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The Lower Cretaceous Dakota Formation produces gas, gas-condensate, and at the basin margins, gas-rich oil. The coal-bearing Upper Cretaceous produces gas with little or no condensate. Delta 13C(PDB) values for methane, measured in dry and condensate-bearing gases, average -43.3 ± 3.4^pmil, indicating derivation from sapropel and petroleum, not coal. Locally, isotopically identical methanes occur in all productive formations over a stratigraphic interval of 1,500 m (4,900 ft). Gas chromatography revealed close similarities in ratios involving the subsidiary alkanes of gases in the Dakota, Mesaverde, and Pictured Cliffs Formations. Both lines of evidence demonstrate extensive vertical migration. In the Dakota Formation there is an approximate gradient from he center of the basin to the margin in the ^dgr13C values of methanes: from -37.7^pmil (Ro = 1.9%) to -51.9^pmil (Ro = 0.7%).
The mean ^dgr13C(PDB) value of three basin-margin oils is -27.7 ± 0.2, whereas the condensates of the central portion of the basin average -27.2 ± 0.6. These facts are interpreted in terms of a derivation of gas condensate from oil. Condensates and oils were compared on the basis of the detailed composition of their gasoline fractions, particularly in terms of paraffinicity (heptane and isoheptane values). Allowing for natural fractionation, the paraffinicity values were very similar, indicating that the condensate liquids and oils had almost identical thermal histories, rather than the oils being of normal thermal aspect and the condensates mature or supermature. This suggests that most of the condensates sampled were formed by merely physical processes. Abundant gas, generated in the central supermature basin region is postulated to have caused entrainment of oil liquids (condensate) in solution, and to have migrated to cooler reservoirs, both vertically and updip. Apparent gas migration pathways are traceable in fluid property (GOR) data in the Chacon Dakota field.
Deuterium/hydrogen ratios were determined in methanes from Dakota and Mesaverde reservoirs in the high-rank, basin-center region. Although both formations contain Type III kerogen or coal, delta D (SMOW) values of -164^pmil and -167^pmil, respectively, are compatible with those of other dry, mature petroleum gases. When considered in conjunction with the carbon isotope ratios, the values did not indicate derivation of the methanes from coal, though some admixture could have occurred.
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