Abstract

Several petroleum source rocks and reservoirs are recognized in the Gaspé Peninsula, Québec, Canada, and numerous geochemical data were published on source rocks for wells drilled in both the Cambrian–Ordovician and Silurian–Devonian series. The aim of this work is to evaluate their respective contribution of oil and gas prospects and to prepare geochemical input data for a further basin modeling study. Rocks samples were collected from both series, 46 from the Cambrian–Ordovician formations and 275 from the Silurian–Devonian formations. They were collected mainly from well cuttings, and a few of them come from outcrops.

Most of the Silurian–Devonian rocks are lean rocks with organic carbon content (Corg) lower than 0.5%. They exhibit systematic abnormal S1 (free hydrocarbons) and S2 (potential hydrocarbons) peaks from Rock-Eval 6 profiles, with compounds detected before and after the S2 peak. However, typical S2 peak profiles were obtained in isolated kerogens after solvent extraction. Kerogens were sorted with increasing depth. Between 1200 and 2600 m (3937–8530 ft), the observed hydrogen index (HI) value decrease is correlated with an H/C ratio decrease and a vitrinite reflectance (Ro) increase. The amount of extract is about 200 mg/g C whatever the burial depth. These results suggest that Silurian–Devonian rocks underwent thermal maturation between 0.8 and 1.4% Ro. Their capacity to expel oil seems to be very limited; however, they have potential as gas shale. Finally, a minimum regional erosion thickness about 4300 m (∼14,108 ft) was derived by comparing the computed HI evolution with the observed trend for a geothermal gradient at 23°C/km.

For the Cambrian–Ordovician rocks, Corg values are richer in both immature and mature samples. The S1 and S2 peaks from Rock-Eval 6 profiles are standard with HI values more than 300 to 400 mg/g C. The Ro being more than 1.6%, these source rocks are more oil than gas prone.

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