Carbon isotope variation in mid-continent Ordovician-type oils: relationship to a major middle Ordovician carbon isotope shift
Detailed organic geochemical comparisons of Mid-Continent Ordovician oils with extracts of potential source rocks show that in the Forest City basin of northeastern Kansas and southeastern Nebraska, oil source rocks are Middle Ordovician shales of the Simpson Group. For the Keota Dome field, Washington County, Iowa, the oil source rock is the Middle Ordovician Glenwood Shale Member of the Platteville Formation. Analyses of saturated and aromatic hydrocarbon fractions of Ordovician-type oils from the Forest City basin, Keota Dome field, and the Michigan basin show that sigma TC of the two fractions are similar and that sigma T varies over a considerable range, from -32.5 per mil to -25.5 per mil (PDB). This large range in sigma TC reflects a major shift in the carbon isotope composition of organic matter during the Middle Ordovician. This shift is shown in a 62.5-ft (19 m) interval of core from the Decorah and Platteville Formations in the E.M. Greene 1 well in Washington County, Iowa, where organic carbon sigma TC changes regularly upward from -32.2 per mil to -22.7 per mil (PDB). The change in organic carbon sigma TC in this core is not related to variations in amount (0.13-41.4% TOC) or type (hydrogen index = 69 to 1000 mg HC/g TOC) of the marginally mature (T/sub max/ = 440 +/- 5C) organic matter. Ordovician-type oils in both the Forest City and Michigan basins show variable sigma TC, suggesting that the sigma TC shift displayed in the Middle Ordovician rocks of southeastern Iowa is a regional and possibly a global effect, related to changes in the sigma TC of the ocean-atmosphere carbon reservoir. Isotopic analyses of coexisting carbonate minerals support this interpretation.
- Research Organization:
- Geological Survey, Denver, CO
- OSTI ID:
- 6216475
- Report Number(s):
- CONF-8506201-
- Journal Information:
- Am. Assoc. Pet. Geol., Bull.; (United States), Vol. 69:5; Conference: AAPG Rocky Mountain Section meeting, Denver, CO, USA, 2 Jun 1985
- Country of Publication:
- United States
- Language:
- English
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PETROLEUM
CHEMICAL COMPOSITION
ISOTOPE RATIO
ALKANES
GEOLOGIC FORMATIONS
IOWA
KANSAS
MATURATION
NEBRASKA
OIL FIELDS
ORDOVICIAN PERIOD
SHALES
SOURCE ROCKS
ENERGY SOURCES
FEDERAL REGION VII
FOSSIL FUELS
FUELS
GEOLOGIC AGES
GEOLOGIC DEPOSITS
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NORTH AMERICA
ORGANIC COMPOUNDS
PALEOZOIC ERA
PETROLEUM DEPOSITS
RESOURCES
ROCKS
SEDIMENTARY ROCKS
USA
023000* - Petroleum- Properties & Composition
020200 - Petroleum- Reserves
Geology
& Exploration