Reservoir Sandstone Bodies in Estuarine Valley Fill: Lower Cretaceous Glauconitic Member, Little Bow Field, Alberta, Canada¹

Over 50% of the established oil reserves (1.325 million m³) of the Little Bow field in southern Alberta are stratigraphically trapped within upper Mannville G, U, and W pools. These pools are hosted by three parallel, elongate sandstone bodies within an estuarine valley fill, which lies at the base of the Lower Cretaceous Glauconitic member. The paleovalley is 2.0-2.5 km wide, 4-30 m thick, and has been mapped over a length of 55 km. The three reservoir sandstone bodies are 3-4 km long, 300-500 m wide, and up to 22 m thick. Reservoir pressure histories and original fluid contacts indicate hydrodynamic isolation of the three pools. Examination of well logs and seismic sections indicates the three sandstone bodies are separated by 100 to 200-m wide shale units. These shale units are interpreted as the fills of mud-prone estuarine channels. The sandstone bodies of G, U, and W pools are interpreted as deposits of longitudinal sand bars that lay between the muddy channels in a configuration similar to that found within the modern Gironde estuary. The data and depositional interpretation presented in this paper provide new information that should help improve modeling of reservoirs within estuarine valley fills of the Glauconitic member.