ABSTRACT: Diagenesis of Wabiskaw Member (Lower Clearwater Formation) Oil Sands, Northeastern Alberta

SHIER, WENDY, and FRED J. LONGSTAFFE, University of Western Ontario, London, Ontario, Canada

Despite shallow burial, Wabiskaw Member litharenites have undergone a complex diagenetic history. Early diagenesis of these oil sands is characterized by formation of glauconite, fluorite, siderite, calcite, clay minerals (illite/smectite and chlorite/smectite), and framboidal

pyrite. Dissolution of detrital K-feldspar, plagioclase and volcanic rock fragments occurred during early to intermediate diagenesis. Vein calcite, pore-filling kaolinite and K-feldspar overgrowths formed during late diagenesis. The unusual occurrence of grain-coating fluorite in both bitumen-rich and bitumen-poor calcite-cemented intervals within the oil sand may be related to NaCl brines derived from dissolution of underlying Paleozoic salts.

Diagenetic siderite rhombs present in cemented intervals have Carbon 13 values (-0.5 to +4.6o/oo PDB) typical of a dominantly inorganic source for carbonate ions; siderite Oxygen 18 values (+21.4 to +22.1o/oo SMOW) indicate influence of meteoric water during early diagenesis. Pore-filling calcite in concretionary intervals occluded all porosity prior to bitumen saturation. The Carbon 13 (-2.3 to +4.0o/oo PDB) and Oxygen 18 (+18.9 and +20.2o/oo SMOW) values of this calcite suggest that its precipitation was dominated by an inorganic carbon reservoir, and by pore fluids containing a sizable fraction of meteoric water. Grain-coating calcite, which occurs in some bitumen-saturated sands, has lower Oxygen 18 (+16.0 and +16.9o/oo SMOW) and much higher Carbon 13 (+10.6 to +12.5o/oo PDB) valu s. Meteoric water and microbial fermentation of organic matter were involved in crystallization of this calcite during burial diagenesis.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)