Devonian Matrix Dolomites and Deep Burial Carbonate Cements: A Comparison Between the Rimbey-Meadowbrook Reef Trend and the Deep Basin of West-Central Alberta

Abstract

The Upper Devonian Rimbey-Meadowbrook reef trend is an extensive subsurface carbonate aquifer system in the southern part of the Western Canada Sedimentary Basin and is compared with Swan Hills, Leduc and Wabamun carbonates of west-central Alberta. These Devonian levels are part of a structural homocline that crops out near its northeastern end and stretches for several hundred kilometers southwestward into the deepest part of the basin. Most Devonian carbonates are pervasively dolomitized into grey matrix dolomite, which commonly comprises about 85-90 vol % of the rocks. Despite its large regional extent and subsurface depths (0 to 6500 m), the Rimbey-Meadowbrook reef trend displays little if any systematic variations in petrography or geochemistry. Stratigraphic, petrographic and geochemical evidence suggest that the grey matrix dolomite formed at depths of about 500-1500 m from marine pore waters with relatively minor modification by water-rock interaction. Volumetrically minor amounts of calcite and dolomite cements, replacive 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-crystalline spars in secondary porosity. Intermediate burial dolomite cements display a slight decrease down the reef trend in delta 18 O values (-4 to -7 per mil PDB) with increasing depth that correspond to slightly elevated 87 Sr/ 86 Sr ratios. Fluid inclusions of these dolomite cements are highly saline and have relatively uniform homogenization temperatures (122 to 131 degrees C), suggesting that they formed from relatively hot fluids before significant basin tilting. Deep burial (>2000-3000 m) saddle dolomites are rare in the Rimbey-Meadowbrook reef trend and the available fluid inclusion homogenization temperatures increase with burial, suggesting that they precipitated under normal geothermal 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 thermochemical 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 Wabamun stratigraphic levels. Replacement matrix dolomites in all three stratigraphic 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 addition, fault-controlled, late-stage burial dolomitization is evident in the Swan Hills, Kaybob South and Simonette buildups, and in the Wabamun Pine Creek Field. Homogenization temperatures from fluid inclusions in saddle dolomites 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, ranging from 104 to 116 degrees C, and suggest secondary alteration of inclusions at present reservoir temperatures and/or that some anhydrite was remobilized at somewhat cooler conditions after maximum burial. Downdip to the southwest in the Obed buildup, deep burial calcite cements in fractures and vugs have highly radiogenic 87 Sr/ 86 Sr ratios (0.7252) and highly depleted delta 13 C values (minimum -27.1 per mil PDB), indicating incorporation of oxidized carbon from Thermochemical Sulphate Reduction (TSR). In other buildups, delta 13 C values of late calcite cements are not as depleted in their carbon isotopic values (0.9 to -10 per mil 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 Creek 0.7140, 0.7195; Wabamun Pine Creek 0.7150, 0.7188). The radiogenic 87 Sr/ 86 Sr signal appears to be derived from fluids that had interacted with the crystalline basement and/or overlying Proterozoic/Lower Cambrian clastics. It is possible that at least some radiogenic Sr was injected into the Devonian section by tectonic loading during the Laramide Orogeny. The Rimbey-Meadowbrook reef trend and the Devonian carbonates in the west-central 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 and calcite cements being more radiogenic, especially along established or inferred faults as in Kaybob South, Swan Hills Simonette and Wabamun Pine Creek fields. The west-central Alberta calcites also contain lighter delta 13 C (-3.5 to -10.0 per mil PDB) and even lower (0 to -27 per mil) in some high sour gas fields like Obed.