Devonian matrix dolomites and deep burial carbonate cements: A comparison between the Rimbey-Meadowbrook reef trend and the deep basin of west-central Alberta

The Upper Devonian Rimbey-Meadowbrook reef trend is an extensive subsurface carbonate aquifer system in the southern part of the Western Canada Sedime ntary Basin and is compared with Swan Hills, Leduc and Wabamun carbonates o f west-central Alberta. These Devonian levels are part of a structural homo cline that crops out near its northeastern end and stretches for several hu ndred kilometers southwestward into the deepest part of the basin. Most Devonian carbonates are pervasively dolomitized into grey matrix dolom ite, which commonly comprises about 85-90 vol % of the rocks. Despite its l arge regional extent and subsurface depths (0 to 6500 m), the Rimbey-Meadow brook reef trend displays little if any systematic variations in petrograph y or geochemistry. Stratigraphic, petrographic and geochemical evidence sug gest that the grey matrix dolomite formed at depths of about 500-1500 m fro m marine pore waters with relatively minor modification by water-rock inter action. Volumetrically minor amounts of calcite and dolomite cements, repla cive and cement anhydrite, and elemental sulfur and hydrocarbons fill some of the remaining porosity. The volumetrically minor intermediate burial (>600-1500 m) dolomite cements and deep burial saddle dolomite and calcite cements occur as coarse-crysta lline spars in secondary porosity. Intermediate burial dolomite cements dis play a slight decrease down the reef trend in delta(18)O values (-4 to -7 % (0) PDB) with increasing depth that correspond to slightly elevated Sr-87/S r-86 ratios. Fluid inclusions of these dolomite cements are highly saline a nd have relatively uniform homogenization temperatures (122 to 131 degrees C), suggesting that they formed from relatively hot fluids before significa nt basin tilting. Deep burial (>2000-3000 m) saddle dolomites are rare in the Rimbey-Meadowbr ook reef trend and the available fluid inclusion homogenization temperature s increase with burial, suggesting that they precipitated under normal geot hermal gradients after significant tectonic tilting. The calcites fall into two groups, a relatively early and a relatively late group. The relatively early calcites follow a 30 degrees C/km geothermal gradient when corrected for pressure, whereas the later calcites that formed as a by-product of th ermochemical sulfate reduction follow a 20 to 25 degrees C/km gradient and may reflect cooler post-laramide conditions. The deep basin of west-central Alberta includes the Obed, Kaybob South, Fox Creek, Pine Creek and Simonette fields from the Swan Hills, Leduc and Waba mun stratigraphic levels. Replacement matrix dolomites in all three stratig raphic levels are similar in terms of textures, trace elements and C, O and Sr isotopes, and to those of the Rimbey-Meadowbrook reef trend. In additio n, fault-controlled, late-stage burial dolomitization is evident in the Swa n Hills, Kaybob South and Simonette buildups, and in the Wabamun Pine Creek Field. Homogenization temperatures from fluid inclusions in saddle dolomit es range from about 127 to 167 degrees C and late calcites have similar T-h ranges from 124 to 164 degrees C. Temperatures for anhydrite are cooler, r anging from 104 to 116 degrees C, and suggest secondary alteration of inclu sions at present reservoir temperatures and/or that some anhydrite was remo bilized at somewhat cooler conditions after maximum burial. Downdip to the southwest in the Obed buildup, deep burial calcite cements i n fractures and vugs have highly radiogenic Sr-87/Sr-86 ratios (0.7252) and highly depleted delta(13)C values (minimum -27.1 %(0) PDB), indicating inc orporation of oxidized carbon from Thermochemical Sulphate Reduction (TSR). In other buildups, delta(13)C values of late calcite cements are not as de pleted in their carbon isotopic values (0.9 to -10 %(0) PDB). Radiogenic Sr occurs updip to the northeast in saddle dolomite and calcite cements (Swan Hills, Kaybob South 0.7221, 0.7310; Simonette 0.7370, 0.7369; Leduc Pine C reek 0.7140, 0.7195; Wabamun Pine Creek 0.7150, 0.7188. The radiogenic Sr-8 7/Sr-86 Signal appears to be derived from fluids that had interacted with t he crystalline basement and/or overlying Proterozoic/Lower Cambrian clastic s. It is possible that at least some radiogenic Sr was injected into the De vonian section by tectonic loading during the Laramide Orogeny. The Rimbey-Meadowbrook reef trend and the Devonian carbonates in the west-c entral Alberta deep basin are similar in terms of overall diagenetic phases and their characteristics. The late-stage cements exhibit somewhat higher temperatures in the deep basin of west-central Alberta. The regions differ mainly in radiogenic Sr with all west-central Alberta late-stage dolomite a nd calcite cements being more radiogenic, especially along established or i nferred faults as in Kaybob South, Swan Hills Simonette and Wabamun Pine Cr eek fields. The west-central Alberta calcites also contain lighter delta(13 )C (-3.5 to -10.0 %(0) PDB) and even lower (0 to -27 %(0)) in some high sou r gas fields like Obed.