Geochemical and petrographic characterization of the Upper Ordovician Utica Shale, southern Quebec, Canada

Abstract

The Utica Shale is a potential unconventional tight gas reservoir from southern Quebec. The Utica Shale is a calcareous shale with intervals of fine-grained siltstone with total organic content of 1.2wt.% (median, n=408). The major organic matter constituents are migrabitumen and zooclasts (chitinozoan and graptolite) macerals. The primary kerogen is characterized as marine due to the presence of chitinozoan and marine-derived prasinophyte algae such as Tasmanites. A significant portion of the bulk organic matter is dominated by migrabitumen. The present hydrocarbon potential (S2, mgHC/g Rock) of the Utica Shale is largely controlled by distribution of solid migrabitumen throughout the unit. Hydrogen Index (HI) declines with increasing maturity, which is controlled by greater burial from shallow to deep sections. Oxygen Index (OI) remains low in most samples, which is attributed to lack of terrestrial input. However, organic lean intervals show significantly inflated OI due to interference of mineral carbon (possibly siderite).Random reflectance measurements on matrix/solid bitumen and chitinozoans are robust indicators of thermal maturity. The maturity ranges from the late oil window in the shallow Utica to the wet and dry gas windows in the intermediate and deep Utica, respectively.The results suggest that matrix bitumen formed during migration and dissemination of hydrocarbon into the clay fraction of the rock. This is associated with significant bacterial sulfate reduction possibly in the early stages of generation and migration of oil. It appears that the porous siltstone facies in the deeply buried section of the Utica Shale acts as a reservoir for the migrated bitumen from organic-rich strata within the section and/or possibly overlying source rock.