Outcrop Analog for Mixed Siliciclastic-Carbonate Ramp Reservoirs--Stratigraphic Hierarchy, Facies Architecture, and Geologic Heterogeneity: Grayburg Formation, Permian Basin, U.S.A.

Abstract

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 (fifthorder) 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.

Foraminiferal biostratigraphy and sequence stratigraphy across the mid-Carboniferous boundary in the Central Taurides, Turkey

“…Lack of fossils and presence of vugs in this internally featureless microfacies indicate a low-energy, restricted lagoonal environment behind the high-energy sand shoals Amirshahkarami et al 2007; Barnaby and Ward 2007;Wanas 2008). This microfacies corresponds to the standard microfacies SMF-23 (non-laminated homogenous micrite and microsparite) and FZ 9 of Flügel (2004), typical of the shallow subtidal zone of a restricted, quiet water (lagoon) setting.…”

“…This microfacies is generally deposited during the regressive phase and records sea-level lowstands (Fischer and Sarnthein 1988;Barnaby and Ward 2007). Cycles at the bottom of the studied section are capped by this facies.…”

“…This facies was deposited under high-energy, shallowwater environments in platform margin sand shoals and platform interior to open-marine settings (Elrick and Read 1991;Flügel 2004;Ahmad et al 2006;Betzler et al 2006;Cózar et al 2006;Armella et al 2007; Barnaby and Ward 2007;Palma et al 2007;Wanas 2008). The oolitic grainstone microfacies is equivalent to the SMF 15 of Flügel (2004) and corresponds to FZ 6 and FZ 7 (platform-margin sand shoal and restricted open marine) of Wilson (1975) and Flügel (2004).…”

“…11). The mudstone-wackestone facies is interpreted to be of low-energy, lagoonal origin on its lack of fossils, presence of vugs, and association with shallow-marine deposits Amirshahkarami et al 2007; Barnaby and Ward 2007;Wanas 2008). …”

Facies, stratigraphy, and evolution of a middle Ediacaran carbonate ramp: Khufai Formation, Sultanate of Oman

“…Previous studies of mixed clastic-carbonate systems have demonstrated the stratigraphic controls on sand-body accumulation (Kerans et al, 1994;McNeill et al, 2004;Barnaby and Ward, 2007;Phelps et al, 2008). In the Permian Basin of west Texas, bypass of eolian sand into downslope environments is well documented (Phelps et al, 2008).…”