Ninth international conference on - Marum

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72 Abstracts of posters Modelling of fluid overpressure beneath a shallow marine gas hydrate province: Hikurangi Margin, New Zealand G. J. Crutchley 1 , I. A. Pecher 2 , S. Geiger 2 , A. R. Gorman 1 , S. A. Henrys 3 1 Department of Geology, University of Otago, PO Box 56, Dunedin 9054, New Zealand 2 Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh EH14 4AS, UK 3 GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand The crest of Rock Garden, a shallow bathymetric expression within New Zealand’s Hikurangi Margin gas hydrate province, is eroded close to the top of the regional base of gas hydrate stability (BGHS). This suggests a strong link between the gas hydrate system and slope failure (Pecher et al., 2005). Ten high-resolution 2D seismic lines, acquired over Rock Garden in 2006 as part of a specialised gas hydrate research cruise, reveal numerous pockets of gas close to the seafloor. They manifest seismically as regions of anomalous amplitudes, and are focused mostly beneath the predicted BGHS (e.g. Figure 1 - examples from north-eastern Rock Garden). This suggests that gas may be at least partially trapped by a permeability contrast between sediments below (free of gas hydrate) and sediments above (partially saturated with gas hydrate, reducing permeability). The gas pockets have the potential to contribute excess fluid pressure and ultimately load pre-existing structures or intact sediments to failure. We are currently conducting 2D finite element modelling to determine the sensitivity of the system to the gas pockets and to permeability contrasts at the BGHS. Our first models are aimed at determining which conditions cause sufficient over-pressure for failure by hydro-fracturing. Initial results show how important the permeability contrast is to generation of overpressure at the BGHS. Partial saturation of free gas within these pockets adds only a minor contribution to the excess fluid pressure. Failure by other modes, such as reactivation of pre-existing faults, will also be considered. Fig. 1. Oblique view looking down on the north-eastern reaches of Rock Garden. (A): Sub-figure showing the tectonic setting of New Zealand and the position of Rock Garden - marked by the ‘X’. (B): Sub-figure showing bathymetry of north-eastern Rock Garden, with the seafloor reflections of five seismic lines overlain. (C): The main figure displaying seismic data from the lines shown in (B) by stripping away the bathymetry – the field of view is the same as in (B). The seafloor is highlighted white, the interpreted BGHS is marked by the broken black line, and major gas pockets are marked. A first assessment of the relationship between sedimentation rates, organic matter preservation, and gas generation; Ría de Vigo, NW Spain E. de Blas 1 , D. Zúñiga 2 , J. Iglesias 3 , F. Alonso-Pérez 2 , N. Martínez 3 , E. Anfuso 2 , C. G. Castro 2 , S. García-Gil 3 1 Dpt. Biología Vegetal y Ciencia del Suelo, University of Vigo, Spain 2 Instituto de Investigacións Mariñas (IIM), CSIC, Vigo, Spain 3 Dpt. Geociencias Marinas, University of Vigo, Spain Ría de Vigo is one of the Galician Rías Baixas, four embayments which lie near the northern boundary of the NW African coastal upwelling system. From May to October prevailing winds cause the upwelling of nutrientrich Eastern North Atlantic Central Waters on the continental shelf and into the rías, promoting a massive increase in phytoplankton biomass. Primary production rises to an average of 1.4 g C.m -2 .day -1 (range 0.1 to 3 g C.m -2 .day -1 ) according to 14 C measurements. This high primary production enables the Rías Baixas to support
Abstracts of posters 73 the highest mussel production in Europe (~243 x 10 6 kg.y -1 of edible mussels). Mussel farming seems to alter the trophic network of the rías’ ecosystem, and affects the environmental conditions of the seabed providing suitable conditions for methanogenesis close to the seabed. In this study, comparisons are made between the sedimentation rates, the preservation of organic matter, and gas generation at two sites in the Ría de Vigo. One site is located in an area which has been occupied by mussel rafts for approximately the last 50 years. In the core collected adjacent to a raft there is a clear boundary between the normal ría sediments and the relatively under-compacted sediments resulting from mussel farming; this anthropogenic-mediated sediment is approximately 60-80 cm thick. Deeper within this core gas voids are present, supporting the interpretation of acoustic turbidity on high resolution seismic profiles as gas. At the control site, outside the mussel farming area, there are no such voids and there is no acoustic turbidity. Sedimentation rates and the rates of deposition of particulate organic carbon (POC) were estimated using sediment traps deployed during the autumn, winter and spring seasons (Fig. 1). Core sample data provide total organic carbon (TOC), and oxidation/reduction potential (Eh; Fig. 2). The total sedimentation rate based on sediment trap data is higher beneath the mussel rafts (~1.32 and 2.35 cm.yr -1 during the autumn and winter, respectively) compared to the control site (~0.17 cm.yr -1 for each of the two seasons). Although the rate of accumulation of POC beneath the rafts (accumulation rates average: ~2,180 mg.m -2 .d -1 ) is high compared to the control site (1,037 mg.m -2 .d -1 ), this equates to only 4.8% of the total sedimentation compared to 17.5% at the control site. At the control site the majority of the organic matter at and near the seabed is utilised under the prevailing sulphate reduction conditions (Eh 0 to -150 mV). In consequence the TOC preserved in the sediment is
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Program overview Monday September 1
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9 th International Conference on Ga
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Wednesday, Sept 17, 2008 09:00 - 09
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Thursday, Sept 18, 2008 09:00 - 09:
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Ninth international conference on - Marum

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