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Scientific Investigations Report 2008–5175

Assessing Gas-Hydrate Prospects on the North Slope of Alaska—Theoretical Considerations

By Myung W. Lee, Timothy S. Collett, and Warren F. Agena

Abstract

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Gas-hydrate resource assessment on the Alaska North Slope using 3-D and 2-D seismic data involved six important steps: (1) determining the top and base of the gas-hydrate stability zone, (2) “tying” well log information to seismic data through synthetic seismograms, (3) differentiating ice from gas hydrate in the permafrost interval, (4) developing an acoustic model for the reservoir and seal, (5) developing a method to estimate gas-hydrate saturation and thickness from seismic attributes, and (6) assessing the potential gas-hydrate prospects from seismic data based on potential migration pathways, source, reservoir quality, and other relevant geological information. This report describes the first five steps in detail using well logs and provides theoretical backgrounds for resource assessments carried out by the U.S. Geological Survey.

Measured and predicted P-wave velocities enabled us to tie synthetic seismograms to the seismic data. The calculated gas-hydrate stability zone from subsurface wellbore temperature data enabled us to focus our effort on the most promising depth intervals in the seismic data. A typical reservoir in this area is characterized by the P-wave velocity of 1.88 km/s, porosity of 42 percent, and clay volume content of 5 percent, whereas seal sediments encasing the reservoir are characterized by the P-wave velocity of 2.2 km/s, porosity of 32 percent, and clay volume content of 20 percent. Because the impedance of a reservoir without gas hydrate is less than that of the seal, a complex amplitude variation with respect to gas-hydrate saturation is predicted, namely polarity change, amplitude blanking, and high seismic amplitude (a bright spot). This amplitude variation with gas-hydrate saturation is the physical basis for the method used to quantify the resource potential of gas hydrates in this assessment.

Version 1.0

Posted November 2008

For additional information contact:

Team Chief Scientist,
USGS Central Energy Resources
Box 25046, Mail Stop 939
Denver, CO 80225

Or visit the Central Energy Resources Team site at: http://energy.cr.usgs.gov/

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Suggested citation:

Lee, M.W., Collett, T.S., and Agena, W.F., 2008, Assessing gas-hydrate prospects on the North Slope of Alaska—Theoretical considerations: U.S. Geological Survey, Scientific Investigations Report 2008–5175, 28 p.



Contents

Abstract

Introduction

Determining Gas-Hydrate Phase Boundary

Tying Well Log Information to Seismic Data

Differentiating Ice from Gas Hydrate in Permafrost

Developing Acoustic Models for Reservoir and Seal

Developing a Method of Estimating Gas-Hydrate Saturation and Thickness

Summary and Conclusions

Acknowledgements

References Cited

Appendix 1. Modified Biot-Gassmann Theory (BGTL)

Appendix 2. Reservoir and Seal Models

Appendix 3. Saturation and Thickness Interpretation Procedure


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