Bob A. Hardage, principal investigator; Milo M. Backus, Michael V. DeAngelo, Robert G. Graebner, Paul E. Murray, Diana C. Sava, and Donald Wagner
The Minerals Management Service (MMS) has contracted EGL to process approximately 1300 km of 2D4C seismic data across an area of offshore Louisiana where water depths range from 500 meters to 1000 meters. The objectives of the data processing are to create high-resolution images of near-seafloor geology and to estimate hydrate concentrations across the hydrate stability zone.
A critical step in the data analysis is to combine resistivity log data with seismic-based VP interval velocities at calibration wells and do a joint inversion of both types of hydrate-sensitive data to estimate hydrate concentration. The relationship between VP velocity and hydrate concentration determined through these joint inversions at several calibration wells is then used to estimate hydrate concentration along OBC profiles where VP velocities are determined across thin sub-seafloor layers via raytrace analyses of common-receiver gathers of OBC data. An example of a joint inversion at one calibration well is shown in the accompanying figure.
(a) Seismic-based VP and VS interval velocities, resistivity log, and their respective estimates of hydrate concentration at calibration well A. The base of the hydrate stability zone (BHSZ) is defined as the top of the layer where VP velocity exhibits a reversal in magnitude. The resistivity immediately below the BHSZ boundary is caused by free gas. (b) Joint inversion of resistivity and VP velocity indicates hydrate occupies 11.4 percent of the pore space (mean value of the PDF). The estimation error is ±2.9 percent (standard deviation of the PDF).
For more information, please contact Mark Shuster, principal investigator.
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