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Characterizing Carbonate Grainstone Geobodies


Oolitic, peloidal, and skeletal grainstones are a key element of carbonate reservoir heterogeneity in many ramp crest and shelf-margin settings. Imaging the aerial and vertical (stratigraphic) distribution of grainstone geobodies is essential for understanding distribution of primary and secondary porosity and fluid flow pathways in reservoirs like the prolific San Andres/Grayburg and Horseshoe Atoll trends of the Permian Basin. Typical grainstone elements are contained within individual depositional cycles of 2–10 m thickness. When stacked vertically there is an opportunity to image these seismically, but commonly intermediate-scale landward or seaward translation of facies tracts keeps porosity zones thin and distributed, thus below seismic resolution. Modern examples like the well-documented Joulter's Cay complex described by Harris (1979) highlight the complexity of these deposits in both time and space, and detailed outcrop mapping of successive intra-cycle bar complexes demonstrates a range of dip-extents rarely > 1 km. Outcrop and subsurface studies of Paleozoic (14) and Mesozoic (7) outcrops and reservoirs (each averaging 10 discrete grainstone geobodies) highlight differences between land-attached and shelf-edge complexes. Land-attached grainstone geobodies include foreshore-shoreface, lagoon-inlet-barrier, and sharp-based shoreface. Fair-weather and storm waves provide current energy; wind-driven longshore currents are secondary. Shelf-edge grainstones include back-reef aprons (skeletal only), tidal bars (oolitic), mixed tide-wave systems, and wind-wave bars. The most extensive ancient examples have a strong tidal influence, stable tectonics and minimal eustatic signal. The presence of a shelf-interior lagoon to allow tidal exchange is critical, and persistent longshore or trade-wind current dominates some settings. Tide-dominated grainstones show complex paleocurrents with distinct ebb-domination, large (>1m) bedforms, and oolitic composition. The Ambergris shoal (Caicos) is an analog for low-energy grainstones with maximum 10–20 cm cross-bed size such as the Jurassic Arab D. Storm-ridge wave-dominated grainstones like the shelf-crest of the Capitan system and the caprinid beaches of the Stuart City trend are characteristically coarse-grained, have significant vadose profiles, and are distinctly strike-elongate and dip limited. Tuning next-generation stochastic models to these depositional assemblages should constrain the universe of realizations.