Geochemical Distinction of Three Valley Fill Sandstones, Mannville Group and Basal Colorado Sandstone, Lower Cretaceous, Southern Alberta
Amelia Wright1, Roger G. Walker2, Brian
Zaitlin3, and David
Wray4
1 Chemostrat Inc, Houston, TX
2 Roger Walker Consulting Inc, Calgary, AB
3 Suncor Energy Inc, Calgary, AB
4 University of Greenwich, Chatham Maritime, United Kingdom
The Basal Colorado Sandstone (BCS) is a prolific gas producer, with individual wells in valley-fill sandstones producing up to 67 BCF. The BCS is bounded by lower and upper transgressive surfaces of erosion (TSE), each with incised valleys hanging from them. The lower TSE truncates the underlying Mannville group, a non-marine unit that also contains valley fill deposits. The Mannville and lower BCS valley fills are relatively clay rich, easily damaged, and cannot be distinguished petrographically. The upper BCS valleys fills are quartzose and prolific gas producers. Chemostratigraphy gives an alternative tool for distinguishing the three sets of valley fills. Analysis for 48 elements was performed, with the results being plotted as elemental ratios. Despite their general similarity, the lower BCS sandstones have relatively high Fe/Al and Mg/Al values compared with the Mannville sandstones, suggesting a subtle change in the detrital carbonate component, or a change in diagenesis. It is not surprising that the distinctions are subtle because the lower BCS valleys are cut into the Mannville floodplain, and are partly filled with Mannville-derived material. The quartzose upper BCS sandstones have significantly lower Ti/Nb and K/Na ratios compared with the lower BCS sandstones, implying derivation from a different area. This is in full agreement with a known change in dispersal directions – the Mannville Group from the west, and the BCS sandstones from the north-east. Heavy mineral analysis also indicates this switch in provenance, from a volcanic influenced source (Mannville) to a metamorphic/sedimentary source (the Canadian Shield, for the BCS).
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005