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

Abstract

Abstract Several large hydrocarbon accumulations in Alberta, Canada are hosted in dolomitized successions of stacked, thin sabkha-capped cycles of Visean age. Porosity is micro-intercrystalline and occurs in dolomitized restricted subtidal and intertidal muds that have their fine primary fabric preserved. Two such fields are here considered, that illustrate divergent and contrasting modes of dolomite stabilization despite initial similarities in facies and textures. The dolomite in the upper Debolt Formation of the Dunvegan Field (NW Alberta) forms planar-e or microsucrosic fabrics with crystals in the 1–20 μm range. The dolomite is non-ferroan, Ca-rich (average of 58 mol% CaCO 3 ), and poorly ordered. Its stable isotopic signatures range from −0.12 to +3.4‰ VPDB for δ 18 O (mean=+1.3‰) and +0.9 to +4.3‰ VPDB for δ 13 C (mean=+2.6‰). The average radiogenic 87 Sr/ 86 Sr ratio for this dolomite is 0.7077. Both sets of values are consistent with dolomite precipitation from Mississippian marine or modified marine (evaporated) seawater. These parameters are strongly reminiscent of Holocene protodolomites and hence suggestive of a sabkha dolomitization process (shallow seepage reflux or evaporitive pumping). This dolomite with its high associated porosity (average of 15%, and up to 38%), relatively unaltered mineralogical and chemical signatures, both preserved despite 4 km of burial depth, suggests a very unique set of relatively non-reactive physico-chemical conditions during burial (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 (open system). Such alteration has affected its crystal size (range δ 18 O ranges between −1.5 in the least altered dolomite and −13.2‰ VPDB; δ 13 C ranges from +3.9 to −1.5‰ VPDB; and 87 Sr/ 86 Sr ranges from 0.7078 to 0.7090). The dolomite has retained a degree of non-stoichiometry with an average Ca content of ∼55 mol% CaCO 3 . The Mount Head dolomites clearly indicate ongoing reactivity between the rocks and basinal fluids during burial, and are in this respect representative of the norm for dolostones in the Western Canada Sedimentary Basin.