Fracture systems and fluid inclusions in Upper Devonian organic-rich shales in the Western Canada Sedimentary Basin and Rocky Mountains Foreland Belt, West-Central Alberta, Canada

Abstract

Unraveling phases of fracture formation and hydrocarbon generation enhances our understanding of the maturation and burial history of organic-rich shales. The Upper Devonian organic-rich shales of the Western Canada Sedimentary Basin (Duvernay Formation) and the Rocky Mountains Foreland belt (Perdrix Formation) experienced distinct structural and burial histories reflected in the orientation and mineralization of developed veins, and the temperature and composition of trapped fluid inclusions. Duvernay shales typically contain vertical and sub-vertical calcite veins with light oil inclusions. The compositions and homogenization temperatures of these inclusions indicate that the trapped fluids are more liquid-rich than the produced fluids and were likely trapped under variable overpressures during fracturing. These inclusions may have formed prior to peak burial (e.g., ∼140 °C), while further burial (maximum <160 °C, at a depth of 4.5 km) resulted in generation of the current reservoir fluid (mostly gas with minor condensate). In contrast, Perdrix shales commonly contain horizontal and sub-horizontal quartz-calcite veins with gas and two-phase aqueous inclusions, where the gas phase is predominantly methane with minor ethane, propane, and benzene indicated by Raman and mass spectrometry. The compositions and temperatures of fluid inclusions in the Perdrix outcrop samples of the Foreland Belt are consistent with highly mature, free-phase thermogenic gas trapped at highly variable temperatures ranging from 180 °C to ≥300 °C (mostly 220–270 °C) and at burial depths of up to 9 km. The results obtained help us analyze the tectonic evolution, stresses, and pressures experienced by the Upper Devonian shales within the mountain ranges and the basin.