International Conference on Gas Hydrates (ICGH) (6th : 2008)

INTEGRATED GAS HYDRATE QUANTIFICATION OFF NICOYA PENINSULA – COSTA RICA Henke, Thomas; Müller, Christian; Marquardt, Mathias; Hensen, Christian; Wallmann, Klaus; Gehrmann, Romina 2008-07

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   INTEGRATED GAS HYDRATE QUANTIFICATION OFF NICOYA PENINSULA – COSTA RICA   Thomas Henke∗, Christian Müller Federal Institute for Geosciences and Natural Resources (BGR) Stilleweg 2, Hannover, 30655 GERMANY  Mathias Marquardt, Christian Hensen, Klaus Wallmann Leibnitz Institute of Marine Sciences at the University of Kiel (IFM-GEOMAR) Wischhofstr. 1-3, Kiel, 24148, GERMANY  Romina Gehrmann University of Leipzig Talstrasse 35, Leipzig, 04103, GERMANY   ABSTRACT The global estimates of methane stored in gas hydrates varied from 1018 to 1015 m3 over the last 4 decades. Each geoscientific discipline has its own quantification methods. The aim of the presented project is the combination of a well proven geochemical approach with a geophysical approach. A transfer function is presented which allows estimations based on geochemical and geophysical parameters. A first application of this combined approach has been performed along seismic line BGR99-44 off Costa Rica. The resulting concentration profile shows a differentiated distribution of the gas hydrate concentration along the slope of the margin with variations of 0 to 3 vol.% of pore space.  Keywords: gas hydrates, quantification, Geophysics, Geochemistry, Costa Rica   ∗ Corresponding author: Phone: +49 511 643 3145 Fax +49 511 643 3663 E-mail: INTRODUCTION The estimated amount of the global volume of hydrate-bound gas decreased over the last decades. One of the first estimations was done in the 1970s with an amount of about 1018 m3 gas. In the late 1980s the estimates were reduced to 1016 m3 gas due to the improved understanding of continental margin processes. The latest estimates were in the order of 1015 m3 gas in the late 1990s. This further decreasing estimate is caused by the results of the ODP/IODP drilling program and better quantification techniques [1]. Every geoscientific discipline as e.g. geophysics and geochemistry has its own approach for gas hydrate quantification. We present the combination of a geochemical and a geophysical approach to combine the advantages and to reduce the disadvantages of each approach. We are combining the geochemical approach of early diagenesis modelling which calculates the microbially mediated degradation of organic matter and transformation into methane, and the geophysical approach based on rock physics modelling which calculates the gas hydrate concentration from seismic velocities by the use of Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. an effective medium theory (EMT). The geochemical approach is a well proven approach with high data density at borehole sites and is able to calibrate the geophysical approach. The advantage of the geophysical approach is the large area covered by multichannel reflection seismic (MCS) data. Off Costa Rica the data base for the approaches are the boreholes of ODP-leg 170 and the 5500 km MCS from cruises SO81 and BGR99 (Figure 1). The main subject of the work is the derivation of a simplified transfer function to quantify the gas hydrate potential off Costa Rica.  87°W 86°W 85°W 84°W 83°W 8°N 9°N 10°N 11°N   SO81  BGR99 ODP-site 1039, 1040, 1041, 1043  plate boundaries ++  Costa Rica   Figure 1: Bathymetry off Costa Rica [2] with tracks of cruises SO81 and BGR99 as well as the borehole sites from ODP-leg 170 and plate boundaries after Barckhausen [3].   THE METHOD In this project a complementary approach has been developed using geochemical reactive-transport models and geophysical rock physics modelling to quantify regional GH inventories. Our preliminary result presented here is a simplified general transfer function which can be used to calculate GH volumes.  Geochemical approach The derivation of the transfer function required a thorough sensitivity analysis to determine the most important key control parameters in a transport- reaction model. The three most important control parameters are the sulphate penetration depth (SPD), sedimentation rate and the thickness of the gas hydrate stability zone (GHSZ). The GHSZ is limited either by the base of the sediment column or by the lower boundary of the GH stability field itself (often marked by the BSR). In addition, several geochemical transport-reaction models constrained on the DSDP/ ODP drill sites 685, 1230, 1233, 1040, 1041 and 1043 (Costa Rica, Peru and Chile) have been developed to obtain the natural variance of the different parameters and the resulting GH concentrations (Figure 2).    Figure 2: DSDS/ODP drill sites used to develop the transfer function to quantify gas hydrate concentration.  In order to derive the general simplified transfer function, the three control parameters and the pertinent resulting GH concentrations from each model have been used to set up a non-linear equation system. Solving that equation system by iteration results in the following function (1) to estimate the potential amount of gas hydrate (depth integrated in g/cm2 seafloor):  SDPe SDPGHSZd SDPSedRatec SDPSedRateb SPDSPDalGHpotentia * ** ** ** )ln(** 2 + + + + −=  (1)  The GH potential is the maximum potential of GH occurrence in g/cm2 seafloor. SPD, SedRate, 1040, 1041, 1043 685, 1230 1233 ODP/ DSDP Sites: GHSZ are the regional parameters and a, b, c, d, e are the respective determined coefficients.  Applying the general function requires incorporation of the aforementioned regional parameters i.e. SPD, SedRate, and the thickness of the GHSZ. The SPD and SedRate have to be obtained by sediments and pore water data of gravity cores. Interpretations of seismic records yield water depth, thermal gradient and sediment thickness (determination of the GHSZ).  0 40 80 120 sedimentation rate [cm/1000a] 0 20 40 60 80 G H  [g  / cm 2 ] 0 200 400 600 800 GHSZ [m] 0 10 20 30 40 50 G H  [g  / cm 2 ] 0 5 10 15 20 25 SO4 -penetration depth [m] 0 20 40 60 80 G H  [g  / cm 2 ]   Figure 3: Input parameters for the application in the transfer function to quantify gas hydrate concentration.  Geophysical approach The geophysical approach is based on rock physics modelling incorporating the Effective Medium Theory from Helgerud [4]. Helgerud calculates the elastic bulk and shear moduli from petrophysical parameters in combination with a gas hydrate formation model. The petrophysical parameters are derived from the ODP-site 1041 from Kimura et al. [5]. The porosity samples were taken from decompressed and degassed cores. These samples do not represent the in-situ conditions of the slope sediments. Therefore the porosity is calibrated by the geochemical approach. The formation model describes the triad between sediment matrix, pore fluid, and gas hydrate or free gas respectively. The gas hydrate can on the one hand be part of the pore fluid or on the other hand a cementation of the sediment grain matrix. The elastic moduli were used to calculate the seismic velocities vp and vs. The gas hydrate concentration is calculated from seismic velocity vp by the use of an inversion algorithm. The inversion algorithm uses a least square method to fit the modelled velocity vp, based on the EMT, to the seismic velocities obtained from seismic data with respect to the gas hydrate concentration. These seismic velocities are derived from Normal Move Out (NMO) analysis or a Prestack Depth Migration (PSDM) method. The NMO analysis generates stacking velocities which represent the velocities that eliminate the deflection of the reflection hyperbola and finally are transformed into interval velocities by the DIX equation. The PSDM method results in migration velocities providing the best possible imaging as a result of several iteration steps that minimise the residual move out.  RESULTS This combined approach was first applied to line BGR99-44 across the ODP-sites (Figure 4).  86°30’W 86°00’W 85°30’W 9°30’N 10°00’N BG R9 9-4 4 BGR99-48 1039 1040 1041 1043      BGR99 ODP-Leg 170++   Figure 4: Location of seismic lines from cruise BGR99 across borehole sites from ODP-leg 170.  Hensen & Wallmann [6] predicted a gas hydrate concentration of 2.5 vol.% of pore space for the area based on geochemical modelling at borehole site 1040. The presented result (Figure 5) applies the transfer function (1) and confirms and details the prediction of gas hydrate concentration across the slope sediments by subdividing them along a seismic line. It can be seen that the gas hydrate concentration varies between 0 and 3 vol.% of pore space. The reason of the variation can be explained by the methane generation. The methane flux, which is responsible for the SPD, is a result of biologic processes with a higher activity in lower water depth at the upper slope. Here we have the lowest SPD, which indicates a high methane production caused by high degradation rates of organic matter. Near the trench the degradation rate of organic material is lower than in the upper part so the concentration is decreasing. This can be caused by the decreasing input of organic material from the shelf to the trench.  REFERENCES [1] Milkov A.V. Global estimates of hydrate- bound gas in marine sediments: how much is really out there? Earth-Science Reviews, 2004, 66, 183-197. [2] Ranero C.R., Weinrebe W. Tectonic processes during convergence of lithospheric plates at subduction zones. In: P. Wille, ed., Sound Images of the Ocean, Springer Publisher, 2005. [3] Barckhausen, U., Ranero, C. R., von Huene, R., Cande, S. C. and Roeser, H. A., Revised tectonic boundaries in the Cocos Plate off Costa Rica: Implications for the segmentation of the convergent margin and for plate tectonic models. J. Geophys. Res., 106(B9), 19,207-19,220, 2001. [4] Helgerud M.B., Dvorkin J., Nur A., Sakai A. & Collett T.S. Elastic-wave velocity in marine sediments with gas hydrate: Effective medium modelling. Geophys. Res. Lett., 26, 2021-2024, 1999. [5] Kimura G., Silver E., Blum P., et al.. Proceedings of the Ocean Drilling Program, Initial Reports, 170. College Station, TX (Ocean Drilling Program), 1997. [6] Hensen, C., Wallmann, K. Methane formation at Costa Rica continental margin—constrains for gas hydrate inventories and cros-décollement fluid flow. Earth Planet. Sci. Lett., 236, 41-60, 2005.   Gas hydrate [vol.% of pore space] : 0-1 1-1.5 1.5-2 2-2.25  2.25-2.5   2.5-2.75   2.75-3 Bottom  of slop e sedim entsGH SZ BSR  Figure 5: Time migrated section of BGR99-44 with color coded gas hydrate concentration as result of the application of the transfer function (1). The blue line which is partly dotted (BSR) describes the base of the gas hydrate stability zone (GHSZ). The black dotted line marks the bottom of the slope sediments.


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