Stabilization of early-formed dolomite: a tale of divergence from two Mississippian dolomites

Several large hydrocarbon accumulations in Alberta, Canada are hosted in do lomitized successions of stacked, thin sabkha-capped cycles of Visean age. Porosity is micro-intercrystalline and occurs in dolomitized restricted sub tidal and intertidal muds that have their fine primary fabric preserved. Tw o such fields are here considered, that illustrate divergent and contrastin g modes of dolomite stabilization despite initial similarities in facies an d textures. The dolomite in the upper Debolt Formation of the Dunvegan Fiel d (NW Alberta) forms planar-e or microsucrosic fabrics with crystals in the 1-20 mu m range. The dolomite is non-ferroan, Ca-rich (average of 58 mol% CaCO3), and poorly ordered. Its stable isotopic signatures range from -0.12 to +3.4 parts per thousand VPDB for delta(18)O (mean = +1.3 parts per thou sand) and +0.9 to +4.3 parts per thousand VPDB for delta(13)C (mean = +2.6 parts per thousand). The average radiogenic Sr-87/Sr-86 ratio for this dolo mite is 0.7077. Both sets of values are consistent with dolomite precipitat ion from Mississippian marine or modified marine (evaporated) seawater. The se parameters are strongly reminiscent of Holocene protodolomites and hence suggestive of a sabkha dolomitization process (shallow seepage reflux or e vaporitive pumping). This dolomite with its high associated porosity (avera ge of 15%, and up to 38%), relatively unaltered mineralogical and chemical signatures, both preserved despite 4 km of burial depth, suggests a very un ique set of relatively non-reactive physico-chemical conditions during buri al (likely a closed system). In contrast, the dolomite from the Mount Head Formation of the Shell Waterton Field (SW Alberta) has undergone measurable neomorphic alteration in several stages in deeper burial environments (ope n system). Such alteration has affected its crystal size (range <10-100 mu m), and isotopic chemistry (delta(18)O ranges between -1.5 in the least alt ered dolomite and -13.2 parts per thousand VPDB; delta(13)C ranges from +3. 9 to -1.5 parts per thousand VPDB; and Sr-87/Sr-86 ranges from 0.7078 to 0. 7090). The dolomite has retained a degree of non-stoichiometry with an aver age Ca content of similar to 55 mol% CaCO3. The Mount Head dolomites clearl y indicate ongoing reactivity between the rocks and basinal fluids during b urial, and are in this respect representative of the norm for dolostones in the Western Canada Sedimentary Basin. (C) 2000 Elsevier Science Ltd. All r ights reserved.