Petrographic analyses of Swan Hills carbonates (Middle-Upper Devonian) from the deeply buried subsurface (2500-4000 m) of west-central Alberta indicate that these carbonates underwent diagenesis in submarine and subsurface environments, without significant modifications from shallow, meteoric waters. Submarine diagenetic products include syntaxial, micritic, microspar, and fibrous calcite cements that precipitated in interparticle pores of some carbonates and significantly reduced porosity. Meteoric waters associated with subaerial exposure had minimal effect due to the short intervals of exposure, the prevailing arid climate, and the original calcitic mineralogy. Subsurface diagenetic products include blocky calcite cement (400-500 microns), stylolites, matrix replacement dolomites and associated dissolution vugs, saddle dolomite, coarsely crystalline anhydrite cement, late stage macrocrystalline (centimetre size) calcite cement, elemental sulphur, and pyrobitumen. Dissolution associated with matrix dolomitization created sufficient porosity and permeability to sustain gas production. Limestones that did not undergo dolomitization lost porosity through stylolitization and cementation as they were more deeply buried and, as a consequence, lack reservoir quality.