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Morphology and Dynamics of Carbonate Tidal Sand Ridges: Schooner Cays, Bahamas*

Eugene C. Rankey1, Stacy L. Reeder2, Scott Ritter3, and Paul M. Harris4

 

Search and Discovery Article #50134 (2008)

Posted November 20, 2008

 

*Adapted from oral presentation at AAPG International Conference and Exhibition, Cape Town, South Africa, October 26-29, 2008.

Click to view list of articles adapted from presentations by P.M. (Mitch) Harris or by his co-workers and him at AAPG meetings from 2000 to 2008.

 

1 RSMAS/University of Miami, Miami, FL, USA; currently University of Kansas

2 Schlumberger-Doll Research, Cambridge, MA, USA  

3 Brigham Young University, Provo, UT, USA  

4 Chevron Energy Technology Company, San Ramon, CA, USA([email protected])

 

Abstract

Understanding and modeling many carbonate reservoir systems requires knowledge of depositional trends, including links between geomorphology and granulometry. To further explore such trends in carbonate systems, this study explores Holocene ooid shoals of Schooner Cays, Bahamas.

In this area, sands occur in geomorphic forms including both parabolic bars and flow-parallel tidal sand ridges and channels between a rocky to skeletal-sand rich outer shelf (5-8 m deep) and the muddy peloidal platform interior (~4-6 m deep). Within this bar-and-channel belt, individual flow-parallel sand ridges are up to 13 km long and 1.5 km wide, and generally radiate outward. Sand ridge crests include bare, rippled sands, with superimposed sand waves of various orientations. The crest sediments are clean, moderately well-sorted oolitic sands, with mean grain size of ~ 600 μm and no mud or silt. In contrast, most bar flank and channel areas are burrowed and partly seagrass-stabilized sediments with moderately- to poorly-sorted peloid-skeletal-ooid sands and silts, with up to ~30% mud and silt. Within the channels, there is a trend of decreasing grain sizes, from oceanward to bankward. Measured depth-averaged current velocities reach a maximum of ~80 cm/sec in channels, whereas velocities decrease to < 40 cm/sec on shallow crests; current velocity generally decreases platformward within channels. Significant wave height is < 1 m, and waves are not powerful enough to initiate large-scale sand transport, although winds may drive some westward transport on the platform.

Collectively, the results illustrate geomorphic-sedimentologic links, driven by physical oceanographic controls. The quantitative information provides metrics and trends for developing more realistic geologic or simulation models.

 

uAbstract

uFigures

uReservoir modeling

uTraining image?

uSchooners cays

uReservoirs

uMorphology

uSedimentology

uKey findings

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFigures

uReservoir modeling

uTraining image?

uSchooners cays

uReservoirs

uMorphology

uSedimentology

uKey findings

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFigures

uReservoir modeling

uTraining image?

uSchooners cays

uReservoirs

uMorphology

uSedimentology

uKey findings

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFigures

uReservoir modeling

uTraining image?

uSchooners cays

uReservoirs

uMorphology

uSedimentology

uKey findings

uReferences

 

Reservoir Modeling Using Multiple Point Statistics (MPS)

MPS is an innovative reservoir facies modeling technique that uses conceptual geological models as 3D training images to generate geologically realistic reservoir models.
            Ability to reproduce “shapes” of object-based algorithms.
            Speed, flexibility and easy data conditioning of variogram-based algorithms.

What is a Training Image?

The 3D training image is a rendering of the geological model that defines relative facies body dimensions and shapes, as well as associations between facies.
            First describe geometry of each facies.
            Then specify relationships between facies.

Morphology and Dynamics of Carbonate Tidal Sand Ridges –Schooners Cays

  • Describe the morphology of tidal sand ridges.
  • Characterize their sedimentology.
  • Relate morphology and sedimentology to the hydrodynamics.
  • Develop quantitative understanding to be used as input for reservoir models.

Tidal Sand Ridges as Reservoirs

Subparallel, elongate oolite bodies are typical of Carboniferous sequences in the Illinois Basin, the Mid-Continent, and the Appalachian Basin.

Tidal Sand Ridge Morphology

  • Muddy peloidal platform interior (~4-6 m deep).
  • Sands occur in both flow-parallel tidal sand ridges and channels and parabolic bars.
  • Rocky to skeletal-sand rich outer shelf (5-8 m deep).
  • Flow-parallel sand ridges are up to 13 km long and 1.5 km wide.

Tidal Sand Ridge Sedimentology

  • Sand ridge crests include bare, rippled sands, with superimposed sand waves of various orientations.
  • Bar flank and channel areas are burrowed and partly seagrass-stabilized sediments.
  • Bar flank and channel sediments are moderately-to poorly-sorted peloid-skeletal-ooidsands with up to ~30% mud and silt.
  • Bar crest sediments are clean, moderately well-sorted oolitic sands, with mean grain size of ~ 600 μm and no mud or silt.
  • Within the channels, there is a trend of decreasing grain sizes from oceanward to bankward.

Key Findings

  • Tidal bar-and-channel belt: ~ flow-parallel tidal sand ridges radiating outward.
  • Tidal velocities reach maximum in channels, decrease toward shoal crests and platformward.
  • Basinward-platformward and on-/off-bar sedimentologic changes.
  • Granulometric trends seem consistent with those documented in subsurface.
  • Collectively, these results illustrate geomorphic-sedimentologic links, driven by physical oceanographic controls.
  • Systematic trends in depositional geometry.
  • Clear linkages to hydrodynamics.
  • Control granulometry.
  • Quantitative information provides metrics and trends for developing more realistic geologic models.
  • Immediate value: Training images for reservoir modeling.

References

Asquith, G.B., 1984, Depositional and diagenetic history of the Upper Chester (Mississippian) oolitic reservoirs, north-central Beaver County, Oklahoma, in N.J. Hyne, ed., Limestones of the Mid-Continent: Tulsa Geological Society, Oklahoma, p. 87-92.

Rankey, E.C., B. Riegl, and K. Steffen, 2006, Form, function and feedbacks in a tidally dominated ooid shoal, Bahamas: Sedimentology, v. 53/6, p. 1191-1210.

Tharp, T.C., 1983, Subsurface geology and paleogeography of the lower Ste. Genevieve Limestone in Hamilton County, Illinois (M.S. thesis): University of Cincinnati, 93 p.

 

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