Faulting of Gas-Hydrate-Bearing Marine Sediments - Contribution to Permeability
Authors: Dillon William , Holbrook W. Steven , Drury Rebecca , Hutchinson Deborah , Booth James , Taylor Michael 
Published: October 2008

Extensive faulting is observed in sediments containing high concentrations of methane hydrate off the southeastern coast of the United States. Faults that break the sea floor show evidence of both extension and shortening; mud diapirs are also present. The zone of recent faulting apparently extends from the ocean floor down to the base of gas-hydrate stability. We infer that the faulting resulted from excess pore pressure in gas trapped beneath the gas hydrate-bearing layer and/or weakening and mobilization of sediments in the region just below the gas-hydrate stability zone. In addition to the zone of surface faults, we identified two buried zones of faulting, that may have similar origins. Subsurface faulted zones appear to act asgas traps.

Introduction

Methane hydrate in continental margin settings, especially in passive margins, commonly occurs in hemipelagic clay/silts. These deposits appear to have some of the highest concentrations of hydrate, but they also have low permeability, presenting an impediment to extraction of hydrate-derived gas. Recently, seismic profiling by the U.S. Geological Survey and drilling by the Ocean Drilling Program (ODP) have been carried out on the Blake Ridge off the coast of the southeastern United States, a location having high gas-hydrate concentrations. Extensive faulting is noted in these deposits, probably partly associated with gas-hydrate processes. Thefaulting may create zones of high permeability that could act as reservoirs for exploitation, or that might contain shallow gas deposits that could represent drilling hazards.

Methods

Most of the profiles shown in this paper were collected with seismic systems designed to image the gas-hydrate zone and the region directly below it (1.5-2.0 s below the sea floor). They are part of a set of 4-km-spaced seismic profile that covera 4,800 square-kilometer area on the crest of the Blake Ridge. Moderately small, pneumatic seismic-sound sources provided adequate power to penetrate below the region of gas-hydrate stability, while still maintaining the best possible resolution. Sources used were a 160-cubic inch (2.62-liter) airgun or a Generator/injector (GI) gun, in which the generator chamber (which generates the primary signal) was 105 cubic inches (1.72 liters), and the injector chamber (which controls bubble pulsing) was also 105 cubic inches. Swept-frequency ("chirp", 2 to 7 kHz)) subbottom-reflection data were obtained from a deep-towed, multisensor system; they provide data on near-surface sediments (upper 60 m).

Blake Ridge

The Blake Ridge (Fig.1) is a broad, generally smooth sedimentary accretionary ridge - a deep-sea sediment drift deposit - that is accreting at the site of interaction of major ocean current1,2,3,4 Deposition on its southern flank and erosion on its northern flank have resulted in slow southward migration of the feature. The ridge is considered to be the area where gas hydrate may be most concentrated on the United States Atlantic margin5.