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A computer simulation model for the reconstruction of the sedimentary processes for carbonates and siliciclastics (Facies-3D) has been developed by the Technology Research Center of the Japan National Oil Corporation. The Facies-3D carbonate model describes carbonate facies in each grid in three dimension based on water depth and current velocity. Water depth is calculated from sea level changes and basement movement. Current velocity at each grid is calculated from boundary current velocity data given at four corners of the simulation area and at any other points. The accumulation rate of sediments is determined from the type of carbonate facies and the calculated water depth. Output data at each step are facies and thickness for each grid. The above-mentioned series of calculations is made at each step for the simulation period. A simulation case study was conducted using the deposition al model of the reefal carbonate facies of the upper Kais Formation, which is the main oil reservoir in the Walio field in the Salawati Basin in Indonesia. The upper Kais Formation is interpreted to have been deposited during a third-order cycle (5.5 Ma-4.2 Ma). Eight carbonate facies were delineated in the Kais and overlying Klasafet Formations as follows; (1) restricted back reef, (2) open back reef, (3) reef flat, (4) reef crest, (5) reef front, (6) shallow fore reef, (7) fore reef talus, and (8) deep fore reef. Parameters for the simulations such as carbonate facies, basement data, sea level curve, basement subsidence, and current conditions were evaluated from various analyses from eight key wells in the Walio field area, regional geology and ODP data. The simulation was conducted for the period of 5.4 Ma to 4.8 Ma. The results indicate that the simulation model describes the back-stepping feature and facies change at the major flooding events such as Q-1 , 0 and K interpreted from well data.

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A numerical model has been developed which successfully simulated not only carbonate precipitation but also re-sedimentation of carbonate material under a strong tidal current environment. Carbonate reservoirs in oil fields, offshore Abu Dhabi, were deposited under such environments, so that there is difficulty to predict the reservoir heterogeneity in the exploitation stage where no more appraisal wells would be available. A numerical process model is the only solution for predicting reservoir heterogeneity under such conditions; however, most of the carbonate models that have been developed in recent years take account only of the generation of carbonate material under specified depositional environments. Sedimentation by tidal currents, which is similar to clastic sedimentation, was also neglected in the models because of its lesser importance for carbonate reservoir characterization. The present model estimates in detail the distribution of tidal current velocity in the field by using a commercial flow model with multi-layered systems. For a simplification, only the bed load transportation was taken into account for transportation of carbonate sediments. It was possible to formulate the effect by bed load transportation as a function of water depth, grain size and current velocity at each point for a given grid system. For simulation of a longer period of ancient sedimentation, the ‘Mean Field Method’ and ‘Error Tolerance Level’ are introduced for effective calculation of the equivalent result using longer time conditions than daily tidal cycles. The model developed was tested against the present-day geologic situation from the Bahamas where many of the input parameters can be observed. Then it is applied to the real oil field, offshore Abu Dhabi. We could reproduce the geologic sequences that are conceptually similar to the observations in both cases. The model thus has the potential to predict ancient reservoir quality at the petroleum exploitation stage.