SHUSTER, MARK W., KSEPL (Shell Research), Rijswijk, Netherlands; and D. W. CHILDERS, Shell Western Exploration and Production Inc., Houston, TX

ABSTRACT: High-Resolution Stratigraphic Forward Modeling: A Case Study of the Lower-Middle San Andres Formation, Permian Basin

The purpose of this study was to attempt to calibrate Shell's two-dimensional (2-D) basin modeling program as an exploration tool by simulating the stratigraphy of a mixed carbonate/clastic third-order depositional sequence. The lower-middle San Andres Formation was selected because available log, core, and outcrop data from the Northwest Shelf area, Permian basin, provided an excellent calibration set. A regional stratigraphic cross section from the Cato-Chaveroo to the Wasson fields was constructed delineating lithology (i.e., dolomite, limestone, anhydrite, and halite) and porosity distribution. Approximately 10 shoaling-upward depositional cycles were interpreted. A higher frequency, five-in-one cyclicity was also interpreted based on core and outcrop data. The observed stratigrap y was simulated using a composite eustasy consisting of third-order (2,000,000 yr), fourth-order (100,000 yr), and fifth-order (20,000 yr) sinusoids each at five-meter amplitudes. Subsidence input was constrained by backstripped tectonic subsidence curves calculated from well data. Sedimentation parameters were interactively derived. New empirically based algorithms were used to model Dunham lithofacies, environmental facies, and sabkha anhydrite distribution. Additionally, synthetic log and 2-D synthetic seismic profiles were constructed from the simulation output. The simulation results suggest that (1) relative sea level is the dominant control on the observed depositional cyclicity, (2) the distribution of regional seal facies (anhydrite) reflects falling sea level and prolonged expo ure, (3) limestone-dolomite trends on the shelf are grossly related to environment (i.e., paleobathymetry), and (4) the distribution

of grainstones and packstones (potential reservoirs) occurs as fourth- and fifth-order offlapping and aggradational pods. The synthetic log signatures compared to "real" logs substantiate the interpreted depositional cyclicity, however, also point out the difficulties in interpreting high-order cycles based on log data alone.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.