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Interpretations of Focused Fluid Flow Leading to Highly Concentrated Gas Hydrate Deposits


Figure 5: Gas Chimney in Line T8 – Seismic Data and Fluid Flow interpretations


400m 1.5 A 400m B hi 2.0


A: Seismic data from line T8 in the trough between Paoanui Ridge and Rock Garden (4x vertical exaggeration at 1,500ms-1). B: Plot of instantaneous amplitudes from the region of data given in A, with various interpretations overlain: thin black lines are faults, thick grey lines mark BSRs, dotted black lines delineate gas chimney extents, and arrows indicate proposed directions of enhanced fluid flow.


Figure 6: Gas Chimney in Line T7 – Seismic Data and Fluid Flow Interpretations


400m 1.5 A 400m B hi lo


secondary permeability resulting from faulting and fracturing is likely to be very important for the transmission of fluids through the GHSZ. We have focused on regions of the ridge top where large pockets of gas, interpreted from seismic data, appear to penetrate the GHSZ (see Figures 3 and 4). These regions represent the most attractive targets for highly concentrated hydrate beneath the ridge top as the gas may provide both a driving force for generation of fluid flow conduits into the GHSZ, via excess fluid pressure-induced structural failure, and high methane concentrations for the precipitation of gas hydrate.


As the rocks of the Rock Garden ridge probably possess (in general) low primary porosity,4,5


fluid flow pathways into and through the GHSZ. In turn, these processes are likely to influence where highly concentrated deposits of gas hydrate are forming.


To the west of the ridge, the seismic sections reveal a much more continuous and coherent character of basin-fill strata. The best evidence for focused fluid flow in this region comes from the low-amplitude chimney structures associated with deformation in the active thrust belt. Both the chimneys themselves and the surrounding strata are attractive from the perspective of anomalous hydrate concentrations, as seismic data suggest that fluids are focused sub-vertically through the chimneys but also away from the chimneys along dipping basin strata. n


Acknowledgements 2.0


A: Seismic data from line T7 in the trough between Paoanui Ridge and Rock Garden (4x vertical exaggeration at 1,500ms-1). B: Plot of instantaneous amplitudes from the region of data given in A, with various interpretations overlain: thin black lines are faults, thick grey lines mark BSRs, dotted black lines delineate gas chimney extents, and arrows indicate proposed directions of enhanced fluid flow.


Conclusions


The relationships between sedimentary strata and structural deformation at Rock Garden are very important for the deflection and focusing of


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We thank Steve Wilkox, Peter Gerring and John Mitchell for the acquisition of CHARMNZ seismic and bathymetry data and we gratefully acknowledge the excellent efforts made by captains and crews of R/V Tangaroa and F/S Sonne during Hikurangi Margin research campaigns. Seismic processing was carried out with an academic licence to the Globe Claritas seismic processing software and some of the figures made use of GMT plotting software. This research was funded through a Royal Society of New Zealand Marsden Grant, contract number GNS0403.


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EXPLORATION & PRODUCTION – VOLUME 8 ISSUE 2


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