About This Item
Share This Item
The Woodbend group of Upper Devonian age in central Alberta is a reef complex characterized by large-scale facies changes. The reefs, which grew in a subsiding basin and were initiated in restricted areas of suitable depth, are surrounded by the calcareous shales and argillaceous limestones of the Duvernay and Ireton formations. Isopach maps indicate relative movements of the basin during deposition. Very fine carbonate clastics derived from the reefs were spread throughout the basin during Duvernay and lower Ireton time. The distribution of these carbonates was detected by mapping the average apparent resistivity of a stratigraphic interval from borehole measurements.
The pore volume of these rocks decreases with increasing depth and carbonate content, and resistivity increases correspondingly. The straight-line relation of carbonate content and porosity suggests that reduction of porosity is directly proportional to the volume of calcite grains present. Other factors affecting porosity aside from carbonate content and depth of burial are small by comparison. Internal redeposition of calcium carbonate has been unimportant.
Resistivity mapping in the subsurface shows promise of being a useful exploration tool for determining the relative amount of coarser grains in shale.
End_of_Article - Last_Page 1101------------