ABSTRACT

The Viking and Cardium Formations occur at relatively shallow depths in central Alberta, and at progressively greater depths toward the Alberta foothills. Because of the abundance of core and reservoir data, certainty of correlation, and undeformed state, these formations provide ideal case studies for establishing depth-porosity relationships in sandstones. Seventeen cores of the Viking, ranging from depths of 640 m to 2640 m, and ten cores of the Cardium, ranging from 1220 m to 2745 m, were used.

Studies of other formations have indicated that, in general, porosity decreases with depth in a linear fashion. The porosity-depth relationship of the Viking is essentially linear and may be termed "normal". The Cardium Formation, on the other hand, shows no distinct trend with depth and is therefore considered to be "abnormal". The scanning-electron microscope has been used to document diagenetic changes with depth, and to attempt to understand the differences between the two formations.

Porosity values for the Viking Formation range from 19 to 36 per cent over a depth interval from 640 m to 915 m as a result of variations in concentrations of such components as authigenic clays, detrital clays and pyrite. Below 915 m, the porosity values gradually decrease to 6 per cent at a depth of 2640 m. The general reduction of porosity with increasing depth of burial in the Viking sandstones is due primarily to mechanical compaction, chemical compaction and quartz-overgrowth development.

In contrast to the Viking Formation, porosity values of 20 per cent can be found in the Cardium sandstones at depths of over 2700 m. However, variations do exist in porosity values at similar depths because of differing concentrations of detrital clays, authigenic clays, calcite and siderite. In the Cardium sandstones, there is some porosity reduction with increased burial depth due to mechanical compaction, chemical compaction and quartz overgrowth development. However, the migration of hydrocarbons into the Cardium sandstones essentially terminated diagenesis and with further burial overpressuring was developed.