Fault-controlled dolomitization at Swan Hills Simonette oil field (Devonian), deep basin west-central Alberta Canada

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 dolomite s that include both cementing and replacive varieties post-date stylolites in limestones (depths > 500 m). These include early planar varieties and la ter saddle dolomites. Fluid inclusion data from saddle dolomite cements (T- h=137-190 degreesC) suggest that some precipitated at burial temperatures h igher than the temperatures indicated by reflectance data (T-peak=160 degre esC). Thus, at least some dolomitizing fluids were 'hydrothermal'. Fluoresc ence microscopy identified three populations of primary hydrocarbon-bearing fluid inclusions and confirms that saddle dolomitization overlapped with U pper Cretaceous oil migration. The source of early dolomitizing fluids prob ably was Devonian or Mississippian seawater that was mixed with a more Sr-8 7-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 f rom the faults. This 'plume-like' geometry suggests that early and late dol omitization events were fault controlled. Late diagenetic fluids were, in p art, derived from the crystalline basement or Palaeozoic siliciclastic aqui fers, based on Sr-87/Sr-86 values up to 0.7370 from saddle dolomite, calcit e and sphalerite cements, and Pb-206/Pb-204 of 22.86 from galena samples. F low of dolomitizing and mineralizing fluids occurred during burial greater than 500 m, both vertically along reactivated faults and laterally in the b uildup along units that retained primary and/or secondary porosity.