Chapter 32
Carbon Isotope Fractionation during Oxidation of Light Hydrocarbon Gases Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on August 6, 2018 at 00:34:37 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Relevance to Thermochemical Sulfate Reduction in Gas Reservoirs 1
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Y. Kiyosu , H. R. Krouse , and C. A. Viau
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Department of Earth Sciences, Nagoya University, Nagoya, Japan Department of Physics and Astronomy, University of Calgary, Calgary, Alberta T2N 1N4, Canada Shell Canada Limited, Calgary, Alberta T2P 3S6, Canada
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Relative enrichments of C in light hydrocarbon gases imply that they have served as thermochemical sulfate reducing agents during the production of H S in some deep Paleozoic carbonate reservoirs of western Canada. This hypothesis was tested with laboratory measurements of carbon isotope selectivity during oxidation of light hydrocarbons. Although some experiments were carried out with sulfate as the oxidant, the accompanying slow reaction rates prompted the use of metal oxides. In flow and sealed tube experiments, CH was oxidized 1.021 to 1.011 times faster than CH over the temperature range 300 to 900°C. In sealed tube experiments, product CO was depleted in C by 8 and 4 ° / о оwith respect to reactants C H and C H respectively in the temperature range 300 to 400°C. CH , relatively enriched in 13c, 2
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was produced as an intermediate during C H oxidation. 2
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It has long been recognized that the high concentrations of H S i n "sour gas" i n deep carbonate reservoir rocks arise from thermochemical sulfate reduction (TSR) (1-4). The remaining problem i s to define the reducing agents and the s p e c i f i c chemical reactions. Organic matter has been implicated i n general terms (4) and a reaction sequence involving H S and sulfate proposed (1,2). Light hydrocarbon gasec were i d e n t i f i e d as sulfate reducing agents i n the Burnt Timber and Limestone Fields (6) i n the Upper Devonian C r o s s f i e l d Member (Wabamum Group) and the Leduc Formation, as well as i n the Mississippian Turner Valley Formation i n the deep (3,500-4,200 m) F o o t h i l l s region of southwestern Alberta, Canada (7). This i d e n t i f i c a t i o n i s based on the fact that these gases are more enriched i n C than expected from data obtained from other Devonian gas deposits of western Canada where H S was not as abundant. In p a r t i c u l a r , the 6 C values for ethane and propane i n deeper sour gas (>3500 m depth) are about 10 /oo higher than those i n shallower sweeter gas (Figure 1). The s h i f t s to more negative 0097-6156/90/O429-O633$06.00/0 © 1990 American Chemical Society 2
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Orr and White; Geochemistry of Sulfur in Fossil Fuels ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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GEOCHEMISTRY OF SULFUR IN FOSSIL FUELS
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Figure 1. Comparison of 6 c values of CO2 and l i g h t hydrocarbon gases i n the Burnt Timber and Limestone Fields (Crossfield Member) to other Devonian gas occurrences at immediately shallower depths i n western Canada. 13
Orr and White; Geochemistry of Sulfur in Fossil Fuels ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
32. KIYOSUETAK
Carbon Isotope Fractionation
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and positive 6 C values f o r C0 and CH^ respectively i n the deeper sour gas are smaller. This phenomenon has been recently observed i n other sour gas formations i n western Canada with even greater C depletions i n the C0 and enrichments i n the hydrocarbon gases. The s h i f t s are attributed to k i n e t i c isotope e f f e c t s whereby C-containing gases were oxidized preferentially, producing C-enriched C0 thereby enriching the unreacted gases i n C . It i s d i f f i c u l t to assess the extent of redox reactions i n gas reservoirs because of several input and removal processes. For example, C0 may be generated by oxidation of organic matter or dissolution of carbonate and removed by carbonate mineral formation. Mass and isotope balances on i n d i v i d u a l gases would be 13 possible i f the kinetic, isotope effects and their
