Petrophysical Heterogeneity of a Pleistocene Oolitic Shoal: Lessons for Ancient Reservoirs
F. Eduardo Cruz, Gregor P. Eberli, Alan P. Byrnes, 2006. "Petrophysical Heterogeneity of a Pleistocene Oolitic Shoal: Lessons for Ancient Reservoirs", Reservoir Characterization: Integrating Technology and Business Practices, Roger M. Slatt, Norman c. Rosen, Michael Bowman, John Castagna, Timothy Good, Robert Loucks, Rebecca Latimer, Mark Scheihing, Hu Smith
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The shallow subsurface at Ocean Cay on western Great Bahama Bank consists of Pleistocene oolitic sand shoals and adjacent facies that exhibit facies-dependent petrophysical heterogeneity and reveal depositional and early diagenetic controls on petrophysical properties in oolitic bank margin reservoirs. Sedimentological heterogeneity is produced by complex facies architecture associated with abrupt lateral changes, while diagenetic heterogeneity is largely the result of patchy dissolution and cementation.
The investigated oolitic interval is the topmost unit in 22 tightly-spaced borings covering an area of about 1 km2. Typical lithofacies include cross-bedded oolitic and oolitic/skeletal/peloidal fine- to coarsegrained grainstones and bioturbated skeletal/peloidal grainstone to packstone ranging in thickness from 0.5 m to 15 m. The architecture of the cross-bedded facies is similar to the modern Cat Cay shoal configuration of bars and superimposed sand waves that are linked to tidal flows. Particularly, the juxtaposition of biotur-bated skeletal/peloidal sediment to cross-bedded oolitic/peloidal sands is mirrored in the Pleistocene section. Intense bioturbation within the skeletal/peloidal grainstone to packstone and bedding in the oolitic/pel-oidal/skeletal grainstone facies produce small-scale heterogeneity that is enhanced by diagenesis.
Petrophysical data and petrographic analysis show that, in addition to facies, diagenesis controls porosity and permeability at different intensities. In particular, permeability is very sensitive to cementation and can be drastically reduced by few rims of fibrous aragonite and/or meniscus cements. Porosity in the Ocean Cay grainstones is high, ranging from 29 to 47%, and permeability ranges from 0.1 to 11500 mD. Permeability is partly controlled by bedding. The average permeability in the massive-bedded oolitic grainstone lithofacies is 2770 mD while it is only 620 mD in the laminated and cross-bedded oolitic/peloidal grainstone lithofacies. Vertical/horizontal permeability ratio decreases with increasing bedding complexity: in massive bedded oolitic grainstone kv/kh 0.34, in oolitic/peloid laminated and low-angle cross-bedded grainstone kv/kh 0.22, and in trough and tabular cross-bedded oolitic/peloid grainstone kv/kh 0.11. The relationship between permeability and porosity can be approximated by power-law functions.
Values for the Archie cementation exponent range from 1.9 to 4.3 (mavg = 2.7), and the cementation exponent generally increases with increasing depth. For Ocean Cay carbonates the influence of microporosity, which is as much as 35-55% of the pore volume in these rocks, on critical water saturation is significant. Critical water saturation ranges from 36% to 95% and decreases with increasing porosity. In general, petro-physical trends for some properties in the Ocean Cay oolitic/oomoldic rocks are consistent with extension of trends for lower porosity/permeability rocks in ancient reservoirs to higher porosity and permeability.