Talk Title: Use of Isotopic Data to Determine Gas Source and Continuity of Gas Accumulations in the Uinta Basin
Speaker: Janell D. Edman, Consulting Geochemist
Talk Date: March 18, 2005
Publication: The Outcrop, March 2005, p. 4-5
Isotopic measurements made on hydrocarbon gas samples can provide information not only on gas source but also on the approximate temperature and maturity (vitrinite reflectance) level at which the gas was generated. In addition, isotopic data can further provide input on the continuity of the gas accumulation. The gas samples needed for these determinations can be collected simply and inexpensively either while drilling or later during completion or production. The techniques used in making the source and continuity determinations are illustrated using four gas samples from the Uinta Basin.
In this Uinta Basin example, four separator gas samples were submitted for both gas compositional and isotopic analyses. These four samples are from the Wasatch, Price River, and Black Hawk formations, with two of the four samples collected from the Price River. First, considering gas source, interpretation of the isotopic data indicates all four of these Uinta gases are thermogenic in origin, and that the Wasatch and Price River samples probably have a single, common source while the gas in the Black Hawk sample likely has a different source. In addition, the isotopic data indicate the Wasatch gas and the two gases from the Price River were generated at different maturity levels from their common source. This interpretation of different maturity levels from a common source is consistent with the compositional data for these three samples as well as the individual temperature and vitrinite reflectance estimates for gas generation derived from the isotopic data. The differences in gas generation temperature and maturity are the result of different episodes of gas generation and expulsion from the source rock.
Second, regarding the continuity of these gas accumulations, the method for determining gas continuity is straightforward. If the carbon isotopes for methane, ethane, propane etc. are similar from one gas sample to another, it is likely the gas distribution is continuous. Conversely, if the isotopic data exhibit notable differences, the gas distribution is discontinuous. In the case of the four Uinta samples, differences in the carbon and deuterium isotopes that arise from variations in both source and maturity for the four gas samples clearly demonstrate that none of these samples are in fluid communication. Application of isotopic data to determine gas continuity can be used on either a local scale as input to reservoir models or on a regional scale as an independent, quantitative constraint on exploration models for basin-centered gas accumulations. To summarize, isotopes represent a powerful additional tool for finding and developing hydrocarbon gases.