Host rock dolomitization and secondary porosity development in the Upper Devonian Cairn Formation of the Fairholme carbonate complex (South-west Alberta, Canadian Rockies): diagenesis and geochemical modelling

The Upper Devonian carbonate reefs in West-central Alberta are important petroleum reservoirs that are well-known for their extensive secondary porosity. An outcrop analogue study indicates that an early matrix-selective dolomitization event occurred which is characterized by a major Late Devonian sea water component with increased salinity because of evaporation. It is interpreted that the matrix (replacive) dolomite formed during the Famennian as the result of a combination of both seepage and latent reflux dolomitization, although an additional type or overprinting of later intermediate burial dolomitization cannot be excluded. Formation of the moulds is attributed mainly to the dissolution of undolomitized fossil cores, most typically stromatoporoids. Geochemical modelling indicates that carboxylic acid fluids have the highest potential for dissolving residual calcite in this case. Geochemical models consistent with this analysis and interpretations can reproduce the secondary porosity and suggest a viable dolomitization process for the localities studied.