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ABSTRACT: Sequence Stratigraphy of the Pennsylvanian Morrow Formation, Northern Delaware Basin, New Mexico

Annell R. Bay, Paul A. Baltensperger

Stratigraphic sequence analysis of the Pennsylvanian Morrow Formation is a new technique useful in predicting and understanding shifts in sand trends that can help locate Morrow gas reservoirs in the mature northern Delaware basin. Morrow sandstone reservoirs are fluvial deltaic channel fill and transgressive beach deposits that typically are 10-30 ft thick and consistently less than 1 mi wide. Detailed mapping and correlation within the systems tracts of each sequence can high-grade specific areas in the basin for exploration.

Based on subsurface log correlations, the Morrow clastics and Atoka carbonates in the northern Delaware basin are interpreted as three stratigraphic sequences bounded by subregional type I unconformities. The post-Mississippian unconformity represents the oldest sequence boundary in the Morrow-to-Atoka succession and formed as the base level dropped and the shoreline shifted at least 50 mi basinward. The uplifted Pedernal Highlands supplied sediment to dip-trending lower Morrow channels that downcut into the exposed Mississippian carbonate ramp surface. The transgressive systems tract in this sequence consists of landward-stepping, wave-dominated deltaic deposits. The Morrow shale, a regionally correlatable organic-rich shale that separates the lower Morrow from the middle Morrow, rep esents the highstand deposits as base level that rose to a maximum.

Another base level drop occurred at the end of Morrow shale deposition and resulted in dip-trending channel-fill sandstones and, stacked, landward-stepping transgressive beach and offshore ridge deposits oriented parallel to strike. The highstand progradational deposits of this sequence formed a terrace that supplied a shelf margin system of deltaic and slope-apron sediments during the succeeding third sequence. The shelf margin deposits are capped by highstand shelf carbonates of the upper Morrow and lower Atoka.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990