--> ABSTRACT: Stratigraphlc Hierarchy and Cycle Stacking, Facies Distribution, and Interwell-Scale Heterogenelty: Grayburg Formation, New Mexico, by Roger J. Barnaby and Bruce W. Ward; #91019 (1996)

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Stratigraphlc Hierarchy and Cycle Stacking, Facies Distribution, and Interwell-Scale Heterogenelty: Grayburg Formation, New Mexico

Roger J. Barnaby and Bruce W. Ward

The Grayburg Formation (middle Guadalupian) of southeastern New Mexico is a mixed carbonate-siliciclastic succession that represents predominantly shallow-water sedimentation on the Northwest Shelf of the Delaware Basin. Exceptional Grayburg outcrops in the Brokeoff Mountains permit high-resolution correlation and mapping of interwell-scale facies complexity both along depositional dip and across strike.

Three orders of stratigraphic hierarchy are recognized using vertical facies successions and transgressive-regressive relationships expressed along a dip cross section. This stratgraphic organization cannot be fully defined from a single vertical section. Cycles (330 ft thick) are the smallest-scale, upward-shoaIing facies successions that can be correlated across facies tracts and constitute the basic chronostratigraphic unit. Cycles are organized into transgressive-regressive cycle sets (15-40 ft thick). High-frequency sequences (100-160 ft thick) are unconformity-bound successions composed of 10 to 20 cycles arranged into transgressive and highstand systems tracts. One-dimensional cycle thickness stacking patterns do not reflect the long-term accommodation trends imposed by high-fr quency sequences because most cycles did not fully aggrade to sea level.

The stratigraphic setting strongly influenced facies distribution and lateral heterogeneity. Permeable ooid grainstones, for example, occur in both transgressive and highstand systems tracts but are volumetrically more abundant in highstand settings. Interwell-scale heterogeneity (hundreds of ft) is high in transgressive ooid grainstones, which formed dip-elongate tidal channels and shoals. Highstand ooid grainstones formed strike-elongate shoals that are laterally continuous along dip and across strike (thousands of ft) due to stacking and amalgamation of grainstone bodies in an accommodation-limited highstand setting. The well defined relationship between stratigraphic hierarchy and facies composition and heterogeneity can be used to improve subsurface correlations and better predic lateral dimensions of reservoir facies.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California