ABSTRACT

The Basal Belly River sands of south-central Alberta are diachronous sand bodies reflecting the eastward (seaward) progradation of a deltaic clastic wedge, being older to the west and younger to the east. In this area, these sands are informally subdivided, in an ascending order, as follows: Basal (traditionally known as the Basal Belly River Sandstone), Lower, Middle and Upper sands.

Detailed sedimentological and palynofacies investigation of over 3000 feet (915 m) of core in south-central Alberta, has led to the recognition of the following lithofacies: 1) nearshore marine bar and interbar; 2) prodelta; 3) distal delta-front; 4) proximal delta-front; 5) swamp; 6) fluvial channel; 7) fluvial overbank; 8) estuarine channel; 9) back-barrier tidal channels; 10) barrier bar; 11) sub-tidal channels; 12) tidal flats; 13) lagoon; and 14) transgressive tidal sand ridges. The following depositional history was responsible for the regeneration of the four sand units of the Basal Belly River:

1. Development of the subaqueous deltaic facies, represented by the Basal Sand unit.
2. Modification of river-dominated into tide-influenced delta, represented by the Lower Sand unit.
3. Dominance of fluvio-deltaic sediments, represented by the Middle Sand unit.
4. Continuation of fluvio-deltaic sediments, represented by the Upper Sand unit, which prevailed until the close of Belly River time.

Considering the thinness, relative position and widespread nature of the subaqueous deltaic lithofacies of the Basal Belly River Sand, one can conclude that: a) Structural effects were minimal during the deposition of Belly River sediments; b) the depositional basin was relatively shallow; c) the rate of sediment supply was high and therefore the fluvial processes overpowered marine processes and subsidence. Consequently, the Basal Belly River delta(s) was river-dominated and tide-wave influenced.

Porosity preservation, to a varying degree, is influenced by geothermal gradient and chlorite grain coatings, while porosity reduction with depth is controlled by diagenesis. Major diagenetic trends appear related to depositional facies. The best exploration targets and reservoir developments are found in fluvial and subtidal channels, and shoreline sandstones.

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