THE DEPOSITION AND HETEROGENEITY OF THE ISLA DE MONA REEF COMPLEX, PUERTO RICO, A RESERVOIR ANALOGUE FOR THE MODELING OF CARBONATE PLATFORMS

This Miocene to Pliocene Mona Reef complex exposed on Isla de Mona, Puerto Rico, provides excellent outcrops for the study of facies distribution and porosity evolution in carbonate platforms. Although the Isla de Mona complex is not an oil/gas producer platform, carbonates are major oil/gas producers in other areas such as: Indonesia, the Gulf of Suez, Mesopotamia, Adana Bay (Turkey), and Vienna basin (Rumania and Moldavia). The reef complex in Isla de Mona, Puero Rico was investigated to define facies distribution, describe strata geometries, and provide insights on porosity distribution. Two units were identified on the platform separated by a Late Miocene to Early Pliocene erosional unconformity: Unit I encompasses a Miocene coral reef framework and Unit II a Pliocene coral reef framework. The Miocene framework is characterized by platform facies that seems to be preferentially dolomitized and reef core facies that appear to be limestone. The Pliocene framework is characterized by reef core facies with less diagenetic alterations. Data from visual porosity survey revealed porosity ranging between 2% and 35% in limestone and 7% and 25% in dolomites, which commonly occurs as moldic, interparticle and intercrystalline porosity. On the basis of geometries, facies and porosity, it is suggested that the best reservoir strata on the Isla de Mona consists of grainstones and rudstones (higher porosity) that are associated with proximal platform and reef core zones with moderate water energy conditions. The best seals are wackestones (less porosity) associated with slightly agitated water energy conditions and paleosols associated with the unconformity (subaerial exposure). In addition, areas with alternations of high and low porosity strata in clinoforms can represent excellent carrier beds. The episodes of subaerial exposure promoted karsification and dolomitization of the strata, increasing the leaching of skeletal fragments, precipitation of cements and the development of secondary porosity. In general, secondary porosity affects the reservoir potential by enhancing the porosity on distal platform facies, however, additional research is needed to better understand the porosity distribution, heterogeneity and evolution on the Isla de Mona complex.