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.

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