ABSTRACT

A post-steam core was recovered from the near-bore region of an Alberta oil sands reservoir that had been subjected to cyclic steam stimulation. The upper portion of the core has remained heavily bitumen-saturated. The lower portion of the core comprises three zones, which are essentially bitumen-free but made up of a melange of shale fragments and partially lithified sand clasts. The upper and lower portions of the core are separated by a calcite-cemented interval, as are two of the three zones within the bitumen-extracted interval. The mineralogy of the bitumen-extracted zones are the focus of this paper.

Quartz and lesser amounts of plagioclase, dolomite, K-feldspar and clay minerals characterize the bulk mineralogy of the bitumen-extracted material. The <2µm separates from both the sand and shale clasts are dominated by illitic, smectitic, kaolinitic and chloritic clay minerals. Berthierine was not detected. Relative to shale separates from the same sample, the sand clasts are generally poorer in smectitic clay minerals, and richer in illitic, chloritic and kaolinitic clay minerals. The smectitic clay abundance in the sand clasts generally increases with depth.

The smectitic clay mineral present in both the sand and shale separates is a mixed-layer phase, containing an average of 65 per cent smectitic layers in the sands and 71 per cent in the shales. Only the sample closest to the top of the uppermost bitumen-extracted portion of the core behaves like a "normal" smectite/illite clay mineral, and it is the only sample with 80 per cent expandibility. All remaining samples exhibit anomalous behaviour, failing to collapse to an illite-type structure upon heating. Such behaviour is characeristic of smectitic clay minerals that have become intercalated with a cationic complex, hydroxy-interlayer material or organic molecule that is not readily removed or destroyed during heating. Formation of this interlayer material may reflect stabilization of smectitic clays in an expanded position by precipitation of "chloriticlike" material in the smectite interlayer region. The formation of such interlayer material would limit smectite swelling.

The resistance to collapse of the smectitic clay mineral within the bitumen-extracted zones increases with depth. The depth-dependent, increased penetration and improved crystallinity of the intercalated material may reflect higher temperatures and(or) more extensive oil-sand/steam interaction at greater depths within the bitumen-extracted interval.

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