Oil Sands Geology - Athabasca Deposit North

The study area encompasses the northern part of the Athabasca Wabiskaw-McMurray oil sands deposit, from Townships 91 to 104 and Ranges 6 to 20 west of the fourth meridian. Most of the bitumen reserves in the deposit are contained in laterally discontinuous, upward-fining channel sand bodies in the McMurray Formation. In the western part of the study area, significant reserves occur in laterally extensive marine bar sands at the top of the McMurray formation and at the base of the overlying Clearwater Formation.

The McMurray Formation was deposited in a north-south trending depression on an erosional surface of Devonian limestone. The highly variable relief on this paleo-topographic surface is the most important single control on the distribution of facies and reserves. Structural movement after deposition involved regional tilting to the northwest, on the axis of the Peace River Arch, and continued subsidence of local areas due to the solution of underlying Devonian evaporites.

Division of the McMurray Formation into the lower, middle and upper members was found to be valid over most of the study area. The lower member is of fluvial origin and fills in the deepest lows on the limestone surface, where the formation is greater than about 60 m thick. Coals and rooted zones are common near the top of the member. Rapid sea level rise during middle member time resulted in a lowering f gradient of the fluvial system and a change in channel type from the shallow (5 to 10 m), commonly coarse-grained channels of the lower member to deeper (20 to 30 m), narrow, sinuous, high suspended load channels of the middle member. The middle member channels were subject to invasion by salt water during low stage and were associated with a coastal plain mosaic of lakes and brackish bays. By upper member time, the open sea had invaded the northern and western parts of the study area, where upward-coarsening offshore marine bar sands are common.

Mapping of the reservoir characteristics with a control of four wells per township provides a regional overview as to the applicability of the various in situ recovery methods to different areas of the deposit. The most basic screening criteria for in situ recovery methods in Athabasca are thickness of uninterrupted rich oil sand, overburden thickness and presence or absence of bottom water sands. Mapping of these screening criteria show 87 at the areas of the deposit suitable for a particular process may be very limited.