Abstract

The seismic resolution of stratal geometries and facies distributions observed in San Andres Formation (Permian) outcrops in Last Chance Canyon, Guadalupe Mountains, New Mexico, is studied by seismic modeling of a published, detailed stratigraphic cross section. The outcrops in Last Chance Canyon are composed of two fourth-order depositional sequences: an aggrading carbonate bank (upper San Andres 3; uSA3) followed by a strongly progradational, offlapping mixed carbonate-siliciclastic succession (upper San Andres 4; uSA4). Each sequence comprises a number of subsidiary high-frequency sequences (fifth-order). Two alternative impedance models were used: Model A, in which all facies transitions are reflecting boundaries, and Model B, in which only time-significant surfaces act as reflectors and lateral facies transitions are represented by horizontal velocity gradients. The vertical-incidence modeling technique was used to compute perfectly migrated time and depth sections with different frequencies. Using a low-frequency wavelet (25 Hz), the sequence boundary separating the two fourth-order cycles (uSA3 and uSA4) is poorly imaged. Instead, one is tempted to incorrectly interpret an onlap pattern generated by a high-frequency cycle within uSA4 as this major sequence boundary. In addition, the 25 Hz runs show toplap and downlap lap-out patterns in an overly oblique fashion, obscuring true asymptotic stratal relationships. Both at 35 Hz and 50 Hz, profiles based on Model B image the genetic structure of both uSA3 and uSA4 relatively well. At 50 Hz, Model A incorrectly shows a transition from a ramp to a rimmed margin within uSA4. The 35 Hz models are qualitatively compared with a published Exxon Production Research Co. seismic line, located approximately 50 km along depositional strike to the northeast. Model A shows an unexpected good match with the Exxon seismic line, whereas Model B comes much closer to the depositional anatomy observed in outcrop. Our results show that the resolution of stratal geometries and facies distributions in Last Chance Canyon is strongly related to carbonate-sandstone alternations and the way impedance contrasts at carbonate-sandstone transitions are represented.

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