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ABSTRACT

With increased understanding and application of sequence stratigraphic principles to shallow marine and nonmarine strata, many hydrocarbon-producing sandstones in the Cretaceous Western Interior Seaway that were interpreted in the 1980s as offshore shelf bar sandstones have been reinterpreted in the 1990s as shoreface sandstones. The Terry (Sussex) Sandstone is such a deposit. Prior studies of the Terry Sandstone in Spindle and LaPoudre-Antelope fields suggested a shelf bar origin. This study of the Terry Sandstone in the Hambert-Aristocrat Field, which lies between Spindle and Antelope fields, indicates a shoreface origin.

In Hambert-Aristocrat Field, a sharp-based sequence boundary, interpreted as a “forced regression,” occurs at the base of the Terry Sandstone. Overlying this sequence boundary is a series of at least six stacked shoreface parasequences, each separated by transgressive marine shale. Individual parasequences are comprised mainly of upper shoreface strata toward the southwest (paleo-landward), which grade laterally toward the northeast (paleo-seaward) into lower shoreface to offshore strata. Detailed mapping of individual parasequences reveals complex northwest- and northeast-trending geometries of upper shoreface strata to the southwest, but more orderly northwest-trending geometries of lower shoreface/offshore strata toward the northeast, probably reflecting variations in marine influence. Parasequences step progressively paleo-landward indicating they record periodic relative sea level stillstands within an overall transgressive systems tract.

Permeabilities, which are rarely >5md and generally <2md, are primarily related to sedimentary facies. Upper shoreface sandstone immediately above the sequence boundary is the major oil- and gas-productive interval. Other, non-perforated upper-lower shoreface/offshore strata might constitute additional opportunities for recompletion, directional or targeted infill drilling. Transgressive shales provide the potential for stratigraphic sealing and vertical isolation of individual sandstones.

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