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A computer simulation model Facies-3D carbonate, developed by Japan National Oil Corporation, describes three dimensional carbonate facies based on water depth and current velocity in conjunction with estimated paleo-topography. The Facies-3D carbonate model also contains a module for evaluation of porosity changes in meteoric diagenetic environments. Output data are facies, thickness, and porosity for each grid for the simulation. The above-mentioned calculations are made at each step for the simulation period. Two simulation case studies were conducted using depositional and diagenetic models of the Pleistocene Ryukyu Group in the Irabu Island in the southern Ryukyus, Southwest Japan and the Upper Miocene Kais Formation in the Walio oil field in Irian Jaya, Indonesia. The study of cores drilled in the Irabu Island clarified that the Ryukyu Group consists of two major lithofacies; the coral limestone facies of shallow shelf environment predominant in the eastern part of the island and the rhodolith limestone facies of deeper shelf environment predominant in the western part. Enhancement of porosity was observed beneath subaerial exposures. The Kais reefal carbonate reservoirs in the eastern Walio field were subdivided into seven shallowing upward cycles, and consist of the fore reef facies in the eastern edge of the area and the back reef facies in the central and western parts. The Kais reservoirs were affected by meteoric diagenesis. The results of the simulations show that the Facies-3D carbonate model approximated to a high degree, the distribution of facies and the occurrence of porous intervals. The Facies-3D carbonate model can be applied as a tool for reservoir characterization in the field. Introduction Carbonate reservoirs are more heterogeneous than siliciclastic reservoirs because of their biological origin, lithological complexity, and the high chemical reactivity of carbonates in the diagenetic processes. Their reservoir properties were related with the original sedimentary facies and their diagenetic alterations. Therefore, it is important to evaluate both the distributions of sedimentary facies and diagenetic features to reconstruct the distributions of reservoir properties. Since 1986, a three-dimensional computer simulation model for the reconstruction of the carbonate sedimentary processes (GEOSIM and Facies-3D) has been developed by Technology Research Center (TRC) of Japan National Oil Corporation (JNOC) (Refs. 1, 2 and 3). The GEOSIM carbonate model was developed based on results of surveys conducted on the Quaternary carbonates in the Ryukyus (Refs 4, 5 and 6) and the Recent sediments from offshore Fraser island, eastern Australia (Ref. 7). Some simulation case studies were carried out using the results of the surveys (Refs. 1 and 2). The results of these case studies indicate that the simulation model reconstructed to a high degree, the distribution of the carbonate facies. Since 1995, TRC of JNOC has developed Facies-3D which is an upgraded version of GEOSIM. From 1996, a case study using the Facies-3D carbonate model has been conducted for the Pleistocene Ryukyu Group in the Irabu Island (Ref. 3) and the Walio field located in the Salawati Basin, Irian Jaya, eastern Indonesia (Refs. 8, 9 and 10). The Facies-3D carbonate model is used to verify reliability of the depositional and diagenetic model constructed using geological and geophysical data. The results of these case studies showed the application of the Facies-3D carbonate model as a stratigraphic simulator on a basin scale and as a tool for reservoir characterization in the field. Based on the results of the case studies, the Faices-3D carbonate model has been upgraded. This paper summarizes the results of case studies for the Pleistocene Ryukyu Group in the Irabu Island and for the Miocene Kais Formation in the Walio field using the up-to-date version of the Facies-3D carbonate model, which describes not only the three-dimensional distribution of carbonate facies but also porosity changes caused by shallow diagenesis.