Pervasive Burial Dissolution of Early and Late Dolomites in Devonian Pools, Alberta, Western Canada

DRAVIS, JEFFREY J., Rice University, Houston, TX, and IAIN D. MUIR, Esso Resources Canada Ltd., Calgary, Canada

Middle Devonian Keg River and Muskeg pools along the northern margin of Rainbow basin in northern Alberta produce mainly from secondary porosity created by dissolution of replacement dolomites and cements, including saddle dolomites. Resultant pores are predominantly vug and moldic and often microporous. Coarse vuggy porosity is also associated with Zebra dolomites.

Several relationships confirm the burial dissolution of dolomites in these sequences. First, dolomites which replaced stylolitic sediment, including saddle dolomites, are leached, as shown by fluorescence microscopy. Second, high amounts of secondary porosity are preserved in highly stylolitic dolomites, often directly adjacent to pressure solution seams. Third, pressure solution seams terminate directly into secondary porosity. And fourth, fractures which cut stylolites also commonly feed directly into secondary pores.

These petrographic relationships are consistent only if dolomite dissolution and secondary porosity development were contemporaneous with or postdated pressure solution. The spatial juxtaposition of fractures and stylolites with secondary porosity implies these diagenetic fabrics served as pathways for burial fluids promoting dolomite dissolution.

Given the regional extent of pervasive dolomite dissolution, dolomite stability in these sequences is not a function of the type or relative age of dolomite crystals precipitated. Rather, pore-fluid chemistry appears to be the major control. Calcium-rich burial fluids, responsible for emplacement of burial anhydrites and fluorite cements, are thought to promote the deep-burial dolomite dissolution and secondary porosity development in these pools.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)