25th International Meeting on Organic Geochemistry IMOG 2011

P-365
Geochemical evaluation of the origin and migration of gases
present in natural gas hydrates – an example from the
Norwegian continental shelf
Tanja Barth 1 , Espen N. Vaular 1 , Irene Roalkvam 2 , Ida H. Steen 2
1 Department of Chemistry, University of Bergen, Bergen, Norway, 2 Centre for Geobiology, University of
Bergen, Bergen, Norway (corresponding author:Tanja.Barth@kj.uib.no)
Introduction
Natural gas hydrates are found widely distributed in
deep sea sediments. Their occurrence is primarily
detected using seismic mapping, with only a limited
number of samples available for geochemical analysis
and interpretation. Thus, the origin of these large
volumes of methane rich gas is still open for investigation
and discussion. Although a thermochemical
source is often assumed, recent results point at
microbial processes in the sediments as providing a
major part of the gas (1, 2). Due to the simple
chemical structure of the hydrocarbon gases, the
tools for assessing their origin are limited to
compositional and isotopic characterisation. Thus the
basis for the source evaluation needs to include any
information available on the sedimentary environment
to support the conclusions. In this context, interdisciplinary
input from especially microbiological
characterisations of the hydrate bearing sediments
are of high value. The interpretation of the isotopic
compositions of the gases is a critical factor, and all
potentially significant types of fractionation processes
must be critically evaluated, including the potential for
isotopic fractionation during the physical-chemical
process of hydrate formation and dissolution.
Samples and analysis
Headspace from seabed samples, and hydrate gases
have been analysed as previously reported (2,3). The
microbial communities in the same sediments have
been mapped at the Centre of Geobiology at UiB. In
addition, laboratory experiments in a stirred, highpressure,
temperature controlled hydrate incubator
have been performed using both a natural gas mix
and pure methane. Both the hydrate gas and the
residual gas phase were analysed for their isotope
values to evaluate the possibility of isotopic
fractionation when hydrates were formed.
Results and discussion
Figure 1 illustrates schematically our initial interpretation
of the fluid flow that provides the gases incorporated
in the hydrates sampled at the seabed at the
G11 pockmark at Nyegga. The free gas system is
believed to include a contribution from the polygonal
faults in the deep sediment over the Helland Hansen
Arch (HHA). The migrating fluid from the expulsions
forms the chimney ending in the pockmark and feeds
the gas hydrate located in the pockmarks. It contains
primarily microbial methane, while a proportion of
microbial ethane, thermogenic ethane and traces of
higher homologues are also indicated. However, this
interpretation can be challenged, and alternative interpretations
where the thermogenic contribution is of
less importance can be proposed. These hypotheses
will be discussed and tested using extensive data,
including evaluation of the pore water geochemistry,
sediment microbiology and laboratory results.
Figure 1: Schematic drawing of fluid flow systems at
the G11 pockmark at Nyegga (from reference 2).
References
1. Pohlman et al (2009) Methane sources and production
in the northern Cascadia margin gas hydrate
system. Earth Planet Sci. Lett. 287 (2009) 504
2. Vaular et al. (2010) Geochemical characteristics of
the hydrate-bound gases from the Nyegga pockmark
field, Norwegian Sea. Org. Geochem. 41 (2010) 437
3.Vaular et al. (2011) Comparison of Vestnesa and
Nyegga Pockmark Fields using light hydrocarbon parameters
for geochemical profiling. Subm. Marine
Geol.
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