ABSTRACT

Regional study of the Lower Mannville Group (McMurray Formation and overlying Wabiskaw Member of the Clearwater Formation) in the Central (Townships 80-90, Ranges 1-18W4 M) and Southern (Townships 67-79, Ranges 10-18W4 M) Athabasca oil sands area, reveals a diverse and complex array of sedimentation styles and resulting reservoir geometries. Over 1000 well logs and 100 cores were examined along with information from palynological and mineralogical samples.

Recognition of distinct sedimentation patterns allows informal McMurray Formation subdivision into the McMurray B (approximating the lower and middle McMurray) and overlying McMurray C (approximating the upper McMurray).

The best reservoirs in this area occur in the McMurray B sandy facies, where up to 65 m of greater than 3 weight percent bitumen-saturated sands occur. The sands show blocky or fining-upward wireline log profiles. Palynological and ichnological indicators in the McMurray B show an upward increase in paleosalinities from continental to marginal marine environments. These deposits are interpreted as fluvial channel systems progressing upward to fluvial-estuary channel systems.

The overlying McMurray C typically shows a 1 to 5 m coarsening-upward wireline log signature and good lateral continuity (tens of km). Paleosalinities of the McMurray C are higher than the McMurray B; however, ichnology, palynology and sedimentary structures such as syneresis cracks suggest abrupt salinity fluctuations. McMurray C deposits are interpreted as shoreface shoaling sequences within brackish bay settings.

The McMurray Formation to Clearwater Formation transition represents a relatively flat ravinement or erosion surface developed during

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transgression by the Boreal sea. An interval with high glauconite concentrations and a locally occurring transgressive ichnology suite overlies this surface. Semi-mature litharenite to feldspathic litharenite marine shoaling sands are regionally thin and randomly distributed in most of the study area. In the North Primrose area of Athabasca South, pronounced relief occurs on this surface as an east-west oriented scour. The North Primrose scour is interpreted to have formed by fluvial incisement during a relative sea level lowstand prior to the region's inundation by the Boreal sea. The best Wabiskaw Member reservoirs occur in the North Primrose scour, where delta front sands reach thicknesses in excess of 40 m with greater than 3 weight percent bitumen saturation.

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