Syndepositional and Postdepositional Salt-Dissolution Structure Traps in Lower Cretaceous Sandstones, Cessford Field, Alberta, Canada

John C. Hopkins, John E. Pollock

Many Cretaceous structure-stratigraphic traps in the Western Canada sedimentary basin formed through regional tilting and/or local salt dissolution (subrosion). Hydrocarbons are trapped in Cretaceous sandstones: (1) along updip pinch-outs as a result of regional tilting, (2) in sediment sinks created by local synsedimentary subsidence due to subrosion, and (3) on local structural highs isolated by subrosion.

Cessford field lies over an area of local salt removal and contains a variety of traps in different Lower Cretaceous stratigraphic zones. Regional pinch-outs and local structural highs in marine shelf sandstones of the lower Colorado Group have been exploited since the early 1950s. Renewed exploration in the last decade resulted in the discovery of several pools in fluvial sediments of the underlying Mannville Group in areas characterized by synsedimentary subsidence.

A seismic line across a portion of Cessford field reveals the presence of a narrow graben bounded by normal faults reaching from Precambrian basement into the Cretaceous section. Salt solution in the vicinity of the graben is demonstrated by thinning of a Devonian salt formation toward the fault. Structure and isopach maps of several Cretaceous markers show that solution proceeded episodically along elongate zones oriented normal to the graben. Episodic salt solution is interpreted to have been controlled by movement of water along basement isostatic faults in response to Cordilleran thrusting and loading of the foreland basin.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.