Collapse‐induced fluidization structures in the Lower Cretaceous Athabasca Oil Sands Deposit, Western Canada

Abstract

AbstractUnique vertical fluidization structures from the Lower Cretaceous Athabasca Oil Sands Deposit in Western Canada are described. The multi‐metre long structures resulted from ascending Devonian karst aquifer waters injected into the overlying unconsolidated sands of the McMurray Formation (Aptian). The dissolution removal of 100–200 m of Middle Devonian salt beds caused the collapse and fragmentation of the 200 m of Upper Devonian limestones underlying the Cretaceous sands. Hypogene karst aquifer waters fingered upwards along the faults and fractures. Disintegrated muddy wall rock sourced hydroplastic mud flows along fractures in the limestone beds below the floor of the central Bitumount Trough. These dykes widened and lengthened collapse‐induced Upper Devonian fault blocks that differentially subsided due to salt dissolution. Conduits plugged by these mud flows diverted over‐pressured aquifer waters upwards along remaining open pipes, some of which vented on the trough floor. Aquifer waters injected into the McMurray sand and mud beds accumulated on the trough floor generated several types of collapse‐induced fluidization structures: (1) stronger aquifer flows mobilized sands into tens‐of‐metres high sand dykes consisting of multiple vertical pipes of fluidized sand contorted around and intertwined with fragmented muddy wall rock; (2) smaller aquifer jets resulted in 1–2 m high sand‐rich pillars on the trough floor; and (3) narrower high‐pressure jets flowed muddy waters along multi‐metre long, 3–5 cm wide, vertical pipes that cross‐cut and wrapped around fragmented in situ beds. The velocity gradient between these narrow but multi‐metre long water pipe flows and the ambient velocity in the surrounding sediments caused water infiltration to radiate outwards. This plugged porosity along the outer wall of the pipe, diminished drainage into the wall rock, and stabilized concentric growth ring depositions along the length of the pipe. These pipe fills appear as unique striped ribbon fabrics when viewed in longitudinal section.