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ABSTRACT: Heterogeneity in Grainstone Reservoirs--Investigation of a Modern Analog, Joulters Cays, Bahamas

D. G. Bebout, P. M. Harris, M. H. Holtz, C. Kerans, R. P. Major, D. C. Ratcliff, N. Tyler, G. W. Vander Stoep

Skeletal, peloidal, and ooid grainstones form major carbonate reservoirs for hydrocarbons throughout the world. Reservoirs in the San Andres and Grayburg formations of the Permian basin provide excellent examples of this reservoir type. Subsurface and outcrop studies have documented extreme permeability variability within thin grainstone units of shoaling cycles that occur within these formations.

The Joulters Cays ooid shoal, located north of Andros Island, Great Bahamas Bank, is the most thoroughly documented modern carbonate sand complex. But even at Joulters, the core spacing of previous studies was approximately one core per square mile, which does not approach the scale needed to address most problems of interwell heterogeneity. Using the existing regional core framework as a guide, a detailed coring program with well spacing approximating that in mature oil reservoirs (between 20 and 40 ac/well) was undertaken to examine the sediment variation and early diagenesis within some subenvironments of the active shoal.

Cores and box cores taken at Joulters are dominantly clean ooid sands (grainstones) with minor muddy sands (packstone). However, changes in depositional style across the shoal and the effects of these changes on the preserved texture are of particular significance. Variations in the penetration of resin through box cores suggest greater permeability differences that are related to minor changes in grain size. These differences are most evident within graded units of spillover-lobe cross stratification on the bankward side of the shoal. Parallel-laminated seaward-dipping units show less variation. Cemented zones (hardgrounds), ranging up to at least 1 mi2 in extent, also may contribute to the permeability heterogeneity. These types of subtle depositional and early diagenetic fabrics influence subsequent diagenesis and resultant permeability variations in grainstone reservoirs.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990