Sedimentologic and Diagenetic Controls on Reservoir Development at Rosevear Gas Field, Swan Hills Formation (Upper Devonian), Central Alberta

Jonathan Kaufman, Gilbert N. Hanson, William J. Meyers

Carbonate strata at the Rosevear gas field consist of three major sedimentological packages: (1) basal platform, (2) platform reef, and (3) capping platform. Gas production is localized within two narrow trends of porous, massive, replacive dolostone occurring in the platform-reef sequence; tight limestones updip form the reservoir seal. Porosity trends are primarily restricted to the margins of a marine channel developed through the platform reef, but not the basal platform. Channel-margin strata consist mostly of dolomitized branching-stromatoporoid floatstones and rudstones.

Massive replacive dolostone is composed of inclusion-rich coarsely crystalline nonferroan euhedral to anhedral rhombs that show a red cathodoluminescence. This dolomite has selectively replaced the lime-mud matrix; fossils were replaced to a much lesser extent. Fossils not dolomitized were selectively leached, resulting in well-developed biomoldic and vuggy porosity that forms the reservoir. Dolomitization occurred after cementation by clear, equant calcite and after early pressure solution. Secondary porosity in the dolostone trends was only partially reduced during later diagenesis, which consisted of, in order of decreasing age, precipitation of saddle dolomite, anhydrite, and coarsely crystalline calcite. Hydrocarbon migration occurred after the saddle dolomites, but before some l te-stage calcite cement.

Replacement dolomites have a mean stable isotopic composition of -7.4 ^pmil ^dgr¹⁸O and 2.5 ^pmil ^dgr¹³C, PDB, ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7079 to 0.7100 (SRM 987 = 0.71014), and Sr concentrations of less than 100 ppm. Preliminary data indicate that the dolomitizing solutions may have been warm, connate waters funneled along the channel during intermediate stages of burial. Possible sources of water and dissolved species for dolomitization include the surrounding platform carbonates and the reef-encasing basinal siliciclastics.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.