Faulting of Gas-Hydrate-Bearing Marine Sediments — Contribution to Permeability

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 as gas 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. The faulting 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 cover a 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.l) 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 currents'-'. 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 margin. The surface of the ridge is generally smooth except for minor differential erosion features on its northern (eroding) flank, but, at the crest of the ridge at about 31° 50'N to 32°N, a complex topographic depression covers an area of about 33 by 22 km (Fig. 2) Faults Shallow Faults. Seismic-reflection profiles clearly show that the complex topographic depression is a faulted structural collapse (Fig. 3). Faulting took place in a surface layer of sediment about 0.5 to 0.6 s thick (about 400 to 500 m). Seismic profiles indicate that these faults consistently extend from the sea floor to near the base of the gas-hydrate stability zone. The base of the gas-hydrate stability zone is assumed to be identified by the Bottom Simulating Reflection (BSR, note