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3D controlled-source electromagnetic imaging of gas hydrates; insights from the Pelotas Basin offshore Brazil

Raghava Tharimela, Adolpho Augustin, Marcelo Ketzer, Jose Cupertino, Dennis Miller, Adriano Viana and Kim Senger
3D controlled-source electromagnetic imaging of gas hydrates; insights from the Pelotas Basin offshore Brazil
Interpretation (Tulsa) (November 2019) 7 (4): SH111-SH131


Mapping of natural gas hydrate systems has been performed successfully in the past using the controlled-source electromagnetic (CSEM) method. This method relies on differentiating resistive highly saturated free gas or hydrate-bearing host sediment from a less resistive low-saturated gas or brine-bearing host sediments. Knowledge of the lateral extent and resistivity variations (and hence the saturation variations) within sediments that host hydrates is crucial to be able to accurately quantify the presence of saturated gas hydrates. A 3D CSEM survey (PUCRS14) was acquired in 2014 in the Pelotas Basin offshore Brazil, with hydrate resistivity mapping as the main objective. The survey was acquired within the context of the CONEGAS research project, which investigated the origin and distribution of gas hydrate deposits in the Pelotas Basin. We have inverted the acquired data using a proprietary 3D CSEM anisotropic inversion algorithm. Inversion was purely CSEM data driven, and we did not include any a priori information in the process. Prior to CSEM, interpretation of near-surface geophysical data including 2D seismic, sub-bottom profiler, and multibeam bathymetry data indicated possible presence of gas hydrates within features identified such as faults, chimneys, and seeps leading to pockmarks, along the bottom simulating reflector and within the gas hydrate stability zone. Upon integration of the same with CSEM-derived resistivity volume, the interpretation revealed excellent spatial correlation with many of these features. The interpretation further revealed new features with possible hydrate presence, which were previously overlooked due to a lack of a clear seismic and/or multibeam backscatter signature. In addition, features that were previously mapped as gas hydrate bearing had to be reinterpreted as residual or low-saturated gas/hydrate features, due to the lack of significant resistivity response associated with them. Furthermore, we used the inverted resistivity volume to derive the saturation volume of the subsurface using Archie's equation.

ISSN: 2324-8858
EISSN: 2324-8866
Serial Title: Interpretation (Tulsa)
Serial Volume: 7
Serial Issue: 4
Title: 3D controlled-source electromagnetic imaging of gas hydrates; insights from the Pelotas Basin offshore Brazil
Affiliation: Electrojagnetic Geoservices Asia Pacific, Kuala Lumpur, Malaysia
Pages: SH111-SH131
Published: 201911
Text Language: English
Publisher: Society of Exploration Geophysicists, Tulsa, OK, United States
References: 75
Accession Number: 2020-011791
Categories: Economic geology, geology of energy sourcesApplied geophysics
Document Type: Serial
Bibliographic Level: Analytic
Annotation: Part of a special section on Brazil, edited by Egorov, V. et al.
Illustration Description: illus. incl. 1 table, geol. sketch map
S32°00'00" - S32°00'00", W51°00'00" - W51°00'00"
Secondary Affiliation: Instituto do Petroleo e dos Resursos Naturais, BRA, BrazilLinnaeus University, SWE, SwedenPETROBRAS, BRA, BrazilUniversity Centre in Svalbard, SJM, Svalbard and Jan Mayen Islands
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute.
Update Code: 2020
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