Fluid Focusing in Compacting Marine Sediments: Implications on the Age and Spatial Extent of the Methane Source for Gas Hydrates
Abstract
Pore fluids in compacting marine sediments are often much older than the host sediments, even when fluid expulsion due to sediment compaction is taken into account. Pore fluid age is measured using its isotopic ratio of iodine,129-I/I, which originates from decaying organic material within the sediments. Therefore, the pore fluid age has been used by many as a guide to locate possible methane sources for hydrate formations, assuming a pervasive upward fluid flow. More complex flows, however, are likely to exist in natural settings due to topography- and fracture-driven pore fluid focusing. We developed a two-dimensional numerical transport model which shows that fluid focusing due to topography and fractures can substantially alter pore fluid pathlines beneath topographic features and in close proximity to fractured zones. We find that focusing can cause significant lateral fluid migration as well as regions where downward flow reverses direction and returns toward the seafloor. Large, looping pathlines can produce pore fluid ages 2-4 times older than that expected without focusing. Ages modeled for Blake Ridge (USA), a well-studied hydrate province, are compared to reported observations. We find pore fluid ages beneath regions of topography and within fractured zones that are between 20-70 million years old. Our results suggest that the source of methane in hydrate deposits such as Blake Ridge may be a mix of new and old sources. However, old sources need not originate at great depths. Methane within pore fluids can be laterally tapped from several kilometers, implying an extensive source region around the deposit. This type of focusing may aid hydrate formation beneath topographic highs.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMOS12A..05F
- Keywords:
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- 3004 MARINE GEOLOGY AND GEOPHYSICS / Gas and hydrate systems;
- 3021 MARINE GEOLOGY AND GEOPHYSICS / Marine hydrogeology;
- 4255 OCEANOGRAPHY: GENERAL / Numerical modeling;
- 4918 PALEOCEANOGRAPHY / Cosmogenic isotopes