Sequence stratigraphy, sedimentology and petroleum geology of the Glauconitic member and adjacent strata, Lower Cretaceous Mannville Group, southern Alberta

The Lower Cretaceous Mannville Group forms part of the fill of the Cordilleran foreland basin in western Canada. Upper Mannville strata in the subsurface of the study area in southern Alberta accumulated on the cratonic side of the basin. Four upper Mannville lithostratigraphic units are informally recognized (in ascending order): Glauconitic member, and upper Mannville divisions A, B and C. Attention is focussed on the Glauconitic member and division A because many cores from oil and gas wells sample these units. Few cores sample division B, and none sample division C. The Glauconitic member and divisions A and B are differentiated based on sandstone composition, presence of valley fills, and nature of sedimentary structures, paleosols and coals.;Upper Mannville strata accumulated in coastal plain settings located landward of fully marine environments. Deposition evolved in response to changes in sediment supply and relative sea level. The Glauconitic member accumulated dominantly in estuarine environments, whereas divisions A, B and C accumulated dominantly in fluvial and deltaic environments. The change in depositional setting coincided with a change in sandstone composition from quartzose to lithofeldspathic. These changes occurred because sediment influx from the orogenic belt began to outstrip accommodation in this part of the foreland basin, and the boreal sea concomitantly regressed generally northward.;Valley fills and paleosols in the Glauconitic member and division A indicate the presence of unconformity bounded sequences that formed through numerous cycles of fluvial incision and subaerial exposure followed by aggradation. These cycles, which are interpreted to have been induced by fluctuations of relative sea level, resulted in significant changes in coastal plain deposition. The Glauconitic member accumulated in inner estuarine environments that were confined within valleys when sea level was low, and in outer estuarine bays that covered interfluves when sea level was high. Division A accumulated in fluvial channels that were confined within valleys when sea level was low, and in shallow deltas that covered interfluves when sea level was high.;Most of the oil produced in the study area comes from the Glauconitic member. Reservoir sandstones in valley fills are thick, elongate pods that formed from longitudinal bars when inner estuarine deposition was confined within valleys. Reservoir sandstones outside valley fills are thin sheets that formed from coalesced sand shoal and tidal channel deposits when outer estuarine bays spread across interfluvial areas adjacent to the valleys. Common updip seals for both types of reservoirs are formed by (1) intrasequence facies changes from sandstone to shale, and (2) younger, cross-cutting valley fills in division A which contain low-permeability lithofeldspathic sandstone.