ABSTRACT

The Upper Shaunavon (15-25 m) is a mixed siliciclastic -carbonate unit. In the Rapdan and Dollard oilfield areas the lower part consists of quartz sands, calcareous shales, skeletal/peloidal packstones and grainstones that probably represent reworked relict sediments of a transgressive barrier island system. In the Instow - Bone Creek area the lower part is dominated by skeletal and ooid grainstones, some of which are producing reservoirs. Reservoir beds in the middle and upper parts consist of quartz and skeletal sands. Their porosity is enhanced by aragonite dissolution, and early freshwater phreatic calcite cement fringes ensured that little reduction of porosity occurred due to compaction during burial. Dolomitic sediments beneath, and on the flanks of reservoir units may have partially originated in the fresh - marine mixing zone, but some show features suggestive of an evaporative origin. The results of this study are consistent with earlier interpretations that envisage a NNE - SSW trending coastal barrier system. However, further detailed study of slabbed cores is needed to establish the extent to which wave and tidal processes sculptured Shaunavon reservoir units.