SEISMIC MODELS OF A SHELF-MARGIN DEPOSITIONAL SEQUENCE - UPPER SAN-ANDRES FORMATION, LAST-CHANCE CANYON, NEW-MEXICO

The seismic resolution of stratal geometries and facies distributions observed in San Andres Formation (Permian) outcrops in Last Chance Can yon, Guadalupe Mountains, New Mexico, is studied by seismic modeling o f a published, detailed stratigraphic cross section. The outcrops in L ast Chance Canyon are composed of two fourth-order depositional sequen ces: an aggrading carbonate bank (upper San Andres 3; uSA3) followed b y a strongly progradational, offlapping mixed carbonate-silici-elastic succession (upper San Andres 4; uSA4). Each sequence comprises a numb er of subsidiary high-frequency sequences (fifth-order). Two alternati ve impedance models were used: Model A, in which all facies transition s are reflecting boundaries, and Model B, in which only time-significa nt surfaces act as reflectors and lateral facies transitions are repre sented by horizontal velocity gradients. The vertical-incidence modeli ng technique was used to compute perfectly migrated time and depth sec tions 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 int erpret an onlap pattern generated by a high-frequency cycle within uSA 4 as this major sequence boundary. In addition, the 25 Hz runs show to plap and downlap lap-out patterns in an overly oblique fashion, obscur ing true asymptotic stratal relationships. Both at 35 Hz and 50 Hz, pr ofiles based on Model B image the genetic structure of both uSA3 and u SA4 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 quali tatively compared with a published Exxon Production Research Co, seism ic line, located approximately 50 km along depositional strike to the northeast. Model A shows an unexpected good match with the Exxon seism ic 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 cont rasts at carbonate-sandstone transitions are represented.