chemical, sedimentological, and downhole logging information from Leg 164 to establish benchmark linkages between seismic signature (reflectivity, P-velocity, S-velocity) amounts and distribution of hydrate and free gas.
This project was a joint DOE-NSF venture to improve the three-dimensional seismic images and deploy a new array of ocean-bottom seismometers to allow never-before-seen seismic images of methane hydrate deposits off the South Carolina Coast at Blake Ridge. DOE funding has allowed acquisition of 3D MCS data at denser line spacing, access to the best available data processing software, and acquisition and analysis of densely spaced OBS data to allow a detailed compressional-wave to shear-wave (P-S) study over the edge of the bottom simulating reflectors (BSRs). Analysis of the data has led to a number of conclusions.
First, that the quantity of missing methane from the Blake Ridge Depression can be estimated at roughly 46 Tcf, a number that is an order of magnitude higher than previous estimates based on a structural collapse model. The implication is that escape of methane from a free gas zone below a hydrate accumulation offers a mechanism for the sustained ocean-wide release of methane. It is possible that continental slope erosion, brought on by circulation changes, could initiate such a release without requiring the sequence of bottom water warming, hydrate dissociation, and mechanical failure of sediments suggested for the “collapse model.”
Second, project results revealed that hydrates at Blake Ridge can be directly detected in at least three ways: 1) paleo-bottom simulating reflectors (BSRs); 2) highly reflective bright spots; and 3) low-amplitude “blanked” strata. These observations provide important indicators of locations where efforts to quantify hydrate deposits should be focused. Amplitude blanking may be useful as a method of detecting methane hydrate in limited cases where lateral contrasts in reflection amplitude occur above the BSR in sediments of the same lithology. The large number of variables affecting reflection amplitude between two different strata makes it difficult to use blanking as a means of accurately quantifying hydrate. Remote quantification of hydrate concentrations, therefore, is best performed through detailed velocity analysis and comparison to rock physics models. Velocity analysis reveals that the hydrate bright spot at Blake Ridge contains higher concentrations (30-42% bulk hydrate) than the blanked lens region (13-23% bulk hydrate). End member estimates of methane trapped within the lens fall between 1.5 and 2.5 Tcf.
Finally, a third discovery revealed mechanical controls on the maximum thickness of the free gas zone and gas escape. Data from Blake Ridge and elsewhere support the idea that the thickness of the free gas zone beneath hydrate deposits in basin settings is limited by the Mohr-Coulomb failure criterion. This new model of hydrate-related free gas zones achieves two important advances. First, by proposing a physics-based upper limit on free gas zone thickness, the model provides a basis for the first estimate of the global inventory of free gas associated with hydrate deposits. Free gas associated with methane hydrate accumulations likely contain between 1/8 and 1/2 of the methane contained in the hydrate. Second, the upper limit on free gas thickness provides a plausible, quantifiable mechanism to explain the release of ~2500 Gt of methane during a five degree Celsius increase in bottom-water temperature, as occurred during the Late Paleocene Period.
Final report issued.
In addition to the information provided here, a full listing of project related publications and presentations as well as a listing of funded students can be found in the Methane Hydrate Program Bibliography [PDF].
Three-dimensional Structure and Physical Properties of Methane Hydrate Deposit and Methane Reservoir - Blake Ridge [External site - University of Texas at Austin] Steven Holbrook (U. Wyoming), Ingo Pecher(UTIG)
Gas Hydrate Research: ODP Leg 164 [External site - University of Wyoming] Steven Holbrook
Topical Report - 3-D Structure and Physical Properties of a Methane Hydrate Deposit, Blake Ridge [PDF-825KB]
Gorman, A., W. Holbrook, M. Hornbach, K. Hackwith, D. Lizarralde, and I. Pecher, 2002, Migration of methane gas through the hydrate stability zone in a low-flux hydrate province, Geology, Volume 30, p. 327-330.
Holbrook, W., D. Lizarralde, I. Pecher, A. Gorman, K. Hackwith, M. Hornbach, and D. Saffer, 2002, Escape of methane gas through sediment waves in a large methane hydrate province, Geology, Volume 30, p. 467-470.
Hornbach, M., W. Holbrook, R. Gorman, K. Hackwith, D. Lizarralde, and I. Pecher, 2003, Direct seismic detection of methane hydrate and the Blake Ridge, Geophysics, Volume 68, n. 1, p. 92-100.
Hornbach, M., D. Saffer, and W. Holbrook, 2004, Critically Pressured Free-Gas Reservoirs Below Gas Hydrate Provinces, Letters to Nature, Volume 427, p. 142-144.
Holbrook, W., A. Gorman, M. Hornbach, K. Hackwith, J. Nealon, D. Lizarralde, and I. Pecher, 2002, Seismic detection of marine methane hydrate, The Leading Edge, Volume 21, p. 686-689.