Internal Geometry of a Modern Carbonate Grainstone Shoal--an Analog for Hydrocarbon Reservoir Heterogeneity

We chose the ooid sand shoals of the Joulters Cays area of Great Bahama Bank for detailed sedimentological study to investigate the patterns of internal heterogeneity within a modern carbonate sand belt and to develop criteria for predicting the lateral extent of carbonate sand facies. Major facies identified from cores were (1) crossbedded, well-sorted ooids, (2) burrowed, poorly sorted ooids, and (3) poorly sorted ooids and mud containing Thalassia. Clast-rich zones and mud layers were also encountered. We propose that upon burial and compaction, the poorly sorted ooids and mud containing Thalassia will likely retain negligible porosity and permeability, whereas both the crossbedded, well-sorted ooids and burrowed poorly sorted ooids will likely maintain their high initial porosity and permeability. However, study of many ancient subsurface reservoirs indicates that the crossbedded, well-sorted ooids can undergo considerable cementation and have low resultant porosity and permeability. Thus, in many settings, the burrowed, poorly sorted ooids could retain the highest porosity and permeability. Additional cementation within the clast-rich zones, which occur in both the crossbedded, well-sorted ooids and burrowed, poorly sorted ooids, will result in thin, low-porosity barriers within a reservoir. Locally the surface configuration of the modern shoal complex at Joulters Cays was altered significantly by the passing of Hurricane Andrew in August 1992. Prominent wash over bars were planed off, and well-sorted ooids were deposited in low areas of the shoal where poorly sorted and mud-rich deposits of ooids had previously accumulated. The post-hurricane configuration of the shoal demonstrates how a single short-term depositional event contributed significantly to the internal heterogeneity of the shoal complex.