Late Laramide dolomite recrystallization of the Husky Rainbow "A" hydrocarbon Devonian reservoir, northwestern Alberta, Canada: paleomagnetic and geochemical evidence

The Rainbow Field is in reefal carbonates of the Middle Devonian Keg River Formation in the Western Canada Sedimentary Basin. Alternating field and th ermal step demagnetization was done on specimens from unoriented core from a vertical, an inclined, and a horizontal well core in the dolomite reservo ir. Although they had no viscous remanent magnetization component to use fo r orientation, most specimens had a well-defined characteristic remanent ma gnetization (ChRM) that resides in single to pseudosingle domain magnetite. By rotating the mean ChRM direction around its core axis, a small circle c an be generated for each core and the small circles intersect in the true C hRM direction of D = 168 degrees, I = -73.5 degrees (alpha(95) = 5.8%, k = 32.2). Its paleopole of 164%E, 83%N (A(95) = 10 degrees) defines a Tertiary age with one sigma limits of Middle Eocene to Middle Miocene age. Petrogra phic examination defines four generations of dolomite. Matrix dolomite has 60-100 mu m diameter crystals that were later recrystallized to 200-400 mu m. Dolomite cements are represented by vug-filling coarse dolomite (100-200 mu m) and saddle dolomite (1000 mu m). All four generations of dolomite gi ve similar delta(18)O values of -10.7 to -16.5 parts per thousand (Peedee B elemnite, PDB), delta(13)C values of +0.7 to +3.2 parts per thousand (PDB), and Sr isotopic ratios of 0.70826 to 0.70846 that do not match the expecte d Middle Devonian carbonate or seawater values. We interpret these data to indicate that mixed pre-Laramide basinal fluids, heated by burial during th e Laramide Orogeny, were present during late Laramide time when the dolomit es were recrystallized and (or) precipitated prior to petroleum migration a nd accumulation in the Rainbow "A" reservoir. Thus the combined use of pale omagnetism, geochemistry, and petrography has been proven to be a useful te chnique to date and identify dolomitization events and pathways for the mig ration of hydrocarbons.