Abstract

Dolomitization of the Mississippian Turner Valley Formation of the Quirk Creek Field, Alberta occurred as a result of multiple dolomitizing events. These events were closely linked to the evolution of reservoir porosity. Petrographic and geochemical studies show the presence of four distinct dolomite types: microdolomite (4-10 μm), patchy dolomite (30-150 μm), pervasive matrix dolomite (30 to 300 μm) and coarse or megadolomite (0.5 to 2 mm). Chemical data of these dolomites indicate a progression from early, nonstoichiometric to late, nearly stoichiometric dolomite. Microdolomite is dense, and locally occurs only in sabkha lithofacies. Its high Sr and Na concentrations and relatively heavy δ¹⁸O values (0 to -2.22% PDB) suggest sabkha evaporative dolomitization. However, its overgrowth rims and variable values of ⁸⁷Sr/⁸⁶Sr (0.70773-0.70874) may reflect recrystallization at later stages, possibly during Pennsylvanian or Early Permian time. Patchy dolomite occurs along dissolution seams and early stylolites (δ¹⁸O= -0.79 to -4.29%) and is interpreted to have formed during early compaction. The third stage of dolomitization represented by pervasive matrix dolomite is volumetrically very dominant. This dolomite is also very porous. It has modified and obliterated most earlier diagenetic fabrics. This dolomite is the result of the mixing of meteoric waters passing through the overlying evaporites of the Mount Head Formation with seawater or fluids of marine parentage. The low values of some trace elements concentrations (e.g., Sr, Na), slightly radiogenic Sr isotopes (0.70834 to 0.70848), slightly depleted δ¹⁸O (-1.11 to -4.81%), and common occurrence with secondary anhydrite support this interpretation. The last stage of dolomitization is represented by coarse, euhedral, replacive dolomite. It has high Fe and Mn concentrations, radiogenic Sr (0.70836 to 0.70875) and the lowest δ¹⁸O values (-2.12 to -6.47%), suggesting later formation in a relatively deep burial setting.

The overall sequence of dolomitization and other related diagenetic events suggest precipitation from diagenetic fluids of changing parentage, progressing from evaporative to mixed meteoric-marine fluids and burial brines with time.