ABSTRACT

The Woodbend group of Upper Devonian age in Central Alberta includes 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 water depth, are surrounded by the calcareous shales and argillaceous limestones of the Duvernay and Ireton formations. A correlation section indicates depositional slopes of several feet per mile to the west for these interreef beds. Very fine carbonate clastics derived from the reefs were spread throughout the basin during the Duvernay and lower Ireton time. The distribution of these carbonates is detected by mapping the average apparent resistivity of a given stratigraphic interval.

The pore volume of these rocks decreases with increasing depth of burial and lime content, and resistivity increases correspondingly. A straight-line relationship of lime content and porosity suggest that reduction of porosity is directly proportional to the volume of lime grains present. Resistivity, however, increases exponentially with a decrease in porosity or an increase in lime content. Other factors affecting porosity aside from lime content and depth of burial are small by comparison. Internal redeposition of lime 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, and hence in predicting the direction to reef or shoal areas.