A seismic dip line through the Eagle D-21 well shows a salt-cored, faulted rollover anticline with two antithetic faults (Figure 2.2.1). The Eagle gas reservoir is located within Late Cretaceous Wyandot Formation with the formation top indicated in yellow on the seismic line. The structure is bounded on the north and northwest by a major northeast-southwest trending growth fault and mapped closure is indicated by the green shading on the Wyandot time structure map.
The reservoir is a thick, continuous package of limestones, marls and chalks representing deposition on a stable, shallow, open-marine continental shelf. Well data indicates that the Wyandot carbonates are generally lime mudstones that are soft, chalky, fossiliferous, pyritic, argillaceous and interbedded with marls and calcareous grey shales and mudstones. The polygonal faulting pattern is obvious over much of the chalk surface (Figure 2.2.2). The acoustic seismic amplitude map draped over the time structure map shows a dimming, indicated in yellow, of the Wyandot reflection that approximates the mapped closure of the gas-charged chalk (Figure 2.2.3). This dimming may result from an increase in porosity within the chalk.
The top seal of the trap is provided by the overlying shales of the Tertiary Banquereau Formation. The lateral extent of the reservoir to the east, south and southwest is probably limited by a combination of structural and stratigraphic trapping due to the generally low matrix permeability of the reservoir. The northern bounding fault seals where the Wyandot reservoir is juxtaposed against shales and tight limestones of the underlying Dawson Canyon Formation.