Seismic models of a mixed carbonate-siliciclastic shelf margin: Permian upper San Andres Formation, Last Chance Canyon, New Mexico

Seismic models based on randomly distributed samples from the Permian upper San Andres Formation (Last Chance Canyon, New Mexico) verify that the most prominent seismic reflections are related to stratal geometry. However, at least some reflections arise from lateral facies transitions that are commonly associated with highly prograding mixed carbonate-siliciclastic sediments. Understanding seismic reflections and reflection terminations in sedimentary rocks requires simulation of at least 2-D cross-sections of the impedance distribution. To investigate the cause of reflections in strongly prograding mixed carbonate-siliciclastic shelf margins, acoustic velocity and bulk density were measured on more than 60 plugs that spatially cover one higher order genetic sequence. ne resulting impedance values were gridded and contoured, and the genetic sequence was repeated to create a cross-section. Averaging impedance values for each lithofacies zone generated a second impedance cross-section. Seismic models of both impedance cross-sections revealed the following observations: (1) reflections associated with the sequence boundaries are subject to amplitude and polarity phase changes and (2) at least one reflection within the high-order sequences is related to subhorizontal facies changes and is associated with two pseudounconformities. The contoured impedance model is suggested to closer resemble the true impedance function in outcrop and shows subtle vertical shifts and significantly higher impedance contrasts.