^PSCan Fractures in Soft Sediments Host Significant Quantities of Gas Hydrates?*

By

Thomas M. McGee ¹, Carol B. Lutken ¹, Rudy E. Rogers ², Charlotte A. Brunner ³, J.S. Dearman ², F. L. Lynch ², and J. Robert Woolsey ¹

Search and Discovery Article #400167 (2005)

Posted August 22, 2005

*Poster presentation at AAPG Annual Convention, Calgary, Alberta, June 19-22, 2005.

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       Poster 1       Poster 2

¹University of Mississippi, University, MS ([email protected])

²Mississippi State University, Mississippi State, MS

³University of Southern Mississippi, Stennis Space Center, MS

Abstract 

Current interest concerning what types of geologic features contain significant accumulations of gas hydrate arises from the expectation that some day commercial quantities of natural gas will be produced from hydrates. Various geologic structures within the hydrate stability zone have been imaged, seismically, but there is little consensus concerning serious candidates for exploratory drilling. Some investigators favor targeting sandy sediments where porosity and permeability are greater than in silts and clays. Others expect fractures within fine-grained sediments may host greater volumes of hydrates. The latter scenario seems to fit better with conditions in the hydrate stability zone in the northern Gulf of Mexico and with laboratory results. Hydrates have been created in the laboratory by adding natural gas, sea water, and naturally occurring microbial surfactants to artificial sediments comprised of smectite, kaolinite and sand under appropriate conditions of pressure and temperature. Findings show that biosurfactants greatly enhance hydrate formation and that hydrates form preferentially on smectite (a known component of soft sediments in the Gulf) rather than kaolinite or sand. Given sufficient natural gas, all that remains to complete the formation of hydrates is a mechanism of producing a dense population of fractures open to gas and water circulation. This presentation postulates that the mechanism is polygonal faulting and provides supporting evidence.

Location map showing Mississippi Canyon (red dot delineates study area).

Conclusions 

The sea floor in the vicinity of MC798 lies within the hydrate stability zone, and the base of that zone occurs about 400 m below the sea floor where it is marked by a prominent negative seismic reflection. Features (“brooms”) associated with polygonal faulting elsewhere are visible on seismic profiles throughout the region within 100 m of the sea floor. Perhaps they are present at greater depths but have not been resolved seismically. Also, the “brooms” have been observed to occur in at least two tiers separated by a mobile layer.

Analyses of core samples have shown that the sea-floor sediments in the region are composed predominantly of clay-sized particles in which polygonal faults could develop. Moreover, the clay consists largely of smectite which has been demonstrated to promote the formation of hydrates by decreasing the induction time.

It is therefore concluded that it is possible, perhaps even probable, that a polygonal fault system exists within the hydrate stability zone in the vicinity of MC798. If so, it could provide the fracture porosity to facilitate circulation of gas and water and thereby host significant accumulations of gas hydrate. The convincing evidence would be a horizontal slice through a 3-D volume that confirms that the “brooms” do, indeed, correspond to faults that intersect in a polygonal pattern. It is planned to collect such a volume during 2005.