Figure Captions (1-8)

	Figure 1. Modern analogs help refine suburface interpretations.
	Figure 2. Modern analogs provide data for reservoir characterization and modeling.
	Figure 3. Study area: Tongue of the Ocean, Schooners (at north end of Exuma Sound), and the Exumas (along the western edge of Exuma Sound).
	Figure 4. Geographic setting of Great Bahama Bank (NASA World Wind Blue Marble (WMS) 2004 image).
	Figure 5. Great Bahama Bank.
	Figure 6. Tongue of the Ocean (TOTO).
	Figure 7. Schooners.
	Figure 8. Exumas.

Modern Analogs

Modern analogs help refine suburface interpretations (Figure 1), and they provide data for reservoir characterization and modeling (Figure 2). They can be used to infer heterogeneity due to depositional facies and provide geobody attributes for Training Images and the Vertical Proportions Curve (VPC). Modern settings show dip and strike variability in great detail; facies (geobodies) can be interrogated for size, shape, and spatial relations.

Study Area on Great Bahama Bank (Figures 3, 4, and 5)

Tongue of the Ocean (Figure 6)

Rimming the southern end of TOTO is the broadest expanse of “high-energy” sands found in the Bahamas, a sand body extending parallel to the platform edge for 155 km and covering 3120 km². This sand body is generally characterized by narrow sandbars separated by wide, deep channels and a lack of islands. Individual sandbars extend onto the shallow platform on average 20 km but up to 27 km.

Schooners(Figure 7)

A variation of the sandbar motif occurs at the northern end of Exuma Sound (Schooners), where a sand body covering 716 km² and extending along the platform edge for 62 km is progressively set back from the platform edge toward the west. In comparison to TOTO, it contains broader and more irregular sandbars up to 17 km long, averaging 12 km, with relatively narrow channels and few small islands.

Exumas(Figure 8)

Sands associated with tidal channels and islands of the Exumas along the western edge of Exuma Sound occur primarily as flood tidal deltas in a linear sand body set back from the platform edge. Lobes or sandbars extend up to 8 km, averaging 6 km, onto the platform and form a deposit extending 100 km along the platform edge and covering 450 km². The islands, often two or three paralleling each other, and the inter-island gaps (= channels) control the distribution of sand.

Delineating the Sand Bodies and Sandbars

Figure Captions (9-14)

	Figure 9. Processed Landsat, TOTO.
	Figure 10. Bathymetric DEM, TOTO.
	Figure 11. Comparison of size of the three sand bodies.
	Figure 12. Sand body domains.
	Figure 13. Sandbars and bar crests: Select contours of DEM.
	Figure 14. Sandbars and bar crests: Portion of domains and total sand body occupied by sandbars.

Tongue of the Ocean (Figures 9 and 10)

Comparison of Sand Bodies (Figure 11)

Collectively, the three sand bodies occur along an ~370-km belt paralleling the platform edge, and they show a range of depositional facies patterns found in modern deposits.

Sand Body Domains (Figures 12, 13, and 14)

Shown in the tabular data with the image of TOTO (Figure 12) are the area (in km²) of each domain and percentage of total for each. ortion of each domain and total sand body occupied by sandbars [areas (km²) and percentage].

Interrogating the Sand Bodies and Sandbars

Figure Captions (15-27)

	Figure 15. Sand body profiles: TOTO.
	Figure 16. Sand body profiles: FFT analysis of strike-oriented sandbar profiles.
	Figure 17. Centerlines of sandbars: TOTO.
	Figure 18. Centerline density.
	Figure 19. The hierarchy of sandbar spacing.
	Figure 20. Size of TOTO sandbars and comparison with those of Schooners and the Exumas.
	Figure 21. Size of sandbars and bar crests.
	Figure 22. Shape of sandbars: TOTO.
	Figure 23. Shape of sandbars: Comparative data—sinuosity, form factor, aspect, roundness.
	Figure 24. Shape and size of sandbars: Form factor vs. area.
	Figure 25. Three-dimensional form of sandbars.
	Figure 26. Three-dimensional form of sandbars: Area vs. variance in water depth over bar.
	Figure 27. Three-dimensional form of sandbars: Distance to platform edge from polygon vs. surface to planar ratio of sandbar. Left: TOTO. Right: Schooners.

Sand Body Profiles

Sand body Profiles of TOTO (Figure 15) llustrate that variable fill of accommodation space equals to heterogeneity and correlation challenges within a sand body.

FFT analysis of strike-oriented sandbar profiles ((Figure 16) shows that the dominant frequency is conspicuously long in the Exumas, where the sandbars are more lobate and wider than for Schooners and TOTO. Secondary frequencies, which are similar between sites, are interpreted as the breadth of the channels that separate sandbars. Therefore, channel breadth is more consistent between the three sand bodies than the width of the sandbars themselves.

Attributes of sandbars that are useful measures of sand-body geometry include:

• Centerline (Figure 17)
• Centerline density (Figure 18)
• Hierarchy of their spacing (Figure 19)
• Size (Figures 19 and 20), together with bar crests (Figure 21)
• Shape (Figures 22 and 23), together with size (Figure 24).

In comparing size and bar crests:

• Small geobodies are more prevalent than large ones.
• Trends are highly comparable for Exumas and Schooners.
• TOTO follows a different trajectory because its sandbars, and bar crests are approximately one order of magnitude larger.
• Trends Trends are robustly exponential (r² > 0.92).

Three-Dimensional Form of Sandbars (Figures 25,26, and 27)

Sandbars with highest S/P values are closest to the margin and, as visually apparent on the Landsat images and DEMs, their geometry is linear and elongated (Figure 27).

Final Comments

• Quantitative-interrogation approach broadens our perspective of the types of information that can be derived from studies of modern environments.
• Certain aspects of the geometry of the three sand bodies are found to behave in a systematic and hence predictable manner.
• Predictable behavior has the potential to impart considerable insight to the characterization of grainstone systems, both modern and ancient.

Reference

Droste, H.J., and M. Van Steenwinkel, 2004, Stratal geometries and patterns of platform carbonates: the Cretaceous of Oman, in G. Eberli, J.L. Massaferro, and J.F.R. Sarg (eds.) Seismic Imaging of Carbonate reservoirs and Systems, AAPG Memoir 81, p. 185-206.

Return to top.

 

Copyright © AAPG. Serial rights given by author. For all other rights contact author directly.