Dolomitization and burial diagenesis of the Devonian west-central Alberta deep basin : Kaybob South and Fox Creek (Swan Hills Formation) and Pine Creek fields (Leduc and Wabamun Formations)

Dolostone gas reservoirs in the Devonian west-central Alberta deep basin exist at three stratigraphic levels in the study area: (1) the Swan Hills Formation (Kaybob South and Fox Creek fields), (2) Leduc Formation (Pine Creek and Pine Creek Northwest fields) and (3) the Wabamun (Pine Creek field). Replacement dolomite occurs in Leduc buildups and their underlying platforms, whereas Swan Hills and Wabamun dolomites are generally limited to narrow N-NW trends. These N-NW dolomite trends in the Swan Hills Formation are associated with the platform reef area while in the Wabamun Formation they occur above the edges of Leduc buildups. Pervasive replacement dolomite has light delta18O values (-9.7 to -3.6‰ PDB), and Sr isotopes slightly more radiogenic (87Sr/ 86Sr = 0.7083 to 0.7118) than Devonian seawater values. Temperatures of dolomite formation of 50°C to 75°C from slightly modified brines during shallow to intermediate burial (800--1500 m) in the Devonian to Mississippian is suggested by the isotopic data. The Leduc (possibly Cambrian and Precambrian strata) likely served as a widespread regional conduit for dolomitizing brines. Presumed fault-controlled fractures, breccias and stratigraphic offsets suggest fault conduits created cross-formational flow to dolomitize the Beaverhill Lake and Wabamun reservoirs.Late-stage cements present in order of decreasing age are saddle dolomite, anhydrite, coarse calcite and quartz. Fluid inclusion homogenization (Th) and melting temperatures (Tm), depleted oxygen and carbon stable isotope ratios and radiogenic Sr suggest that brines that precipitated late-stage cements were hot and saline, and these brines probably had interacted with either the Precambrian basement or clastic sequences derived from the basement. The control on the distribution of late-stage cements appears to be structural, with reactivation of Devonian-Mississippian faults in the Late Cretaceous to Early Tertiary creating vertical fluid pathways for brines moving from downdip conduits.Measured %Bro (Bitumen reflectance) from isotropic bitumens are estimated to equal values of 1.75--2.02 %Ro (vitrinite reflectance). This agrees with (%Ro) reflectance of organic matter values of 1.73--1.95 found from Duvernay source rock intervals in the area. Maximum burial temperature estimates of (~180°C) are derived from %Ro ranges (182--194°C), maximum fluid inclusion temperatures of about 180--185°C from calcite cements and from burial history and maturation modeling (175--180°C).Maturation modeling and fluid inclusion gas analysis suggest that cementation overlapped with hydrocarbon emplacement and destruction and thermochemical sulfate reduction (TSR) processes. Hydrocarbon emplacement and saddle dolomite precipitation occurred at about 140°C--150°C and 4000--4500 in burial depths. Hydrocarbon destruction, anhydrite cementation, and pyrobitumen formation began at temperatures greater than 150°C and greater than 4500 m burial depths, with most calcite cementation occurring after hydrocarbon destruction and maximum burial (about 5400 m) during uplift in the Early Tertiary