The Lower Cretaceous Mannville Group comprises a 200-m succession of poorly consolidated, fine-grained sandstones and shales in the Lloydminster heavy oil producing area of west-central Saskatchewan. Detailed study of closely spaced cores from several oil fields indicates the presence of six major lithofacies (here denoted by letters), and suggests some provisional interpretations of depositional environments.

Facies L consists of well-sorted sandstone characterized by low-angle cross-lamination and hummocky lamination. Nearly all oil production is from multistory facies L sandstone bodies, which average 5 m in thickness and commonly pinch out over a few hundred meters. Facies T includes moderately sorted sandstones with multidirectional trough cross-lamination. Facies M is composed of massive, fine to medium-grained, poorly sorted sandstone. Facies B comprises bioturbated sandstone-shale sequences with abundant oscillation ripples, desiccation cracks, and flaser, lenticular, and wavy bedding. Facies S includes two subfacies: massive shales (S₁) are commonly associated with laminated shales (S₂) that display desiccation cracks as well as flaser, pinstripe, and tidal be ding. Facies C comrises thin lignite beds composed of terrestrial plant debris.

Sedimentary structures in facies B and S indicate shallow-water deposition with intermittent exposure typical of tidal flats. Characteristic structures of facies L and T suggest beach, offshore bar, or sand-flat depositional environments. Facies M may represent thin channel fills. Intimate association of the lithofacies in vertical section indicates a nearshore depositional setting for the Mannville Group. A substantial number of petroleum reservoirs in the Lloydminster area appear to be intertidal sand bodies rather than the fluvial channel fills suggested in some previous studies.

End_of_Article - Last_Page 740------------