Shear modulus distribution at ODP Site 994; new evidence of gas hydrate in the absence of BSRs

Author(s): Pretzschner, Carsten; Goldberg, Dave; Lindner, Harald; Guérin, Gilles
Author Affiliation(s): Primary:
Freiberg University of Mining and Technology, Freiberg, Federal Republic of Germany
Other:
Lamont-Doherty Earth Observatory, United States
Volume Title: American Association of Petroleum Geologists 2001 annual meeting
Source: Annual Meeting Expanded Abstracts - American Association of Petroleum Geologists, Vol.2001, p.162; American Association of Petroleum Geologists 2001 annual meeting, Denver, CO, June 3-6, 2001. Publisher: American Association of Petroleum Geologists and Society of Economic Paleontologists and Mineralogists (AAPG), Tulsa, OK, United States. ISSN: 0094-0038 CODEN: APGAB2
Note: In English
Summary: Shear velocity logs in gas hydrate-bearing sediments were acquired for the first time along a transect of three ODP holes across the Blake Ridge. Shear wave data at Sites 995 and 997 indicate an increase in Vs in the presence of hydrate. Irregular borehole conditions at Site 994, however, did not allow Vs to be extracted using standard techniques. In this study, we use a statistical approach to identify shear waves by the local minimum of each waveform energy envelope. The resulting Vs profile is very similar to those at Sites 995 and 997, extending the observed increase in formation rigidity that is associated with the presence of hydrate beyond the lateral extent of the seismic BSR. In addition, we use these results to generate a 2-D tomographic inversion of the waveforms and determine the shear modulus distribution as a function of distance into the formation. The drilling process often influences measurements around a borehole due to invasion of fluids into the permeable formation. In the presence of hydrate, which is only stable under limited thermodynamic conditions, invasion is commonly thought to dissociate hydrate into free gas. At Site 994, however, the inversion suggests that hydrate does not decompose significantly due to fluid invasion into the formation. These results confirm that Vs logs can accurately measure the in situ properties of hydrate-bearing sediment and that hydrate deposits are not confined by the lateral extent of a BSR.
Year of Publication: 2001
Research Program: ODP Ocean Drilling Program
Key Words: 20 Geophysics, Applied; 29 Economic Geology, Energy Sources; Accuracy; Atlantic Ocean; Blake-Bahama Outer Ridge; Body waves; Bottom-simulating reflectors; Drilling; Elastic waves; Gas hydrates; Geophysical methods; In situ; Inverse problem; Leg 164; Measurement; Natural gas; North Atlantic; ODP Site 994; Ocean Drilling Program; Petroleum; Properties; S-waves; Seismic methods; Seismic waves; Thermodynamic properties; Tomography; Two-dimensional models; Velocity; Waveforms; Well logs
Coordinates: N314708 N314708 W0753245 W0753245
Record ID: 2003023442
Copyright Information: GeoRef, Copyright 2017 American Geosciences Institute. Reference includes data supplied by American Association of Petroleum Geologists, Tulsa, OK, United States