Thermal History of Alberta Deep Basin: Comparative Study of Fluid Inclusion and Vitrinite Reflectance Data

The thermal history of the Alberta Deep Basin, the gas-saturated westernmost part of the Western Canadian sedimentary basin, has been studied by analyzing aqueous and hydrocarbon fluid inclusions in diagenetic cements and comparing the results with coal maturity data. Results indicate that most of the diagenetic quartz druse precipitated from low-salinity fluids (2-3 wt. %) at a temperature of 170°-195°C. Calcite cement precipitated after quartz druse during uplift and cooling at temperatures of 108°-169°C. Inclusions rich in larger hydrocarbon molecules occur in cements that precipitated at 150°C, whereas methane-rich inclusions occur in cements that precipitated at 165°C or at lower temperatures after significant uplift and cooling of the b sin. Maximum burial temperatures (Tmb), calculated from vitrinite reflectance data using the Lopatin-Waples TTI-Ro calibration, are significantly lower (150°C) than the temperature at which quartz druse precipitated in the same area (190°C). An equation directly relating Tmb and Ro, proposed by Barker and Pawlewicz in 1986, yields maximum burial temperatures (200°-210°C) only slightly higher than the fluid temperatures. The results from this study suggest that hot fluids migrated updip along permeable conglomerates and bedding-plane fractures to produce unexpectedly high geothermal gradients (~38°C/km) in the western part of the study area. The apparent restriction of hot fluids to the western part of the study area sugge ts that convective heat transfer by fluids may have operated on a scale of only tens of kilometers in Lower Cretaceous rocks of the Alberta Deep Basin.

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