Assessment of Thermal Maturation in Outcrop Samples of the Utica Shale, Northern Appalachian Basin, New York
Reliable interpretations of thermal maturation provide critical data needed for unconventional resource assessments. In the Appalachian Basin, the Upper Ordovician Utica Shale Formation is a widespread source rock that has been targeted for both tight-oil and shale-gas potential. The absence of vitrinite in pre-Devonian sediments such as the Utica Shale eliminates one of the most commonly measured geothermometers from consideration. Programmed pyrolysis related parameters like Tmax are of limited utility given the thermal maturity regime. However, other organic macerals are potentially available for constraining thermal maturity. The current field site and laboratory study has been undertaken to provide a very detailed comparison of parameters like graptolite and pyrobitumen reflectance and to place these into a regional context with regard to natural fractures, veins, fluid migration and hydrocarbon generation.
The Utica Shale is widely exposed in Mohawk River Valley of New York, where E-W trending fractures and coarse calcite filled veins are thought to have developed early in the evolution of the Taconic foreland basin. Geochemical analyses of Utica Shale outcrop samples reveal moderate levels of organic richness (1.1-1.7 wt% TOC) and low remaining hydrocarbon generation potential (30 Mcf/a-ft) due to elevated thermal maturity. Graptolite reflectance measurements average 2.7% Ro in the western to 3.5% Ro in the eastern Mohawk Valley. These trends are consistent with reported fluid inclusion homogenization temperatures. Calcite filled veins which were thought to contain predominantly solid bitumen were instead found to have entrapped thin lenses of the adjacent source rock complete with preserved bedding plane. Reflectance measurements on rare and dispersed pyrobitumen in both regions average 2.5% Ro. Orientation of graptolite and pyrobitumen was found to have a significant influence on reflectance measurements due to anisotropy which, as well as being caused by vertical stress from burial load, is also likely to be a consequence of non-vertical tectonic stress. The results of this investigation provide additional geochemical data to assist geologist interpreting thermal maturation in the Utica Shale and offer further insights into the tectonic history of the northern Appalachian Basin.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013