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Organic Matter Characterization of the Upper Ordovician Utica and Lorraine Shales, Southern Quebec, Canada


In the eastern Canada, significant industry interest has recently focused on the Upper Ordovician black shales in southern Quebec and Anticosti Island that is the Utica and Lorraine shales and Macasty Shale, respectively. For the Utica Shale, extensive testing through high pressure hydraulic fracturing has shown that the calcareous shales of the Utica have the capacity to release significant volumes of natural gas. The present study reports the organic matter characterization of core samples of the Upper Ordovician Utica and Lorraine shales in southwestern Quebec. Samples are from deep Utica and near surface samples of both the Utica and Lorraine shales. Sample lithology varying from shale to fine grained siltstone has present TOC content of ranging from 0.08 to 2.25%. The current TOC content of samples represents only the remaining 92–98% of the residual carbon in the sample. The Tmax values obtained from the Rock-Eval analysis appear to be unreliable for theses overmature samples due to low S2 values. The major organic matter constituents are matrix and migrated bitumen and pyrobitumen (for overmature samples of Utica) and chitinozoan skeleton particles. The reflectance has been measured on matrix and solid bitumen and chitinozoan skeletons. There is a strong agreement between bitumen reflectance and chitinozoan reflectance when they are converted to vitrinite reflectance. The results show that the samples from the deeper parts of Utica Shale have equivalent VRo of 2.1% and are in the dry gas zone while shallower samples of Utica and Lorraine show equivalent VRo of 1.1% and are in the oil-liquid gas window. This is in agreement with Rock-Eval data, and the reported well production. Organic matter comprises of up to 4.7% in volume of total rock. A portion of organic matter in samples may generate porosity, such as matrix pyrobitumen, which is likely resulting from the formation of gas by secondary cracking of bitumen compounds. Porous matrix solid bitumen appears to be formed during migration and dissemination of bitumen into the porous clay fraction of the rock. This is often associated with significant bacterial sulfate reduction possible in the early generation and migration of bitumen. Based on organic petrology and Rock-Eval data it seems that the organic-lean siltstone facies of the Utica Shale in this area act as a reservoir and bitumen migrated from organic-rich intervals within Utica Shale or overlying strata.