The partly dolomitized Swan Hills Formation (Middle‐Upper Devonian) in the Simonette oil field of west‐central Alberta underwent a complex diagenetic history, which occurred in environments ranging from near surface to deep (>2500 m) burial. Five petrographically and geochemically distinct dolomites that include both cementing and replacive varieties post‐date stylolites in limestones (depths >500 m). These include early planar varieties and later saddle dolomites. Fluid inclusion data from saddle dolomite cements (Th=137–190 °C) suggest that some precipitated at burial temperatures higher than the temperatures indicated by reflectance data (Tpeak=160 °C). Thus, at least some dolomitizing fluids were ‘hydrothermal’. Fluorescence microscopy identified three populations of primary hydrocarbon‐bearing fluid inclusions and confirms that saddle dolomitization overlapped with Upper Cretaceous oil migration. The source of early dolomitizing fluids probably was Devonian or Mississippian seawater that was mixed with a more 87Sr‐rich fluid. Fabric‐destructive and fabric‐preserving dolostones are over 35 m thick in the Swan Hills buildup and basal platform adjacent to faults, thinning to less than 10 cm thick in the buildup between 5 and 8 km away from the faults. This ‘plume‐like’ geometry suggests that early and late dolomitization events were fault controlled. Late diagenetic fluids were, in part, derived from the crystalline basement or Palaeozoic siliciclastic aquifers, based on 87Sr/86Sr values up to 0·7370 from saddle dolomite, calcite and sphalerite cements, and 206Pb/204Pb of 22·86 from galena samples. Flow of dolomitizing and mineralizing fluids occurred during burial greater than 500 m, both vertically along reactivated faults and laterally in the buildup along units that retained primary and/or secondary porosity.
New thermal maturity (%Ro, Rock-Eval ® pyrolysis), shale compaction and apatite fission track (FT) data were integrated into thermal history models for the East MacKay I-77 petroleum exploration well located approximately 80 km southeast of Norman Wells, Northwest Territories. The study well is in the Keele Tectonic Zone where multiple phases of anomalous Phanerozoic subsidence and uplift have resulted in an Upper Cretaceous-Cenozoic foreland succession resting unconformably on Devonian strata. This major unconformity, which developed during pre-and post-Albian time, displays a thermal maturity discontinuity (0.55-0.75%Ro) and represents approximately 270 m.y. of missing geological record. Linear shale compaction across this unconformity suggests that maximum burial occurred during the Cenozoic, whereas thermal maturity data imply that maximum temperatures were reached sometime between the Devonian and Early Cretaceous. Detrital apatite grains from a single sample from the Upper Devonian Imperial Formation of the I-77 well yielded two FT age populations (90.4±6.1 Ma and 222.2±22.5 Ma; ± one standard deviation) with different thermal annealing properties based on their chlorine content.An inverse multi-kinetic FT annealing model was developed and used to determine thermal histories that are consistent with the FT parameters and other geological constraints. Model results suggest that hydrocarbon generation from Devonian rocks at the I-77 well location occurred during the early Mesozoic prior to the development of Late Cretaceous-Cenozoic structures. Peak FT model temperatures are 124±10ºC within the Early Triassic to Middle Jurassic (250-170 Ma), <75ºC during the Albian (112-100 Ma) and 97±9ºC during the Paleocene-Early Eocene (65-50 Ma). The Cretaceous-Cenozoic thermal history was modelled using a simple burial history with the present geothermal gradient (32ºC/km) held constant; a range of geothermal gradients (31-42ºC/km) and maximum burial depths for the pre-Aptian thermal history fit Devonian maturity data. If maximum burial was during the Cenozoic, then Mesozoic peak maturity was achieved under a higher geothermal gradient than present. Although hydrocarbon generation pre-dates structural trap development near the I-77 well, Devonian source rocks retain significant hydrocarbon potential. Given the complicated geological history of the central Mackenzie Valley, deeper Cenozoic burial elsewhere in the region may have generated hydrocarbons from Cretaceous and reactivated Devonian source rocks.
RÉSUMÉUne nouvelle maturité thermique, (%Ro, pyrolyse Rock-Eval ® ), un compactage d'argile litée et des données de trajectoires de fissions apatites (FT) ont été intégrés à des modèles thermiques historiques pour l'exploration pétrolière du puits East MacKay I-77 situé approximativement à 80 km Sud-Est de Norman Wells, dans les Territoires du NordOuest. L'étude du puits se trouve dans la zone tectonique de Keele dans laquelle des phases multiples de subsidences et d'ascendances anomales du Phanérozoïque ont abouti à une...
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