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Seismic Characterization of Stratigraphic Architecture: Outcrop- and Well-Based Three-Dimensional Forward Seismic Modeling of Permian San Andres-Grayburg Strata


The Permian Basin of West Texas remains the most active petroleum basin in the US. In-field drilling and reservoir development could benefit from better understanding on fine-scale seismic stratigraphic architecture through integration of seismic interpretation and iterative forward modeling. We target 2 Permian composite sequences (CS-10 and 11) corresponding to productive Upper San Andres (uSA) and Grayburg formations. This transitional icehouse-greenhouse Guadalupian interval is characterized by complex carbonate-dominant ramps as well as mixed carbonate-siliciclastic clinoforms near the shelf margin punctuated by a basinward shift of facies associated with a sea-level drop at 268 Ma. The 3D geologic model presented here was built based on outcrop data for the uSA mixed clastic/carbonates at Last Chance Canyon and related exposures in Guadalupe Mountains, combined with cores, logs, and seismic from a Grayburg reservoir on the eastern flank of the Central Basin Platform. We created a 3D geocellular facies model by a single/combination of 4 geostatistic methods, including Truncated Gaussian Simulation with Trend, Stochastic Object Modeling, Plaurigaussian Simulation and Constant Facies Value, for each individual high frequency sequence/cycle (HFS/HFC). Acoustic velocity measurements of 68 outcrop plugs of uSA Fm. and from 6 wells of Grayburg Fm. provided the basis for a facies-based impedance analysis. The seismic from the Grayburg reservoir has a frequency band up to 100 Hz with peak frequency at 35 Hz, thus we create 3D synthetic seismic model at 17.5, 35, 60, 70-Hz 90o-phase seismic model as its low to high-frequency representations. A 3D exploding-reflector algorithm was used to simulate wavefields from sources, and then the migrated seismic model can be directly compared to the geologic model. Initial analysis shows potential pitfalls for seismic stratigraphic characterization. At HFS scale in uSA interval, progradational HFS cannot be distinguished from aggradational HFS at 17.5 Hz. In addition, maximum flooding surface of G9 HFS at 60 Hz is imaged over-oblique near shelf-margin promontory by transecting 3 HFCs. At CS scale, seismic sequence boundary (SB) of uSA-Grayburg Fm. could also be locally diachronous, especially where uSA peloid sandstone mingle with Grayburg sand-/grainstone to create a reflector following low-impedance lithofacies. This seismic SB should be use with cautious if chosen as the model base for Grayburg reservoir modeling.