Diagenesis and isotopic evolution of porewaters in the Alberta Deep Basin: The Falher Member and Cadomin Formation

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Diagenesis and porewater evolution in the Alberta Deep Basin have been studied by combining results from petrologic, stable isotope, and fluid inclusion analyses of diagenetic minerals. Four stages of diagenesis and porewater evolution, corresponding to burial and subsequent uplift and erosion, have been identified in the Falher Member of the Spirit River Formation and the Cadomin Formation. Stage 1 (deposition and burial) is marked by early precipitation of hematite, siderite, or chlorite and the dissolution of unstable detrital grains, followed by later albitization, precipitation of illite and local calcite cement, and pressure solution of chert grains. During this stage, δ18O values of porewaters in the Falher Member increased from −10 ± 3%. to +3%. Infiltration of saline fluids from pre-Cretaceous units may have produced porewaters in the Cadomin Formation with δ18O values as high as +4 to +7%. Stage 2 (maximum burial and relief) is dominated by precipitation of quartz druse in conglomerates and horizontal fractures. Porefluids during Stage 2 had δ18O values of +3%. at 190°C (2–3 wt% dissolved solids) in the Falher Member and at 150°C in the Cadomin Formation. During Stage 3 (uplift and erosion) precipitation of dickite occurred in both the Falher Member and the Cadomin Formation from evolved meteoric water as temperatures and δ18O values of the porewater decreased. In addition, precipitation of ankerite and calcite was initiated in the Falher Member as the influx of evolved meteoric water continued. Finally, in Stage 4 (maximum generation of methane from interbedded coals), methane saturation of the porespaces marked the end of diagenesis in the down-dip, gas-saturated part of the Deep Basin.