Abstract: Seismic Modeling of a Shelf-Margin Depositional Sequence: Upper San Andres Formation, Last Chance Canyon, New Mexico

Jan Stafleu, Mark D. Sonnenfeld

The San Andres Formation (Permian) in Last Chance Canyon, Guadalupe Mountains, New Mexico is composed of two third-order depositional sequences: upper San Andres #3 and upper San Andres #4. Each sequence comprises a number of subsidiary high-frequency, sequences (fourth to fifth order).

This study addresses the seismic resolution of stratal geometries and facies distributions observed in Last Chance Canyon outcrops by seismic modelling of a published, detailed stratigraphic cross-section. The seismic models are subsequently compared with an Exxon Production Research seismic line, located 50 km along depositional strike.

Two different impedance distributions were used within the cross-section. The first distribution (Model A) assumes that each of the facies units has its own impedance. This implies that both time-significant and diachronous facies boundaries are potential reflectors. The second impedance model (Model B) uses horizontal velocity gradients to represent diachronous facies transitions, implying that only time-significant boundaries will act as impedance boundaries. Impedances were calibrated by velocities measured on samples.

The Vertical-Incidence modelling technique was used to compute migrated synthetic seismic profiles with different frequencies.

Profiles based on Model B, image the genetic structure of both upper San Andres #3 and #4 relatively well. Model A, however, shows a better match with the Exxon seismic line. This implies that time-transgressive reflections may play an important role in the generation of seismic reflection patterns.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994