The ooid-sand shoals of the Joulters Cays area of Great Bahama Bank were chosen for detailed sedimentologic study to investigate patterns of internal heterogeneity within a modern carbonate sand deposit and to develop criteria for predicting the lateral extent of porous and permeable carbonate grainstone facies in hydrocarbon reservoirs. Using aerial photographs, surface observations, and shallow coring, we documented three sedimentary facies in a 2.7-km2 (1-mi2) study area dominated by mobile ooid sands. Cores were collected at the spacing characteristic of wells in mature hydrocarbon reservoirs.
The shoal crest at this locality had aggrading and northward-prograding (parallel to depositional strike) washover bars composed of crossbedded, well-sorted ooid sands. Burrowed, poorly sorted ooid sands were present seaward of the washover bars, and poorly sorted ooid sand and mud occupied a stabilized area bankward of the actively migrating shoal and local areas between washover bars on the crest of the shoal. Intraclast-rich zones and mud layers were also present. The shoal was crosscut by tidal channels, and older washover bars were in the process of being dissected by tidal currents. We anticipate that, upon burial and compaction, the poorly sorted ooid sand and mud facies will most likely retain negligible porosity and permeability, whereas both the well-sorted ooid sand and poorly sorted ooid sand facies will maintain their high initial porosity and permeability. However, in many ancient subsurface reservoirs, rocks with depositional textures similar to the well-sorted ooid sand facies have undergone considerable cementation and have resultant low porosity and permeability. Thus, in many settings, the poorly sorted ooid sand facies could retain the highest porosity and permeability. Additional cementation of intraclast-rich zones will most likely result in thin, low-porosity barriers within a reservoir. Hurricane Andrew, a category 4 hurricane with wind velocities of approximately 240 km/h (150 mi/h), passed over Joulters Cays in a westerly direction in August 1992, subsequent to our coring program. The hurricane profoundly changed surface features in the study area by eroding washover bars on the crest of the shoal and by transporting ooid sand seaward, leaving a nearly flat shoal crest overlain by a laterally continuous, decimeters-thick (foot-thick) lens of well-sorted ooids that thins seaward and bankward. Posthurricane tidal currents deposited a centimeters-thick (inches-thick) discontinuous layer of carbonate mud over this lens of well-sorted ooids. This mud, although more likely to be preserved in tidal channels than on the shoal crest, has the potential to form low-permeability layers that will define reservoir compartment boundaries. Modern sediment analog studies are an important addition to subsurface reservoir and outcrop analog characterization. Knowledge of the internal geometry of a sand shoal is critical for geologically targeted deployment of production technology and for predicting the efficiency of waterflood and other enhanced-oil recovery operations.
Major, R. P., Bebout, D. G., and Harris, P. M., 1996, Facies Heterogeneity in a Modern Ooid Sand Shoal--An Analog for Hydrocarbon Reservoirs: THe University of Texas at Austin, Bureau of Economic Geology, Geological Circular 9601D, 30 p. doi.org/10.23867/gc9601D.