Dolomitization and Porosity Development in the Middle and Upper Wabamun Group, Southeast Peace River Arch, Alberta, Canada

Abstract

Dolomitization and fracturing are critical to porosity development in most Wabamun Group reservoirs in the southern part of the Peace River arch. Cores from Peoria, Normandville, and Eaglesham fields and adjacent areas were examined to determine the relationship between depositional facies, dolomitization, fracturing, and porosity. Three-dimensional (3-D) seismic data indicate that most upper Wabamun dolomites are coincident with mound-like structural highs at the top of the Wabamun, with structural relief due largely to differential compaction between dolomite and adjacent limestone. The dolomite bodies generally are small, only 100-300 m across, which makes 3-D seismic data very helpful in Wabamun exploration. When this depositional/diagenetic model for dolomitization w s integrated into 3-D seismic interpretations, Canadian OccidentaPs discovery rate in the Wabamun increased from approximately 25 to 80%. Dolomitization and porosity are laterally quite variable in the Wabamun with the 200-250-m Wabamun section changing laterally from porous dolomite to nonporous limestone in less than 1 km (sometimes as little as 150 m). Two generations of replacive dolomite were identified, early near-surface and late hydrothermal. The early dolomite is facies, fabric, and mineralogy selective with some burrow fills and aragonitic mollusks being preferentially dolomitized in dolomitic limestones. Complete dolomitization is common in unfossiliferous carbonates (mudstones and peloid wackestones and packstones), which are interpreted as being deposited on paleotopographic highs and dolomitized by seawater or slightly evaporated seawater during shallow burial. Most fracturing occurred after early, repla ive dolomitization. Late hydrothermal dolomitization was not strictly facies or fabric selective. Features associated with hydrothermal dolomites include vugs, fractures, collapse breccias, white dolomite cement, anhydrite, and coarse calcite cement, although anhydrite and coarse calcite cement precipitated distinctly after hydrothermal dolomitization. Late dolomitization, karst-like dissolution and collapse apparently were localized near the margins of the early dolomites where adjacent limestones were dolomitized or dissolved. Dissolution of calcite in partially dolomitized limestones resulted in geopetal accumulations of dolomite rhombs (porous sucrosic dolomite) in some fractures and burrow fills. Hydrothermal fluids apparently moved updip through the underlying Winterburn and/or Leduc formations, an then upward through porous, early dolomites in the Wabamun, causing dolomitization of adjacent limestones and karst-like dissolution. Stable oxygen isotope ratios of the dolomites are quite variable (^dgr18O ranges from -2.0 to -11.4^pmil, PDB), supporting multiple stages of dolomitization during progressive burial.