Abstract

Devonian reservoirs in the Swan Hills, Leduc and Wabamun formations in the west-central Alberta deep basin occur at present day depths of 3000 m to 3600 m and are generally restricted to dolomitized strata. Pervasive dolomite occurs; 1) along bank margins (Kaybob South Field and Fox Creek Field) of the Swan Hills Formation; 2) in the Leduc Formation rimmed reefs (Pine Creek and Pine Creek Northwest fields) and; 3) in the Wabamun Formation above the up dip edge of the Pine Creek Leduc Field.

Dolomites have variable crystal size (50 to 800 μm) and morphology (planar euhedal rhombs to non-planar anhedral). Type I dolomite (< 5% of all dolomite) is a fine-grained (planar subhedral–euhedral) dolomite that overlaps with low amplitude stylolites and is prevalent as a burrow replacement at the Wabamun Pine Creek Field. Type II pervasive dolomite that postdates low amplitude stylolites is a medium to coarse-grained (non-planar, subhedral–anhedral) dolomite that occurs as the predominant replacement of matrix and fossils in all the fields.

The pervasive replacement dolomite Type II has low δ¹⁸O values of −7.9 to −3.6‰ (relative to PDB) and low to moderate Sr ratios (⁸⁷Sr/⁸⁶Sr of 0.7083 to 0.7118). Wabamun Fm Type I dolomite of burrow mottled intervals (Type IA), have depleted δ¹⁸O (−8.78 to −9.77 ‰, mean −9.15 ‰) and moderately radiogenic ⁸⁷Sr/⁸⁶Sr signatures (0.7095). Type I dolomite from pervasive dolomite intervals (Type IB), have less depleted δ¹⁸O (−3.86 to −7.34 ‰ PDB, mean −5.67 ‰) and moderate ⁸⁷Sr/⁸⁶Sr signatures (0.7099).

Although there is a wide spread in oxygen isotope values, estimated temperatures of Type II dolomite formation range from 45 to 75°C, equivalent to burial depths of 700–1500 m. Samples with ⁸⁷Sr/⁸⁶Sr ranging from Devonian sea water values to more radiogenic (0.7093 to 0.7118), indicate that pore waters were modified by water-rock reactions during burial and dolomitization. Type IA dolomites may have undergone neomorphism during burial at high temperatures. Type IB dolomites may have been altered from fluids that precipitated the Type II dolomites, or formed from fluids that precipitated the Type II dolomite.

Seismic evidence, detailed log marker correlation and fractured and brecciated dolomite occurrences indicate that fault-controlled conduits may have controlled local and restricted dolomite occurrences in the Swan Hills and the middle Wabamun Fm. Widespread dolomite in the Leduc level indicates that porous Leduc dolomites may also have been connected via the underlying dolomitized part of the Lower Leduc platform, forming a basin-wide conduit system.

Basin fluid flow could have been initiated by tectonically stacked thrust sheets and sedimentary loading by a clastic wedge causing mechanical compaction that squeezed the fluids out of the still compactable basin sediments. This may have occurred during two orogenies, the Late-Devonian to Early-Carboniferous Antler Orogeny, and the Late-Jurassic to Early-Tertiary Columbian/Laramide Orogeny. Presumably, the basin fluids flowed laterally updip along the conduit systems and vertically up fault systems until they reached stratigraphic conduits along the eastern flank of the Alberta Basin.