Ninth international conference on - Marum

90
Abstracts of posters
Fig. 2: Compressive strength of natural sediment containing gas hydrates. The sediment is confined in a piston
cell and it is subjected to weak compressive stresses at constant pore pressure of 20MPa. The strength of the
sediment, which is assessed by its modulus of elasticity at each temperature, increases from 5MPa at 12 o C to
maximum value of 14MPa at 18 o C – close to the stability limit of the hydrate phase - and then reduces sharply
down to 2MPa at 26 o C, where all hydrates are dissociated.
A simple applicable transfer function to estimate (2D) marine gas hydrate inventories derived
from precise geochemical modelling
M. Marquardt 1 , T. Henke 2 , R. Gehrmann 3 , C. Hensen 1 , C. Müller 2 , K. Wallmann 1
1 IFM-GEOMAR Kiel
2 Federal Institute for Geosciences and Natural Resources (BGR)
3 University of Leipzig
Offshore gas hydrate inventories have been estimated so far either by the use of available pore water data
(usually restricted to ODP drill sites) or by the interpretation of seismic records. However, the results derived
from these two methods very often revealed significant differences in terms of quantity and distribution for
identical locations.
The project HYDRA aims at a complementary
approach using geochemical reactive-transport
models and geophysical rock physics modelling to
quantify regional GH inventories. Geochemical
transport-reaction models has been constrained on
DSDP/ ODP drill Sites 685, 1230, 1233, 1040, 1041
and 1043 (Costa Rica, Peru and Chile) to derive a
simplified general transfer-function. The application
of that general function requires incorporation of
regional parameters i.e. sediment thickness,
sedimentation rate, thermal conditions and SO4
penetration depth (depends on CH4 flux and is in
relation to the degradation of organic carbon). SO4
penetration and sedimentation rate were derived from
sediment and pore water data of gravity cores.
Seismic interpretation yields both the thermal gradient
Fig. 1: Map of Costa Ricans Pacific coast. On the
red line (seismic line BGR-99 44) a first
application of the transfer function has been
performed.
and the sediment thickness. Therefore a velocity
analysis calculating the seismic velocities vp and vs
from the elastic moduli and the rock density (effective
medium theory), planar information of sediment
thickness, and the thermal gradient has been applied.
Imprecise parameters in the geophysical model (e.g.
porosity) were adjusted before with the geochemical
model in order to maintain two coherent and valid models.
The main target of the project is the accurate estimation of margin-wide gas hydrate inventories. A first
application has been performed on the continental slope offshore Costa Rica (see figure 1). First results include a
2D-distribution along the seismic profile BGR99-44 across ODP sites 1040 and 1041. All data used (SO4
penetration depth, sedimentation rate, sediment thickness and BSR distribution) are derived from ODP leg 170,
the seismic profile BGR-99 44 and the cruises M-54 and SO-173. The resulting profile show the formation