The Guayanilla superhighway outcrop section of southwestern Puerto Rico offers an unusual chance to examine the development and diagenesis of an Oligocene fringing reef and carbonate platform. The stratigraphic section embodies a complex sequence of three cycles of reef development. Reef cycle 2, some 48 m (150 ft) thick, comprises the development of a major fringing or barrier reef at this locality. Reef growth in cycle 2 was inhibited by intermittent ecological stress, perhaps by upwelling colder waters, so that rhodolite-paved ramps replaced shelf margin framework. Tectonic downwarp drowned the reef tract, replacing it by deep open shelf conditions, but a shallowing cycle due to uplift led to establishment of a third cycle of reef growth. Despite the substantial thickness of reef framework, reservoir quality porosity and permeability are low, primarily due to pervasive lime mud matrix. The paleoecological and sedimento-logical models derived from a study of the Guayanilla reef sequence will serve as a standard of comparison for other Tertiary reefs, both in the subsurface and surface.