SALLER, ARTHUR H., Unocal Science and Technology, Brea, CA; and KEN E. YAREMKO, Canadian Occidental Petroleum, Ltd., Calgary, Alberta

ABSTRACT: Dolomitization and Porosity Development in the Wabamun (Devonian), Peace River Arch, Alberta

Reservoir porosity in the Wabamun Group (Upper Devonian) in the Peace River Arch area (Alberta) is related to dolomitization and fracturing. Cores from Normandville, Peoria, and Eaglesham areas were examined to determine the relationship between depositional facies, dolomitization, fracturing, and porosity. Dolomitization and porosity are laterally quite variable with a 200 m Wabamun section changing from porous dolomite to nonporous limestone in 1 km. Two stages of dolomite are present: early near-surface, and late "hydrothermal." Early dolomitization is facies, fabric, and mineralogically-selective with some burrow-fills and aragonitic fossils preferentially dolomitized. Early dolomitization is common in carbonate mudstones and peloid wackestone/packstones deposited on paleotopograp ic highs. Most fracturing occurred after the first stage of dolomitization.

Late hydrothermal dolomitization was not facies or fabric selective. Features associated with hydrothermal dolomites include caverns, fractures, collapse breceias, white saddle dolomite, anhydrite, and coarse calcite cement. Karst-like dissolution and collapse were localized near the margins of the early dolomites where interbedded limestones were preferentially dissolved. The location of late "hydrothermal" dolomite and associated porosity is largely controlled by the early dolomite. Hydrothermal fluids apparently moved updip through the underlying Winterburn and/or Leduc formations and then upward through the early Wabamun dolomites causing dissolution and dolomitization of adjacent limestones. Stable oxygen isotopes of the dolomites are variable (isotope{18}O from -2.0 to -11.4 o/o ) supporting multiple stages of dolomitization.

When this depositional/diagenetic model for dolomitization was integrated into the seismic interpretation, rate of discoveries increased from approximately 25% to 80%.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.