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Geostatistical Modeling Trends for Oolitic Tidal Sand Shoals

Abstract

To assess prospective modeling parameters and trends for oolitic tidal sand shoals, this study examines the cycle-scale architecture of the mobile, oolitic tidal bar belt at Schooner Cays, Bahamas. Process-based, stratigraphic trends are captured in quantitative, geocellular models of the shoal from analyses of satellite imagery, 2-D high-frequency seismic (Chirp), and sediment cores. Observations reveal recurring trends in geomorphic shapes and sedimentation patterns across this oolitic bar belt. For example, longitudinal tidal sand ridges repetitively extend up to 8 km along depositional dip gradually transforming backward into channel-bound, compound barforms consisting of linear, parabolic, and shoulder bars before terminating into a laterally extensive (<10 km), strike-elongate, sand sheet. Detailed geologic models of the shoal integrate facies probability curves, facies probability maps, facies probability cubes, bar and channel centerlines, locally varying azimuthal trends, satellite imagery, 2-D seismic, and core. To enhance confidence in this approach, we first quantify and demonstrate spatial trends in grain size, type, sorting, and barform orientation across the ooid shoal. A geocellular facies model—conditioned to multiple 3-D trends across the shoal—is built from which end-member barforms are extracted for 3-D visualization, quantitative analyses, and validation of geostatistical trends at different scales. Co-rendering of satellite imagery and bathymetric grids reveals that barforms are largely low-angle (<6°) and laterally gradational with adjacent shoal environments. As such, sequential indicator simulation (SIS) techniques are preferred. Facies proportion curves are well-ordered with respect to water depth facilitating distribution of upward-shallowing trends within barforms across the shoal. Inclusion of geometrical trends, anisotropic variograms, and 3-D facies probability grids during SIS generates gradational lithofacies tracts that are appropriately juxtaposed and consistently elongate parallel to bar crests and channels. Analyses of bootstrapped end-member barforms validates modeling techniques and reveals locally predicted facies distributions consistent with field and core observations. Although the modeling trends and methods presented here are for a specific style of modern oolitic sand shoal, similar trends might be discovered for ancient oolitic reservoirs through integrative studies of core and advanced seismic attributes.