In the Deep Basin of Alberta, a northwest-trending boundary separates hydrocarbon prone rocks in the west (downdip) from water-bearing rocks in the east (updip). This boundary represents a discontinuity or interruption in reservoir character. Otherwise, the updip water would exchange positions with the downdip hydrocarbons. Possible mechanisms include facies change, diagenetic effect or faulting. An analysis of the Cardium main sand based on thin section, capillary pressure data and structural information was performed in an effort to determine the controlling factors on the location of the oil/water boundary. In the area of study the Cardium main sand forms a quasi-continuous sand unit across the oil/water boundary, minimizing facies change as a trapping mechanism. Capillary pressure information and analysis of thin sections has shown that the sandstones on the downdip (oil side) of the boundary are more cemented than those updip resulting in distinctly smaller pore throat sizes downdip. This pattern of capillarity is the reverse required for using a diagenetic change to explain the present fluid configuration, indicating that a diagenetic change is not the trapping mechanism. Structural analysis involving trend surface residuals has shown that the location of the hydrocarbon/water boundary coincides with a change in structural style. A structural interpretation should therefore be considered as a viable explanation of the hydrocarbon/water boundary of the Deep Basin.