Controls on Sand Body Orientation in Upper Morrow Formation, Deep Anadarko Basin

A. R. Bay, J. M. Casey, M. A. Foster

Upper Morrow Formation (Lower Pennsylvanian) chert-and-quartz pebble conglomerates and coarse sandstones, in the depositional axis of the deep Anadarko basin of western Oklahoma and the eastern Texas Panhandle, collectively represent a wedge deposited during lowstands of base level. These sediments were deposited in nonmarine to shallow marine environments as fluvial channels, reworked sand ridges, and braid deltas sourced from the Amarillo-Wichita uplift to the south and west. Changes in base level are interpreted from regionally mappable sand trends, subsurface log character, core and micropaleontological sample interpretation, and seismic data analysis. Understanding the relationship between relative changes in base level and sand orientation can lower the risk in dril ing successful exploitation wells in these thin, discontinuous, overpressured gas reservoirs in the upper Morrow Formation.

Sandstones and conglomerates comprise only a minor portion of the seismically defined wedge and occur in two packages; one is a general upward-coarsening package locally capped by thin, chert sandstones and conglomerates, informally called the Puryear sands. In the second package stratigraphically above the Puryear sands, the minor occurrences of sandstone are laterally discontinuous and not mappable beyond 3-4 mi.

Regional interpretations suggest that during maximum lowstands, chert sand- and conglomerate-filled fluvial channels, which are oriented normal to the basin axis, incised both fan deltas along the mountain front and marine shales distal to the basin margin. As base level rose, sands and conglomerates were reworked into longitudinal sand ridges parallel with the basin axis. Puryear sands that cap the upward-coarsening sequence are interpreted as retrogradational channel-fill deposits.

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