Abstract

The Lower Cretaceous McMurray Formation comprises 35 to 70 m of uncemented quartz sand and associated shale, saturated with heavy oil in virtually all zones where there is significant porosity and permeability. Because the sands have suffered very little post-depositional textural change (cementation, clay authigenesis, pressure solution, etc.), the porosity/permeability patterns in the reservoir can be viewed as a direct function of the primary facies distribution. Achieving an understanding of the depositional environments is thus the single most important step in seeking to map and project the zones of high grade bitumen pay.

Initial infilling of the McMurray trough appears to have taken place in a wide variety of fluvial-deltaic environments, many of which are not yet fully understood. Subsequently, there developed a regime marked by the presence of very deep channels, trending north and northwest, which locally incised through the pre-existing sedimentary sequences and deposited a characteristic, fining-upward cycle that can be recognized in many areas, particularly in the northern half of the deposit: trough cross-bedded channel-bottom sands at the base; giving way upwards to solitary sets of epsilon cross-strata, deposited on the sloping flanks of channel-margin bars; passing upward into argillaceous sands of floodplain origin. The channels which produced this sequence were up to 45 m deep and many hundreds of metres wide. Where they eroded and reworked the pre-existing sedimentary pile, apparently along discrete meander-belt trends, they left behind a sand-dominated sequence that today constitutes amongst the thickest and richest oil pay zones in the entire deposit.