DCO Project Summary

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Project Title
Stable and Noble Gas Isotopes in Putative Natural Gas Fields
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The isotopic compositions of terrestrial fluids can identify the provenance of fluids in the Earth’s crust, and identify potential mechanisms for hydrocarbon production. We are constructing and populating a database of existing stable and noble gas isotope measurements of crustal fluids in order to interpret isotopic variability with respect to space and time. We are studying fluids from the Witwatersrand Mine district in South Africa, as well as the Kidd Creek mine in Canada, in order to understand how inorganic processes produce hydrogen and hydrocarbons that support subsurface microbial life.
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Reporting Year 2012 Click to expand

  • RY2012-1 - submitted on Jan 01, 2012

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    [2012-00-01] Developing software to interact with data base.
  • RY2012-2 - submitted on Mar 01, 2012

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    [2012-03-01] Re-commissioning prototype HELIX multi-collector for dedicated DCO work.
  • RY2012-3 - submitted on May 01, 2012

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    [2012-05-01] Populating the database (>1600 entries).

Reporting Year 2014 Click to expand

  • RY2014-1 - submitted on Oct 01, 2013

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    Manchester Univ./Kola Centre (Chris Ballentine & Igor Tolstikhin, Leads) The major aim of this sub-project is to prepare the data base of noble gas isotope abundances in terrestrial fluids, i.e., ground waters, gases, oils, gas hydrates, etc., and apply these data to study sources of hydrocarbons, their formation and evolution.  At present the data base consists of more than 6700 samples from all around the world (the most representative collection), all of which include He isotope composition and, when available, isotope compositions of other noble gases and carbon, as well as chemical gas compositions.  This collection allows “the most statistically supported” relationships between the data. 


    As example, compositions of mantle emanations can be revealed using He as their conservative isotopic tracer.  The general conclusion from helium isotope geochemistry is that 3He /4He is relatively constant, (12 +/- 2) × 10-6 in major magmatic input into the crust, i.e., MORB magmatism.  In some cases helium emanated from ocean island magmas show even higher ratios.  Defining gases without any crustal / atmospheric contributions of helium as those having 3He /4He ≥ 10 × 10-6, we select from the data base 255 samples (fitting to the above condition), in which the major gas compositions are available.  This sub-set of data presents the major gas species, delivered to the upper reservoirs (crust, hydrosphere and atmosphere) from the mantle.  Far the major volatile component, transferred by mantle melts to the surface reservoirs, is carbon dioxide, CO2, which is the major gas constituent in 173 gas samples (or in ≈ 70 % of the total amount of the samples).  Nitrogen dominates in 66 samples (25 %); these are mainly samples from which CO2 has been somehow removed from the fluid phase (e.g., by microbial carbon fixation or by formation of C-bearing inorganic minerals, such as calcite).  Hydrogen dominates in 2 samples, and the residual are mixtures of CO2, N2 and H2.  The highest methane content in a gas from this selection is 2 %: helium of hydrocarbon bearing fluids always shows contribution of radiogenic crustal helium (or it is totally radiogenic). 


    Also possible is to consider helium isotope signature in hydrocarbon bearing samples, in which CH4 is the major constituent, CH4 > 50 volume % (number of samples = 1,150).  More than a half of these samples show 3He /4He < 10-7 indicating a major contribution of crustal helium and thus pointing on the preferentially crustal origin of hydrocarbons. 


    This issue is highlighted further by relationships between helium isotope ratio in hydrocarbon gases and the capacities of gas / oil regions indicating general decrease of the capacities along with increase of 3He / 4He ratios.  Thus, hydrocarbon rich regions, such as West Siberia, Turanian, Volga – Ural, show quite radiogenic 3He / 4He ratios below 10-7.  Considerable contributions of mantle helium are observed in small deposits situated in tectonically active regions, e.g., ≈ 20% contribution of mantle helium is observed in Sakhalin hydrocarbons.  However, enhanced ratios are observed in several large gas fields as well.  For example, helium isotope ratios in hydrocarbon-rich natural gas samples from Cretaceous and Devonian sedimentary reservoirs in the Alberta, Canada, vary in a wide range approaching 7 × 10-7.  Enhanced 3He / 4He ratios are also observed in Hugoton-Panhandle field, USA.  These cases are generally explained as a result of mixing of crustal hydrocarbons with a 3He-bearing mantle component.


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