Abstract

The Saskoil-Gulf Senlac Heavy Oil Pool, located in Townships 38-39, Ranges 26-27 W3M, west-central Saskatchewan, produces from the Lloydminster Member ‘Sands’ of the Lower Cretaceous Mannville Group. The Dina-Cummings Member unconformably overlies the Devonian Duperow Formation and is composed of a 10-30 m fining-upward sequence of sandstone, siltstone and shales containing pyritized rootlets and a restricted trace fossil assemblage (Palaeophycus heberti, Conichnus, Lockeia and Thalassinoides). The overlying Lloydminster Member consists of a basal 1-5 m thick coal, and acritarch-bearing, pinstripe laminated, tidal and lenticular bedded shales, containing a diverse ichnofossil suite (P. heberti, P. tubularis, Planolites, Teichichnus, Conichnus and Cylindrichnus concentricus), that underlie the 0-8 m thick Lloydminster Sand.

The Lloydminster Sand is divisible into two distinct lithofacies: Type 1 sand consists of a 2-5 m thick, unconsolidated, fine to very fine grained sand arranged in 5-150 cm fining-upward cycles. Each cycle is composed of a basal erosional surface overlain by medium angle, planar to trough crossbedded sands that grade upward through trough cross-laminated to rippled silts and silty shales, capped by thin carbonaceous shales. The Type 2 sand is 4-7 m thick, fine grained and exhibits undulating and ripple bedding, with minor amounts of flaser bedding. Bioturbated sandstone, siltstone and silty shales of the Rex Member containing dinoflagellate cysts, foraminifera and a wide diversity of ichnofossils (including Ophiomorpha), overlie the Lloydminster Member.

The Lower Mannville Dina-Cummings to Rex sediments in the Senlac area were deposited within a 25 km (N-S) by 6 km (E-W) northward opening paleo-topographic embayment. The sequence is interpreted, based upon its paleogeomorphic setting, vertical stratigraphic sequence and inferred lithofacies distribution, to represent the progressive change from terrestrial valley-fill deposits in inferred northward draining paleo-valley systems (Dina-Cummings), to estuarine-embayment fill deposits, with associated salt marsh, tidal flat, lagoonal and subtidal (channel and tidal delta/shoal) environments (Lloydminster), to marine deposits of the Rex Member, resulting from an Early to Mid-Albian transgressive event.