Gas Hydrate Occurrence Across the Cascadia Margin From Pore Water Chlorinity and Downhole Geophysical Logs (IODP Exp. 311)
Abstract
IODP Exp. 311 drilled a transect of four sites crossing the northern Cascadia convergent margin in a slow, diffuse fluid flow environment. These transect sites are an ideal data set to compare variation in gas hydrate (GH) occurrence with distance from the deformation front. In this study, we quantify the variation with depth of GH saturation (fraction of pore space occupied by GH) from pore water chlorinity and downhole logs of porosity and electrical resistivity. At each site we determine a GH occurrence zone (GHOZ), defined as the depth interval where gas hydrates are actually observed. The GHOZ is not necessarily the same as the GH stability zone (GHSZ), which is the depth interval where gas hydrates are thermodynamically stable. In three of the Exp. 311 transect sites gas hydrates occur down to the base of GH stability, but the top GHOZ is not at the seafloor (the top of the GHSZ). The top GHOZ deepens moving away from the deformation front, from 47 mbsf at Site U1326 (5 km from the deformation front), to 73 mbsf at Site U1325 (11 km), and 111 mbsf at Site U1327 (21 km). Site U1329 (38 km from the deformation front) shows no clear evidence of GH. Fluid expulsion from the accretionary wedge has been proposed as the main process to advect methane and form GH at the Cascadia margin. Published studies have quantified fluid expulsion rates from the compaction of sediments that are scraped off the subducting plate and are incorporated into the accretionary wedge. These compaction models generally show fluid expulsion rates that decrease sharply moving inland from the deformation front. The Exp. 311 transect sites show a GHOZ that thins moving away from the deformation front and correlates with the decrease in fluid expulsion rates predicted by the compaction models. Our results are consistent with fluid expulsion being a primary control on the formation of GH in the Cascadia convergent margin.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFMOS11C..05M
- Keywords:
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- 0915 Downhole methods;
- 1051 Sedimentary geochemistry;
- 3004 Gas and hydrate systems;
- 3036 Ocean drilling