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Multi-Scale Geocellular Models of a Holocene Bahaman Oolitic Tidal Bar Belt: How Geologic Resolution Impacts Simulation Studies

Rush, Jason; Rankey, Eugene C.; Holubnyak, Yevhen

Construction of actualistic geocellular facies and property models requires information on multiples scales of heterogeneity. To explore 3-D variability in ooid shoals and its potential impact on fluid flow, this study examines the cycle-scale architecture of 15 km by 8 km part of a mobile, oolitic tidal bar belt near Schooner Cays, Bahamas. Geocellular facies and property models of this area use: 1) morphometrics and facies maps from remote sensing analyses; 2) descriptions of sedimentology and stratigraphy from shallow cores and surficial samples; 3) stratal geometries interpreted from high-resolution seismic reflection (Chirp) data, and; 4) petrophysical and dynamic properties derived from oolitic subsurface reservoirs. Sensitivity tests were performed using a range of grid-block sizes (x/y: 1-100 m; z: 0.1-10 m) to determine the level of geologic detail required for realistic reservoir performance predictions.

Interpretation of multiple horizons and stratal terminations combined with facies mapping reveal a complex array of potential geobodies within this Holocene ooid tidal bar belt. Geometries and sediment change systematically onto the platform. For example, 3 km platformward from the margin, dip-elongate (1-8 km) flow-parallel tidal sand ridges (<600-m wide) are stratigraphically isolated, separated by a barren, or thinly veneered, current-swept outer platform (bar spacing: 400-1400 m). These same flow-parallel bars curve northwestward and gradually transform into channel-bound, compound barforms consisting of linear, parabolic, and shoulder bars before terminating into a laterally extensive (<10 km), strike-elongate, sand sheet. This platformward region (extending from 9 to 16 km onto the platform) is laterally and vertically complex, and includes lagoonal sediment overlain by ooid-skeletal sand sheets, multiple hardgrounds, broad channel-form geometries, and barforms. At a finer scale, bar-internal stratal geometries (0.1-1000 m) exhibit a broad range of bedform orientations and facies distributions (well to poorly sorted sand/gravel), which creates a complex array of spatially anisotropic geobodies. These discrete spatial patterns illustrate that oolitic shoals are not constructed of simple scalar, or nested, reservoir attributes that can be straightforwardly averaged. As such, caution should be used when translating discrete reservoir properties into progressively larger (i.e., upscaling), simulation-scale grid-blocks (e.g., 100 x 100 x 10 m).


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013