ACCESS NUMBER: 39076 (M05AC12295) 

STUDY TITLE: Multicomponent and Multifrequency Seismic for Assessment of FluidGas Expulsion Geology and Gas Hydrate Deposits: Gulf of Mexico 

REPORT TITLE: Multicomponent and Multifrequency Seismic for Assessment of FluidGas Expulsion Geology and Gas-Hydrate Deposits: Gulf of Mexico Hydrates 

CONTRACT NUMBER: 1435-01-05-39076 (M05AC12295) 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREAS: Central and Western 

FISCAL YEARS OF PROJECT FUNDING: FY 2005: $150,000; FY 2006 $100,000; FY 

2007: $49,974 

COMPLETION DATE OF REPORT: December 2010 

COSTS: $499,974 

PROJECT MANAGER: H.H. Roberts 

AFFILIATION: Coastal Studies Institute, Louisiana State University 

ADDRESS: Louisiana State University, Coastal Studies Institute, Baton Rouge, 

Louisiana 70803 

PRINCIPAL INVESTIGATORS*: Hardage, B.A. and H.H. Roberts 

KEY WORDS: Hydrates, four-component ocean bottom cable seismic, seismic, Gulf of 

Mexico 

BACKGROUND: The northern Gulf of Mexico continental slope is a well-documented 

province where fluids and gases migrate to the modern seafloor at numerous sites. 

Many of these sites are accompanied by exposed gas hydrate deposits. Migration 

pathways and gas hydrates have been difficult to image because of the presence of 

bubble-phase gas and very complex geology (the product of a dynamic interaction 

between sedimentary loading and salt deformation). In the complex geology of the 

northern Gulf’s hydrate stability zone there are very few reflection events that can be 

interpreted as bottom simulating reflectors (BSRs). These BSR’s generally represent 

the phase change between gas hydrate and free gas at the base of the hydrate stability 

zone, but they are difficult to identify in existing p-wave seismic data sets from the 

northern Gulf. In simple geologic settings, BSRs clearly cut across subsurface 

stratigraphy. However, as the geologic complexity of the subsurface increases, BSRs 

become more difficult to identify and trace. Even though the entire continental slope is 

covered with high quality 3D-seismic, migration pathways for fluids and gases, as well 

as gas hydrates, are not well-imaged and details of these features have generally 

alluded geoscientists. 
ACCESS NUMBER: 39076 (M05AC12295) 

This study provides an unparalleled research opportunity to employ a new 

multicomponent seismic data set collected from across a large area of the northern Gulf 

of Mexico’s upper continental slope (to a depth of 1000 m). These data were acquired 

to provide a new imaging capability. WesternGeco has collected the long offset 

multicomponent seismic data (four-component ocean bottom cable seismic, 4C-OBC) 

from which both shear wave and conventional compressional wave (p-wave) data can 

be derived. Not only will this long offset 4C-OBC data allow deeper imaging within the 

sediment column, but the c-wave data (up-going SV shear wave created by p-to-SV 

mode conversion) made possible with this acquisition strategy images through gas and 

image different stratal surfaces and boundaries, unlike standard p-wave data. In 

addition, lithofacies data, as well as shear moduli and bulk moduli, can be calculated 

from 4C-OBC data. We therefore consider this research to be a great opportunity to 

image migration pathways from the deep subsurface and to provide stratal information 

that can help define the base of the hydrate stability zone and properties of sediments 

invaded by gas hydrate. Analysis of 4C-OBC data also provides us with a means to 

assess c-wave data, with a focus on cross-over information that may be important for 

interpreting p-wave data of various frequency contents (8-100 Hz 3D-seismic to 2-8 

kHz). Three test sites were established, based on relative fluid-gas expulsion rates: 

slow flux, intermediate flux, and rapid flux. These qualitative classes of flux rate are 

based on geologic response at the modern seafloor. The three study sites are located 

in the Green Canyon lease area in lease blocks 204, 237, and 240. Seismic profiles 

with three different frequency contents were collected along common track-lines across 

each experimental site. One of these three seismic types was 4C-OBC data. The 

others are standard exploration-scale 3D-seismic and much higher frequency “chirp 

sonar” subbottom data. The c-wave data from the 4C-OBC data provide valuable new 

insight critical for resource evaluation in the case of gas hydrates and imaging of fault 

systems necessary for constructing realistic numerical models of fluid and gas migration 

from the deep subsurface. In addition, critical geologic differences between slow flux to 

rapid flux settings may be revealed. Linking very high resolution images of the shallow 

subsurface using AUV and chirp technology with the 4C-OBC data is one goal of this 

investigation. 

OBJECTIVES: The overall objective of the research project described in this report 

was to investigate the use of 4-C OBC data from our three study sites for identifying 

properties within the shallow stratigraphic section that relate to the presence and 

amount of gas hydrate. 

DESCRIPTION: Assessment of gas hydrate within the shallow stratigraphic section of 

deep water areas has been a problem without having direct evidence provided by 

drilling and associated core samples. The use of four-component ocean-bottom-cable 

(4-C OBC) technology offers an alternative to standard surface-towed 3D-seismic 

technology for indirect and remotely sensed evaluation of the geologic configuration of 

the subsurface and for investigating the occurrence of gas hydrate that occurs in the 

shallow stratigraphic section. This technology involves a surface acoustic source 

(usually an air-gun) and long lines of ocean bottom sensors that are capable of 

recording three-dimensional vector motion of the seafloor with 3-component geophones, 

as well as scalar pressure variations with hydrophones. The advantage of the 4-C OBC 
ACCESS NUMBER: 39076 (M05AC12295) 

data-collection is that standard compressional wave data (P-P), as well as converted 

shear wave data (P-SV), can be acquired simultaneously as backscatter from 

subsurface reflection horizons. These data are acquired in approximately the 10-200 

Hz frequency range. From a regional grid of data acquired by WesternGeco in the 

northern Gulf of Mexico, outer-shelf to upper slope depth range, this study was given 

access to 4-C OBC data associated with three sites within the Green Canyon Lease 

Area: GC 204, GC 237, and GC 240. Direct observations from manned submersible 

dives confirm gas hydrate deposits as surface exposures at two of these sites, GC 204 

and GC 237. Only the GC 240 site has no direct confirmation of gas hydrate at the 

modern seafloor. 

The GC 204 site is adjacent to Chevron’s Genesis Field, and BHP-Billiton’s Typhoon 

Field is adjacent to our GC 237 site. Both of these fields had well logs available for use 

in our study. Well log data were extremely important to this study for calibrating 

acoustic properties of the subsurface to rock properties. Most well log data spanned at 

least part of the gas hydrate stability interval and were therefore important calibration 

data used to make estimations of gas hydrate concentrations, especially the resistivity 

log measurements. Only the GC 240 site did not have supporting well log data from an 

adjacent oil and/or gas field. 

A ray tracing method was used to adjust the thicknesses of equivalent stratigraphic 

intervals derived as separate P-P and P-SV images. Since representations of the 

subsurface are different using P-P as compared to P-SV data, the ray tracing method 

was employed to adjust layer thickness and interval velocities (Vp and Vs) so that other 

datasets such as those from resistivity logs could be compared to converted shear wave 

data (P-SV) images of the subsurface. The interval from the seafloor to the base of the 

hydrate stability zone was analyzed at each of our three study sites. Using this 

methodology, velocity layers or stratigraphic units were defined as a function of depth 

below the seafloor. 

SIGNIFICANT CONCLUSIONS: Estimates of gas hydrate were made for calibration 

wells using primary data from resistivity logs and seismic velocity estimates (Vp and Vs) 

for given stratigraphic intervals defined from the ray tracing method. Depth dependent 

increases in Vp and Vs were incorporated and used in the gas hydrate assessment 

employing normal sediment compaction theory. Stratigraphic intervals within the gas 

hydrate stability zone where both interval velocities and resistivity values were greater 

than those expected with normal compaction were interpreted as zones of hydrate 

concentration. Using a probability function described in this report, the amount of pore 

space occupied by gas hydrate was calculated. At intervals of 10 receiver stations (250 

m) along each OBC profile, velocity layering and estimated gas hydrate concentrations 

were calculated. Our results indicate: (1) gas hydrate was pervasive throughout the 

sites we studied in the Green Canyon lease area; (2) concentrations of gas hydrate 

ranged from zero to one-third of available pore space, but values of 10-20% of the pore 

volume were typical of most areas; (3) free-gas was found immediately underlying the 

base of the gas hydrate stability zone (not easily determined from resistivity logs alone); 

and, (4) the estimation of gas hydrate stability zone thickness from resistivity logs alone 

produced an overestimation of gas hydrate zone thickness and amount present. 
ACCESS NUMBER: 39076 (M05AC12295) 

STUDY RESULTS: A key to interpreting the presence of gas hydrate and perhaps 

bubble-phase gas in stratigraphic intervals defined by velocity data was the integration 

of several primary datasets: (1) resistivity (well log data), (2) interval velocity, and (3) 

seismic images (P-P and P-SV). However, the relationships between gas hydrate 

occurrence/concentration with resistivity data, as well as the relationships between 

seismic propagation velocity and gas hydrate occurrence/concentration, were found to 

be non-unique. Spatial variations in host sediment properties and the nature of gas 

hydrate (e.g., small crystals in pore spaces to massive layers) within the hydrate 

stability zone accounted for much of the uncertainty in interpretations. By combining 

gas hydrate-dependent geophysical data (e.g., velocity estimates and formation 

resistivity measurements), predictions of gas hydrate concentrations were constrained. 

A rock physics model that related seismic velocity to gas hydrate concentration was 

used (methodology described in Appendix C). This model accounted for the uncertainty 

of all parameters used in the calculation of gas hydrate concentration. 

Applying our analysis methodologies to 4-C OBC data calibrated to well log data in 

Typhoon Field area (lease blocks surrounding GC 237) the following conclusions can be 

drawn: 

1. Gas hydrate occurs in the shallow subsurface throughout the 

Typhoon Field area and it manifests itself at the seafloor in GC 237 

where flux rates of gas and fluids toward the modern ocean bottom 

eliminate the sulfate reducing zone where gas hydrate does not form. 

2. Depth of the hydrate-bearing stratigraphic interval beneath the 

Typhoon Field is approximately 460 m. 

3. Resistivity relationships indicate that gas hydrate concentrations 

within the approximately 460 m thick zone of hydrate stability range 

from approximately 20-40 % of available pore space. 

Our results from analyzing 4-C OBC seismic data and well logs from the Genesis Field 

(lease blocks surrounding GC 204) lead to the following conclusions: 

1. Data suggest that the shallow subsurface associated with the 

Genesis Field supports more gas hydrate than the Typhoon Field 

area. 

2. The gas hydrate stability zone ranges in thickness across the 

Genesis Field area from approximately 365 m to 760 m. 

3. Within the variable zone of gas hydrate stability, we estimate that 20-

40 % of the pore space is filled with gas hydrate. 

Because no well logs were available for our third study site, GC 240, gas hydrate 

concentrations were interpreted from seismic interval velocities only. The following 

conclusions can be stated: 

1. The gas hydrate stability zone is highly variable and ranges from less 

than 800 to nearly 1400 m. 
ACCESS NUMBER: 39076 (M05AC12295) 

2. Compression of the hydrate stability zone is related to a prominent 

expulsion center in the study area where fluid/gases and heat are 

transported toward the modern seafloor. This process moves the 

base of the hydrate stability zone (BHSZ) stratigraphically upward. 

3. Estimates of gas hydrate in the pore space of host sediments based 

on velocity data alone range from approximately 12 to 30% of the 

pore space. 

STUDY PRODUCTS: Hardage, B.A. and H.H. Roberts. 2010. Multicomponent and 

multifrequency seismic for assessment of fluid-gas expulsion geology and gas-hydrate 

deposits: Gulf of Mexico hydrates. U.S. Dept. of the Interior, Bureau of Ocean Energy 

management, Regulation and Enforcement, Gulf of Mexico OCS Region, New Orleans, 

LA. OCS Study BOEMRE 2010-046. 96 pp. 

Hardage, B.A. H.H. Roberts, D.C. Sava, P.E. Murray, M.V. DeAngelo, M.M. Baskus, 

and R.J. Graebner, 2008, Assessing fluid-gas expulsion geology and gas hydrate 

deposits across the Gulf of Mexico with multicomponent and multifrequency seismic 

data: proceedings of 6th International Conference on Gas Hydrates, July 6-10, 

Vancouver, BC, Canada, 12 p. 

Roberts, H.H., B.A. Hardage, W.W. Shedd, and J. Hunt, Jr., 2006, Seafloor reflectivity: 

An important seismic property for interpreting fluid/gas expulsion geology and the 

presence of shallow gas hydrate: Transactions GCAGS, v. 3, p. 725. 

Hardage, B.A., P. Murray, D. Sava, M.M. Backus, R. Remington, and R. Graebner, 

2006, Evaluation of deepwater gas-hydrate system: The Leading Edge, Society of 

Exploration Geophysicists, v. 25, p. 572-576. 

Backus, M.M., P.E. Murray, B.A. Hardage, and R. Graebner, 2006, High-resolution 

multicomponent seismic imaging of deep-water gas hydrate systems: The Leading 

Edge, Society of Exploration Geophysicists, v. 25, p. 578-596. 

Hardage, B.A. and H.H. Roberts, 2006, Multicomponent seismic technology for 

assessment of fluid-gas expulsion geology and gas hydrates: Gulf of Mexico: 

Proceedings of AAPG Hedberg Research Conference, Vancouver, British Columbia. 

Roberts, H.H., B.A. Hardage, W.W. Shedd, and J. Hunt, Jr., 2006, Seafloor reflectivity – 

An important seismic property for interpreting fluid/gas explosion geology and the 

presence of gas hydrate: The Leading Edge, Society of Exploration Geophysicists, v. 

25, p. 620-628. 

Hardage, B.A. and H.H. Roberts, 2006, Gas hydrate. A source of shallow water flow? 

The Leading Edge, Society of Exploration Geophysicists, v. 25, p. 634-635. 
ACCESS NUMBER: 39076 (M05AC12295) 

 

Hardage, B.A., P. Murray, D. Sava, R. Remington, R. Graebner, and H.H. Roberts, 

2006, Evaluation of deepwater gas-hydrated systems: The Leading Edge, Society of 

Exploration Geophysicists, v. 25, p. 572-576. 

Hardage, B.A. and H.H. Roberts, 2006, Gas hydrate in the Gulf of Mexico: What and 

where is the seismic target: The Leading Edge, Society of Exploration Geophysicists, v. 

25, p. 566-571. 

Hardage, B.A., H.H. Roberts, D.C. Silva, P.E. Murray, M.V. DeAngelo, M.M. Backus, 

and R.J. Graebner, (Submitted), Assessing Fluid-Gas Explution Geology and Gas 

Hydrate Deposits Across the Gulf of Mexico with Multicomponent and Multifrequency 

Seismic Data: Proceedings ICGH and Marine Geology. 

*

 P.I.’s affiliation may be different than that listed for Project Managers.