Resistivity Mapping and Petrophysical Study of Upper Devonian Inter-reef Calcareous Shales of Central Alberta, Canada

Abstract

The Woodbend group of Upper Devonian age in central Alberta is composed of a reef complex characterized by large-scale facies changes. The reefs, which grew in a subsiding basin, were initiated in restricted areas of suitable water depth. They are surrounded by the calcareous shales and argillaceous limestones of the Duvernay and Ireton formations. Correlation sections and isopach maps indicate a slightly greater subsidence on the eastern side of the basin during Duvernay deposition, after which the basin tilted so that the Ireton sea floor sloped gently west. Very fine carbonate clastics derived from the reefs were spread throughout the basin during Duvernay and lower Ireton time by a current probably flowing southeast. The distribution of these carbonates was detected b mapping the average apparent resistivity of a stratigraphic interval. The basin reached its maximum depth when the calcareous shales of the middle Ireton were deposited. It then began to shallow and eventually the thinly interbedded coquinoid limestone and shale of the upper Ireton were laid down around and over the reefs. Fine laminations indicate fairly quiet-water deposition of the Ireton and Duvernay sediments. Limestone nodules in these rocks are probably pull-apart structures. 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 carbonate grains present. Other factors affecting porosity aside from carbonate content and depth are small by comparison. Internal redeposition of calcite has been unimportant. Resistivity mapping in the subsurface shows promise of being a valuable exploration tool for determining the relative amount of coarser sedimentary grains in shale.