Figure 5. Carbon and oxygen stable isotope data for all calcite vein samples from the Utica Formation. Chuctanuda Creek is the easternmost locality; Delta Reservoir is furthest to the west. Note the positive carbon isotope excursion for the Chuctanuda Creek locality. The highly positive values reflect carbonate derived from microbial or thermogenic degradation of petroleum hydrocarbons during the later stages of vein development at this locality. The negative carbon signature of one sample from Reall Creek may result from carbonate derived from oxidation of methane. Note the relatively wide range of oxygen values, particularly for the Flat Creek and Town of Minden localities, resulting from vein formation over a range of temperatures, or mixing of waters with different oxygen signatures.