Abstract: Shoreface Sequence Stratigraphy of the Upper Cretaceous Terry Sandstone in Denver Basin, Colorado

SLATT, ROGER M., DWAINE H. EDINGTON, and ANNA A. FURSOVA

With increased understanding and application of sequence stratigraphic principles to shallow marine strata, many hydrocarbon-productive 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 Sandstone in the Denver Basin is such a deposit Prior published studies in the Spindle Field and Antelope-LaPoudre Field areas indicated the Terry is a shelf sandstone, but our study in Hambert-Aristocrat field, which lies between these fields, indicates the Terry Sandstone is a shoreface sandstone.

Here, the Terry Sandstone consists of a series of stacked shoreface parasequences separated by laterally continuous transgressive marine shales. Individual parasequences consist of upper/lower shoreface strata toward the southwest which grade into lower shoreface/shelf strata toward the northeast. A sharp-based 'forced regression' sequence boundary comprises the base of the Terry Sandstone in the southwest area. A 'transition zone' separates southwesterly 'blocky/fining-up' from northeasterly 'coarsening-up' well log patterns within individual parasequences. Within these parasequences, isopach geometries to the southwest of this zone are complex, with diverse orientations, while those to the northeast exhibit a more orderly, northwest orientation. Individual transition zones step progressively landward, indicating the stacked parasequences comprise a series of progradational events within an overall transgressive systems tract.

The main perforated interval is upper shoreface, relatively permeable sandstone immediately above the sequence boundary. Thinner-bedded lower shoreface strata and other non-perforated sandstones might constitute 'missed pay'. Transgressive shales at the base of parasequences provide the potential for vertical isolation of individual sandstones.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado