A New Paleobiological Model for Origin of Dolomites in Mississippian Reservoirs, Midale Beds, Weyburn Oilfield, Saskatchewan

Arjun D. Keswani and George Pemberton
Earth & Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada

Biogenic textural modfications have played a key role in dolomitization and origin of Mississippian reservoirs within dolomudstones. Reservoir strata shows common occurrence of Zoophycos, Planolites and Chondrites, some Helminthopsis, Asterosoma, Teichichnus and Palaeophycus, and rare Siphonichnus. These ichnofossils define a diverse suite, characteristic of a distal-Cruziana ichnofacies, indicating mostly deposit-feeding behaviors in a stable fully-marine, offshore paleoenvironment. These activities have resulted in burrow-fabrics defined by concentric and tangential alignments of fecal pellets, and abundance patterns ranging from clustered-interpenetrating to homogenized textures. These are associated with bioturbate texture-selective dolomites, consistent with origin of carbonate reservoirs.

Textural variations associated with dolomites suggest genesis of intergranular voids in pelletization of mud has facilitated percolation of diagenetic fluids, within otherwise impermeable substrates. Distribution patterns in intergranular porosity indicates both intraburrow- and interburrow-fabric voids contributed to enhancement of permeability. Fluid flow dynamics was influenced by connectivity of voids linked to both grain-size selection- and alignment-defined permeability. Such bioturbate textural controls on permeablity enhancement resulted in a cross burrow-fabric fluid flow regime in dolomitization of fecal pellet material. This implies pellets represented sites for nucleation, crystallization, and re-crystallization of dolomite and evolution of intercrystalline porosity. Variations in porosity resulted from ingrowth (host-to-burrow) and outgrowth (burrow-to-host) patterns in dolomite crystal growth; and, origin of moldic voids indicate burrow-fabrics also provided conduits for dissolution fluids that leached skeletal fragments aligned in bioturbation.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas