Abstract: Sequence Stratigraphy, D Sandstone, Denver Basin

WEIMER, R. J. and T. L. DAVIS

The D Sandstone is a widespread petroleum-productive unit on the east flank of the Denver basin. Having an eastward source, the sandstone intertongues with and passes into marine shale within the basin. The formation varies from O to 90 ft thick, occurs 40 ft above the J (Muddy) Sandstone, and lies between the Huntsman and Graneros marine shales.

From a sequence stratigraphic viewpoint, the D system is unique for the Upper Cretaceous because marine deposits of a sea level lowstand can be evaluated and traced landward into the lowstand erosional surface (unconformity).

The D has two types of sandstones designated D-1 and D-2 members. The older D-2 is a widespread very-fine to fine-grained sandstone deposited as shoal water delta environments. The D-1 Member is a fine- to medium-grained sandstone with shales and siltstones deposited as narrow valley-fills. Fluvial and estuarine channels are the main reservoirs.

The 1/2 to 1 mi-wide paleovalleys and an interfluve unconformity were created by a relative sea level drop approximately 96 Ma. The depth of incisement of drainages into the D-2 Sandstone and Huntsman Shale is up to 50 ft.

The D-1 paleovalleys have been explored for with 2-D seismic for many years with mixed economic success. The new 3-D seismic systems give a much better definition of the paleovalleys and in predicting the lenticular reservoir channel sandstones. When integrated with modern geologic models, the new seismic lowers risk and, therefore, provides new exploration incentives.

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