Abstract

One of the largest geological features in the deep part of the Alberta Basin in western Canada is the Middle to Upper Devonian Southesk-Cairn Carbonate Complex (SCCC). The SCCC is located at depths of about 5000 m adjacent to the disturbed belt of the Rocky Mountains, rising to depths of 3000 m in a northeastward direction over a distance of 150 km. The SCCC produces oil and gas with up to about 30% H2S from several stratigraphic levels.

The diagenesis of the SCCC was investigated by means of petrography and geochemistry. Stable isotope compositions were determined for fibrous calcite cements, matrix dolomite, saddle dolomite, and sparry calcite. Fibrous calcites have stable isotope values of δ18O = −7.5 to −4.5‰ PDB and δ13C = +2.0 +/− 1.0‰ PDB. Matrix dolomite samples have stable isotope values of δ18O = −8.1 to −4.9‰ PDB and δ13C = 0 to +1.0‰ PDB. Saddle dolomite samples range from δ18O = −9.8 to −7.1‰ PDB and δ13C = −1.0 to 0‰ PDB. Sparry calcite samples have δ18O = −13.7 to −7.2‰ PDB and δ13C = −24.0 to 0‰ PDB. The Sr-isotope values of all analyzed samples span a broad range from 0.7080 to 0.7320, showing considerable scatter but also one distinctive pattern, i.e., values higher than 0.7220 occur only at depths in excess of about 4000 m, whereas lower values scatter throughout the entire depth range. Furthermore, the Sr-isotope values of sparry calcites decrease northeastward. Similarly, the fluid inclusion homogenization temperatures of these calcites display a general decrease updip, whereas the salinities show no general spatial trends. The uncorrected homogenization temperatures range from about 85 to 180 °C, and the NaCl-equivalent salinities range from about 17 to 24 weight %. The δD values of brines obtained from seven wells range from −79 to −20‰ SMOW, and their δ18O values range from −1.5 to +8.7‰ SMOW. The sulfur isotope values of the dissolved sulfate ranges from δ34S = +10.1 to +24.5‰ CDT. The Sr-isotope values of the brine samples range from 0.7109 to 0.7255. Elemental sulfur isotope values cluster tightly around δ34S = +18.0 to +18.4‰ CDT. The values of secondary anhydrites range from δ34S = +18.4 to +26.7‰ CDT. Values for solid bitumen vary from δ34S = +19.5 to +20.2‰ CDT.

Integration of all data reveals a complex diagenetic evolution from the Middle Devonian to the Recent. The paragenetic sequence consists of 24 distinct diagenetic phases, which have been grouped into 5 diagenetic stages: Stage I: density-driven convection in near-surface settings, likely superimposed on and/or aided by tidal pumping, facilitating marine cementation; Stage II: cellular convection, facilitating pervasive matrix dolomitization, aided by compaction; Stage III: hydrocarbon migration; Stage IV: thermochemical sulfate reduction forming sour gas, and regional squeegee flow forming sparry calcite cements, and localized flow through faults; Stage V: fluid flow driven by erosional rebound in the overlying Mesozoic strata, and "remnant" squeegee flow.

This study yielded several important results. (1) The prolific repertoire of diagenetic textures found in the Southesk-Cairn Carbonate Complex serves as a means of comparison for petroliferous carbonate complexes elsewhere, especially in sour gas provinces close to orogenic fronts. (2) The diagenetic evolution contains long periods of relative tranquility that were punctuated by several episodes of intense diagenetic activity. (3) Both thermochemical sulfate reduction and squeegee-type fluid flow are recorded in late-diagenetic sparry calcite cements. (4) The effect of the various burial-diagenetic episodes on the hydrocarbon reservoir properties throughout the entire reef complex was highly variable. Specifically, matrix dolomitization, thermochemical sulfate reduction, and the emplacement of secondary and tertiary anhydrites affected the hydrocarbon reservoir properties throughout the entire reef complex. On the other hand, squeegee fluid flow was insignificant in terms of affecting the reservoir properties.

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