Investigating the geochemical relationship between the Ordovician Guttenberg Carbon Isotope Excursion and the Trenton and Point Pleasant reservoirs
The Trenton Limestone and Point Pleasant interval of the Utica Shale of the Appalachian Basin are prolific oil- and gas-bearing zones, and although the Trenton reservoir appears to be conventional, thin organic-rich shale beds of this unit may have contributed to its hydrocarbon inventory. In southeastern, Ohio methane carbon isotopes (δ13C) recorded within the Trenton reservoir display a sharp 2‰ increase making the Point Pleasant and Trenton reservoirs isotopically distinguishable. Hydrocarbon δ13C values can serve as unique geochemical fingerprints controlled in part by thermal maturity due to enrichment of 13C with increasing thermal stress. However, hydrocarbon δ13C values are also known to be impacted by the isotopic value of the precursor organic matter (δ13Corg); i.e., as δ13Corg values become enriched so too do hydrocarbon δ13C values. A global isotope excursion known as the Guttenberg Carbon Isotope Event (GICE) recognized in Ordovician deposits as a sharp positive shift of both carbonate (δ13Ccarb) and δ13Corg can be traced throughout the basin immediately above the Millbrig Ash bed. It is thought that increased global productivity influenced the exchange of 13C enriched pCO2 with the global ocean, which may have been connected with the Appalachian Basin through the Sebree Trough. The introduction of seawater recorded by the transition of tropical to temperate carbonates is readily observed in wireline logs. The GICE has been documented throughout Ohio and δ13Corg values recorded from a core located in Highland county display a sharp increase at the stratigraphic horizon we observe the anomalous δ13C methane signature. Also, long chain hydrocarbon (n-C17, n-C20, n-C28) δ13C values have been observed to mimic the isotopic trend of the parent material (δ13Corg) from the GICE interval throughout the Appalachian Basin and from the Kahula and Variku formations of Estonia. The positive covariance of hydrocarbon δ13C and δ13Corg suggests the GICE not only impacted isotopic values recorded by carbonate and organic matter, but isotopic values of hydrocarbons as well. What remains to be determined then, is the reason for the anomalous δ13C methane signature documented from southeastern Ohio. Did hydrocarbons migrate into the Trenton reservoir from a location that experienced greater thermal stress, or were they self-sourced from organic matter that was affected by the positive excursion resulting in the anomalous signature we observe today?
AAPG Datapages/Search and Discovery Article #90335 © 2018 AAPG 47th Annual AAPG-SPE Eastern Section Joint Meeting, Pittsburgh, Pennsylvania, October 7-11, 2018