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Diagenesis of the Lower Cretaceous Clearwater Formation, Primrose Area, Northeastern Alberta
The Clearwater Formation (excluding the basal Wabiskaw Member) is generally considered to have been deposited in a subaqueous deltaic environment. The δ18O and δ13C values of unaltered shell material (+29.0±1% SMOW and +3.6±0.1% PDB, respectively) from prodelta sediments indicate that the depositional water was normal marine to slightly brackish. The δ18O values of similar material from marginal delta sediments are slightly lower (+27.8±1.4% SMOW), indicative of brackish depositional water. The δ13C values of this shell material are unusually high (+5.6δ0.5% PDB), perhaps indicating high organic productivity in this environment.
Diagenesis of Clearwater sands in the Primrose area was characterized by dissolution and alteration of framework grains, and by crystallization of clays (glauconite, berthierine, chlorite - smectite, kaolinite, hydroxy-interlayer clay), pyrite, clinoptilolite, K-feldspar, and several generations of carbonates (calcite and siderite). Diagenesis can be divided into four stages, all of which occurred at temperatures of less than 70°C.
Stage 1 occurred within a few metres of the sediment-water interface, and was strongly influenced by depositional water. In the Primrose area, brackish to marine depositional water favoured the formation of chlorite-smectite (proximal delta) and glauconite (marginal delta). Rare berthierine (δ18O = +10.5 to +13.1% SMOW) crystallized where depositional waters were fresh to slightly brackish. Fine-grained, pore-filling calcite (δ13C up to +6% PDB; δ18O up to +24.6% SMOW) also crystallized at this time. Alteration and dissolution of framework grains (initiated during Stage 1) contributed to crystallization of clinoptilolite during late Stage 1 and K-feldspar during Stage 2 diagenesis.
During Stage 3, siderite and Fe-rich, pore-filling calcite crystallized, forming carbonate-cemented zones. Siderite is much more abundant in the Primrose area than in the Cold Lake area where Fe availability was limited by earlier precipitation of Fe-rich clays such as berthierine. The δ18O values of Stage 3 siderite and calcite (+19.4 to +24.1% SMOW) indicate crystallization from a porewater that contained a sizeable amount of meteoric water. The wide range of δ13C values for these carbonates (−7.4 to +6.1% PDB for siderite; −10.5 to −2.8% PDB for calcite) indicates the involvement of carbon from a variety of sources.
Kaolinite, hydroxy-interlayer clay and late, pore-lining calcite crystallized during Stage 4. Formation of kaolinite was controlled by porosity and permeability and δ18O data (+8.3 to +10.9% SMOW) suggest crystallization from predominantly meteoric porewater. With further rock-water interaction, late calcite (δ18O = +14.1 to +19.4% SMOW) and hydroxy-interlayer clay precipitated. The high δ13C values of the late calcite (+9.3 to +14.2% PDB) indicate that methanogenesis, perhaps related to biodegradation of the hydrocarbons, occurred during Stage 4. Diagenesis was terminated once the reservoir was filled completely by hydrocarbons.
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