Seal Mechanisms in Shallow Sediments: Implications for Shallow-Water Flow and Gas-Hydrate Hazards

T. J. Katsube¹, K. O. Horkowitz², I. R. Jonasson¹, and D. Piper^3
1 Geological Survey of Canada, Ottawa, ON
² Resource Analysts, Sugar Land, TX
³ Geological Survey of Canada, Dartmouth, NS

A study on possible shallow seal mechanisms has been carried out to analyse the impact they could have on shallow water flow (SWF) and gas-hydrate hazards and on the sediment textures that could lead to their detection.

Laboratory experimental studies have shown that clay-rich sea floor sediments can have permeabilities as low as 100-1000 nD (10^-19-10^-18 m²) under confining pressures equivalent to sub-bottom depths of 200-500 m. This implies that seals of poor quality could exist at relatively shallow depths below the mud-line. Such seals would likely be of poor integrity, but sufficient as a source of SWF with overpressures of 1-4 MPa above hydrostatic pressure. In areas of up-flowing gas, the quality of these seals may also be sufficient to move the temperature-pressure regime of pore pressures into the gas-hydrate stability zone for gas-hydrates to form in the shallow sediments. Once gas-hydrates form, they would enhance the seal quality of the sediments and promote further gas-hydrate formation. There are also indications that bacterial activity can produce carbonate and sulphide cement by oxidation of methane gas and reduction of sea water, respectively, and that these could form effective seals in areas of up-flowing gas. In addition to these seal mechanisms, shallow seals may be formed due to bound-water layer thickness increase by decreased temperatures and fluid expulsion from high sedimentation rates. These seal mechanisms could have significant effect on underlying formation texture that would affect the geophysical signals related to SWF and gas-hydrate hazard risks.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005