Geochemical Assessment of the Utica and Point Pleasant Shales, Ohio, USA
Daniel M. Jarvie
Energy Institute, Texas Christian University, Fort Worth, TX
The Ordovician Utica and Point Pleasant shales of Ohio generate paraffin-rich petroleum with low adsorption affinity and also low non-hydrocarbon content (resins and asphaltenes). With lithofacies often containing higher carbonate content, there is increased storage capacity with some dissolution enhanced porosity, but also reduced petroleum retention capacity. Permeability is also good for such a tight resource system and thus key limiting factors are oil content and pressure drive. Typical of Ordovician oils Utica and Point Pleasant oils have modest gas pressures at oil window thermal maturities. However, thermal maturity indicators are less reliable proxies for oil production than production potential, oil saturation and gas-to-oil (GOR) ratios.
Overall, hydrogen indices are very high sometimes exceeding 800 mg kerogen/g TOC and a best fit trend line suggests an original HI value of 789 mg oil/g TOC. This would suggest about 67% convertibility of original TOC values based on 85% carbon in crude oil or about 173 boe/acre-ft for every 1% original TOC. However, when the total data set are analyzed, a P50 value for original hydrogen index is 449 mg/g, which indicates 38% convertibility. As such this indicates for every 1% TOC (original) that 98 boe/acre-ft would be generated upon full maturation.
A combined forward and reverse calculation of oil in place requires detailed analytical work including fingerprinting of retained oil. Detailed work shows that there are two components to organic matter that can be extracted from an oil window mature source rock. The extractable organic matter is composed of hydrocarbons and nonhydrocarbons (resins and asphaltenes). Hydrocarbons consist of gases and liquids derived from both kerogen and bitumen cracking, the latter of which may decompose to gas if sufficient temperatures are reached in the source or reservoir rock.
Based on paraffin cracking kinetics, it would be expected that increased GOR values would require higher temperatures (maturities) than typical later Paleozoic generated oils. Calculated temperatures of generation and expulsion are higher than many other shale oil systems that are typically 110-120oC, whereas the Utica and Point Pleasant oils range from 125-130oC.
Prediction of gas-to-oil ratios (GOR) on dead oil samples and on source rocks themselves during reconnaissance is made possible by a special analytical technique. This allows stating GOR windows rather than maturity windows as proxies for pre- and post- drill assessment of liquids versus gas volumes.
AAPG Search and Discovery Article #90154©2012 AAPG Eastern Section Meeting, Cleveland, Ohio, 22-26 September 2012