SEDIMENTARY ROCKS/Dolomites 83 Figure Uncemented Smithiphyllum and Phacelophyllum with calcite preservation of the delicate chamber walls (trabeculae) in partially dolomitized matrix Sample is from the Devonian Nisku Formation, Alberta, Canada by-product of thermochemical sulphate reduction, and from hydrothermal fluids Saddle dolomite commonly occurs as gangue in MVT-type metal sulphide deposits Porosity and Permeability Comparison of the molar volumes of calcite and dolomite reveals that about 13% of porosity is generated in the so-called ‘mole-per-mole’ replacement of calcite by dolomite according to reaction [1] (whereby two moles of calcite are replaced by one mole of dolomite) However, several other processes are involved As a generalization, dolostones can have higher, the same, or lower porosity and permeability than their precursor limestones, and the poroperm evolution has to be investigated on a case-by-case basis Many/most dolostones have higher porosities than limestones, and this fact may be the result of one or several of six processes (Figure 13): (1) moleper-mole replacement; (2) dissolution of unreplaced Figure Vuggy dolostone that resulted from (macro )dissol ution of unreplaced calcite matrix and fossils, similar to the sample shown in Figure Connection of pores is intercrystalline pervasive Sample is from the Devonian Nisku Formation, Alberta, Canada calcite (solution undersaturated for calcite after all Mg in excess of dolomite saturation is exhausted); (3) dissolution of dolomite (without externally controlled acidification); (4) acidification of the pore waters (via decarboxylation, clay mineral diagenesis, etc.); (5) fluid mixing (‘Mischungskorrosion’); and (6) thermochemical sulphate reduction, which may generate porosity under certain circumstances Dolomitization almost invariably involves the reorganization of permeability pathways Commonly, permeability increases along with porosity, and vice versa, such as in the Upper Devonian Grosmont Formation in eastern Alberta, which hosts a giant heavy-oil reservoir, and in the Cambrian–Ordovician Bonneterre Formation of Missouri, USA, which hosts one of the world’s largest MVT-type sulphide deposits Planar-e dolomites tend to have the highest porosities and permeabilities, the latter caused by well-connected pore systems with low pore to throat size ratios (as indicated by mercury injection curves);