Traps Associated with Paleovalleys and Interfluves in an Unconformity Bounded Sequence: Lower Cretaceous Glauconitic Member, Southern Alberta, Canada (1)

Abstract

The Glauconitic member in Badger, Little Bow, Retlaw, and Turin fields is an unconformity bounded sequence that formed on an ancient coastal plain in response to relative sea level fluctuations. The member consists of valley-fill and inter-valley strata. Valley-fill sandstone bodies are thick elongate pods that formed from inner estuarine bars when sedimentation was laterally confined between valley margins. Inter-valley sandstone bodies are thin discontinuous sheets that accumulated during highstands when outer estuarine embayments covered interfluvial areas adjacent to associated valleys. Numerous oil pools are stratigraphically trapped within quartzose sandstones in valley-fill and inter-valley strata of the Glauconitic member in the study area. Common updip seals for these reservoirs are (1) intra-sequence facies changes from sandstone to shale, and (2) low-permeability lithic sandstones that fill the cross-cutting paleovalleys of a younger sequence. Traps associated with many valley-fill pools are enhanced by differential-compaction anticlines. Several oil pools in the study area are hosted by discrete quartzose sandstone bodies that lie beneath a valley filled with low-permeability lithic sandstone. These quartzose sandstone bodies are interpreted to be remnants of older Glauconitic deposits that escaped erosion when a younger valley incised into, and followed the trend of, one or more older Glauconitic valleys.