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Gas hydrate quantification using full-waveform inversion of sparse ocean-bottom seismic data; a case study from Green Canyon Block 955, Gulf of Mexico

Wang Jiliang, Priyank Jaiswal, Seth S. Haines, Patrick E. Hart and Wu Shiguo
Gas hydrate quantification using full-waveform inversion of sparse ocean-bottom seismic data; a case study from Green Canyon Block 955, Gulf of Mexico
Geophysics (August 2018) 83 (4): B167-B181

Abstract

We present a case study of gas hydrate quantification using dense short-offset multichannel seismic (MCS) and sparse long-offset ocean-bottom-seismometer (OBS) data in lease block Green Canyon 955 (GC955), Gulf of Mexico (GOM), where the presence of gas hydrate was interpreted using logging while drilling (LWD) data acquired by the GOM Gas Hydrate Joint Industry Project Leg II expedition. We use frequency-domain full-waveform inversion (FWI) of seven OBS gathers to invert for a P-wave velocity model of an approximately 7 km long MCS profile connecting two LWD sites, GC955-H and GC955-Q. We build the starting model for FWI using traveltime inversion (TI) of the MCS and OBS data. In addition, we use the TI model for depth migrating the MCS stack. At the LWD sites, we constrain the hydrate saturation (Sgh) using sonic and resistivity logs. Unfortunately, as is typical of seismic quantification problems, the FWI model resolution is not sufficient to extrapolate the LWD-based Sgh. Therefore, we apply Backus averaging to the sonic log, at 60 m wavelength, bringing it within approximately 8% of the FWI model and make the assumption that averaging the sonic log is same as redistributing the gas hydrate within the Backus wavelength. In this manner, instead of Sgh, the FWI model is able to estimate the total gas hydrate volume. In the end, we use the FWI model and the migrated stack to constrain the locations and bulk volumes of free gas and gas hydrate. Our results demonstrate that with careful processing, reasonable estimates on locations and bulk volumes of submarine gas hydrate accumulations can be achieved even with sparse seismic data that are not adequate for amplitude-based assessments.


ISSN: 0016-8033
EISSN: 1942-2156
Coden: GPYSA7
Serial Title: Geophysics
Serial Volume: 83
Serial Issue: 4
Title: Gas hydrate quantification using full-waveform inversion of sparse ocean-bottom seismic data; a case study from Green Canyon Block 955, Gulf of Mexico
Affiliation: Chinese Academy of Sciences, Institute of Deep-Sea Science and Engineering, Sanya, China
Pages: B167-B181
Published: 201808
Text Language: English
Publisher: Society of Exploration Geophysicists, Tulsa, OK, United States
References: 82
Accession Number: 2018-090958
Categories: Applied geophysicsEconomic geology, geology of energy sources
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, sketch map
N26°00'00" - N30°00'00", W94°00'00" - W89°00'00"
Secondary Affiliation: Oklahoma State University, USA, United StatesU. S. Geological Survey, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute.
Update Code: 201849
Program Name: USGSOPNon-USGS publications with USGS authors
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