ABSTRACT: Compartmentalization of Shoreface Sequences in the Cretaceous Gallup Sandstone West of Shiprock, New Mexico: Implications for Reservoir Quality and Continuity

David W. Valasek

Cretaceous Gallup Sandstone exposed west of Shiprock, New Mexico, is a seaward-stepping progradational event. This event is divided into eleven 1-3-km-wide compartments bounded by time-significant surfaces characterized by well-cemented, fossiliferous or mud-rich beds. Depositional dip widths of compartments have a wide-narrow-wide width pattern similar to thickening and thinning patterns of vertical profiles. Compartments are further divided into 0.1-0.5-km-wide subcompartments termed shingles. Wave-dominated to tidal-dominated facies transition occurs seaward within compartments. Slumping and dewatering structures are common at seaward terminations of compartments.

Capping the progradational event is a condensed section. Above the flooding surface are tidal-influenced bioturbated coarse-grained and sand-rich sigmoidal beds intermixed with pebbly mudstones interpreted as ravinement deposits deposited during major transgression. They are thickest where the flooding surface truncates clastic fluvial and eolian coastal plain deposits. The boundaries between compartments are small-scale condensed sections. Two compartments that prograded seaward were subsequently eroded, yielding remnant lower shoreface and ravinement deposits.

The concept of compartmentalization of progradational events suggests that reservoir characteristics are complex yet predictable. The concept allows for accurate and detailed correlation of wells spaced greater than 0.5 km. Individual compartments may have different petrophysical characteristics and capability to permit fluid flow. Facies thickness variations may be due to small-scale compartmentalization within compartments.

Eustasy, subsidence, and sediment supply are inferred to control distribution of lithologies, shingles, and compartments and hence reservoir quality and continuity. Facies organization within shingles are small-scale analogs of compartments. Genetic sequence analysis coupled with detailed facies analysis improves the understanding of facies organization at multiple scales. Observations of scale-independent self-similar sedimentologic geometries at one scale (e.g., small scales) may be applicable to understanding those at different scales (e.g., seismic scale)

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