25th International Meeting on Organic Geochemistry IMOG 2011

P-357
Mechanisms of thermochemical sulphate reduction: Insights
from redox buffered laboratory experiments
Svenja Germerott 1 , Christian Ostertag-Henning 2 , Harald Behrens 1
1 Leibniz University Hannover, Institute for Mineralogy, Hannover, Germany, 2 Federal Institute for
Geosciences and Natural Resources, Hannover, Germany (corresponding
author:s.germerott@mineralogie.uni-hannover.de)
Cai et al. (2003) showed that high H2S concentrations
in the Wolonghe Field (Sichuan Basin, China) of up to
32% have been generated by thermochemical
sulphate reduction (TSR). This finding is just one
example that stresses the relevance of TSR for
petroleum reservoirs. TSR refers to the abiotic
reduction of sulphate by the oxidation of
hydrocarbons. The accumulation of the major reaction
products (H2S and CO2) reduces the quality of the
reservoir. In spite of its importance, the understanding
of the TSR process is not adequate and the
extrapolation of many experimental results to natural
systems is critical because key chemical variables,
e.g. the redox conditions, were not adjusted to
geologically reasonable values [2].
In this study experimental findings on TSR in a
controlled, redox buffered system are presented in
order to elucidate fundamental mechanisms of the
process and thus to improve the general
understanding of TSR.
The experimental approach involves the use of a
redox-mineral buffer, consisting of pyrite (FeS2),
pyrrhotite (FeS) and magnetite (Fe3O4), in order to
constrain the chemical environment during the
experiments with respect to H2 and H2S fugacities.
Na2SO4, dissolved in water, was used as sulphate
source and C8H18 as model hydrocarbon.
Furthermore, the influence of elemental sulphur on
the reaction mechanisms and progress was
investigated. Experiments were conducted in coldseal
pressure vessels using gold capsules as sample
containers. Temperature was adjusted to either 300
or 350 °C at a pressure of 350 bar. Duration of the
experiments ranged from 24 to 336 h. After the end of
the experiments the samples were analyzed by
different gas chromatographic techniques. In addition
to CO2, gaseous and highly volatile hydrocarbons, we
focused on the analysis of ketones and organosulphur
compounds.
Main findings of this study are:
1) We observed an increase of the CO2 yield
[Fig. 1] with an enhanced TSR reaction
which is in line with the reaction proposed by
Orr (1974). However, our results show that
the oxidation of carbon proceeds via one or
more intermediate steps, such as the
formation of ketones [Fig. 1].
2) Aromatisation and isomerisation are also
promoted with an increased TSR reaction.
The formation of aromatic compounds
reflects an oxidation of hydrocarbons. The
raised yield of branched hydrocarbons
correlates with an increase in the formation
of radicals that are suggested to play a
significant role for the TSR process.
3) Last but not least, consistent with previous
TSR studies, our results confirm the catalytic
influence of elemental sulphur and show that
organosulphur compounds facilitate the TSR
reaction.
Fig. 1: Abundance of CO2 (left) and selected
ketones (right) after experiments for 168 h.
Pyrolysis (PPM+H2O+C8H18), TSR (PPM+H2O+
C8H18+Na2SO4) and TSRcat. (PPM+H2O+
C8H18+Na2SO4+S) refer to the three different
educt combinations used for the experiments.
Error bars are 10%.
References
[1] Cai, C., Worden, R.H., Bottrell, S.H., Wang, L.,
Yang, W. (2003) Chemical Geology 202, 39-57.
[2] Seewald, J.S. (2001) GCA 65, 1641-1664.
[3] Orr, W.L. (1974) American Association of
Petroleum Geologists Bulletin 58, 2295-2318.
487