Major heavy oil deposits are present in Lower Cretaceous strata of west-central Saskatchewan. The Winter Heavy Oil Pool (approximately 566 044 mmbl) consists of bitumen-rich sands from the Aptian-Albian Dina and Cummings members of the Mannville Group. This succession unconformably overlies Paleozoic carbonates and is conformably overlain by the Lloydminster Member. Lower Cretaceous deposition in the Winter area was influenced by topography on the regional sub-Cretaceous unconformity, including the Unity uplands south of the study area and major paleovalleys opening to the northwest. The overall depositional framework consists of a lowstand and transgressive fluvial sandstone deposit (Dina Member) that evolved into a brackish embayment system (Cummings Member). Upon sea level fall, a valley was incised into the mudstone-dominated marginal marine deposits, which filled with sandstone during a subsequent sea-level rise (Cummings Member). Exploitable heavy oil reservoirs are contained within these incised valley sandstone beds. Further transgression led to a rising water table with widespread deposition of an organic-rich shale and coal followed by marine shale of the Lloydminster Member. The new depositional model for the area should lead to more optimal placement of horizontal wells in the reservoir, which is vital to continued bitumen extraction from the Winter Pool.