25th International Meeting on Organic Geochemistry IMOG 2011

P-359
A laboratory study of H2S production from the thermal reactions
of magnesium sulfate and sulfur with a hydrocarbon reactant:
implications for thermochemical sulfate reduction
Hong Lu 1 , Paul Greenwood 2,3 , Tengshui Chen 1 , Jinzhong Liu 1 , Ping'an Peng 1
1 State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, China Academy of
Sciences, Guangzhou, China, 2 WA Biogeochemistry and John De Laeter Mass Spectrometry Centres,
University of Western Australia, Crawley-6009, WA, Perth, Australia, 3 Western australia Organic Isotope
Geochemistry Centre, Curtin University GPO Box U1987, Bentley, WA 6845, Australia (corresponding
author:luhong@gig.ac.cn)
The reactions of pure n-C24 with MgSO4 elemental
sulfur were investigated by monitoring the yields,
stable carbon and hydrogen isotopic composition of
gaseous products from their separate gold–tube
pyrolysis treatment at a series of temperatures over
the range 220–600 ℃. Thermal cracking via pyrolysis
of just n-C24 were also conducted as a Control. H2S
from the thermochemical sulfate reduction (TSR) of
MgSO4 was initiated at 431 ℃, coincident with the
evolution of C2–C5 hydrocarbons. Whereas the yields
of H2S increased progressively with pyrolysis
temperature, the hydrocarbon yields decreased
sharply above 490 ℃ due to TSR consumption.
Ethane and propane were initially very 13 C depleted,
but became progressively heavier with pyrolysis
temperature and were more 13 C enriched than Control
values above 475 ℃.
TSR of MgSO4 also led to progressively higher
concentrations of 13 C depleted CO2 —due to
preferential reaction of 12 C–bonds and precipitation of
13 C rich MgCO3 —at pyrolysis temperatures above
430 ℃. Sulfur reacted with n-C24 to produce H2S at
the relatively low temperature of 250 ℃, implicating
S–hydrocarbon reactions at practical geological
conditions as a potentially important source of H2S
subsurface deposits. Sulfur produced only low
amounts of CO2 to 430 ℃, indicating abstraction of
the H source for H2S occurred in the absence of C–C
bond cleavage. Higher yields of 13 C depleted CO2—
sulfur also showing a reactive preference for 12 C
bonds—and low MW hydrocarbons were evident from
431 ℃, although a moderate reduction (i.e., not as
rapid as with MgSO4 -TSR) of hydrocarbon levels
above 492 ℃ was indicative of their TSR participation,
consistent also with the increased yields of H2S and
13 C depleted CO2 at high temperatures. The reaction
of low MW hydrocarbons with sulfur—added in the
Sulfur treatment and also produced by sulfate
oxidation in the MgSO4 treatment—may also account
for the elemental sulfur (S8, S7, S6 and S4) and
organic sulfur products detected in the solvent
extractable fraction of their pyrolysis residues.
Key words: TSR, H2S, MgSO4, elemental sulfur,
sulfur radical, catalytic hydrogen abstraction.
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