The Grayburg Formation (Late Permian, Guadalupian) is a shallow-marine succession flanking the Delaware and Midland basins of Texas and New Mexico, U.S.A. The Grayburg exemplifies the facies heterogeneity imparted by cyclic interbedding of siliciclastic and carbonate rocks from lithologically diverse, inner-ramp to outer-ramp facies assemblages. High-resolution correlation and mapping of laterally continuous Grayburg strata exposed in the Brokeoff Mountains, New Mexico, allow the stratigraphic architecture, facies distribution, and lateral variability to be characterized in detail. This study provides an outcrop analog for stratigraphically equivalent subsurface reservoirs and comparable carbonate-ramp reservoirs that accumulated during periods of low-amplitude sea-level fluctuations.
Vertical and lateral facies successions in the Grayburg record four hierarchical scales of cyclicity. The entire Grayburg is a composite sequence that initiated with transgression of the San Andres platform and culminated with subaerial exposure, followed by a major basinward shift in deposition. This third-order cycle contains four high-frequency sequences defined by transgressive outer-ramp facies overlain by aggradational ramp-crest to inner-ramp facies capped by an unconformity. Each high-frequency sequence contains several composite cycles, intermediate-scale cyclic successions. The high-frequency (fifth-order) cycles constitute the smallest-scale upward-shoaling facies successions that can be recognized and mapped, comprising the basic correlation entity to delineate lithofacies bodies.
Lateral heterogeneity in the Grayburg reflects both systematic facies transitions and interwell-scale (meters to hundreds of meters) variability due to geologic complexity. Larger-scale systematic facies changes that reflect primary environmental and/or depositional controls (e.g., water depth, platform position, accommodation trends) can be characterized using well and/or seismic data abetted by appropriate depositional models. Interwell heterogeneity due to geologic complexity, however, is difficult to recognize from subsurface datasets. Appropriate outcrop analogs provide information on lateral facies dimensions and heterogeneity architecture that is essential for constructing more realistic three-dimensional reservoir models, rather than oversimplified models based on lithofacies correlations forced between wells by linear interpolation. An understanding of geologic heterogeneity exhibited in outcrop analogs is crucial for geoscientists involved with characterizing and modeling subsurface heterogeneity.