The Cretaceous Cardium Formation, Alberta, Canada, which produces oil and gas from thin stratigraphic traps comprising coastal and

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offshore shelf sand-ridge deposits, appears as "railroad tracks" on seismic sections. Ninety-seven seismic lines were examined over a 10,000 km² area. Here, the Cardium is divided into the Cardium Sand and the overlying Cardium Zone, both of which are 15-50 m thick. The Cardium Sand systematically grades eastward from (a) shoreface-strandplain massive sandstones to (b) inner-shelf sandstones encased in shale. The Cardium Zone grades eastward from (a) marginal marine/inner-shelf sandstones (< 10 m thick) encased in shale to (b) shelf shales.

Two major reflection patterns characterize the Cardium Formation. One consists of two high-amplitude reflections spaced 20-30 m apart, and the other consists of a single reflection; further subdivision is possible on the basis of reflection amplitude. Areally, these patterns correlate with the regional distribution of sedimentary facies described above.

Reflection patterns of 26 1-D seismic models generated from sonic logs correlate with those of the field seismic data thus allowing interpretation of the field data in terms of sedimentary facies. Thickness of the Cardium Zone and number and thickness of sandstone beds in the Zone were found to control seismic reflection patterns. The double reflection pattern occurs where the Cardium Zone is more than 24 m thick and contains shelf sandstone beds encased in shale. A single reflection, generated from the Cardium Sand, occurs where the zone is less than 24 m thick and lacks sandstones. These relationships can be used to detect and map potential sandstone reservoirs on seismic records.

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