Abstract

A computer simulation model for the reconstruction of the sedimentary processes for carbonates and siliciclastics (GEOSIM) has been developed by the Technology Research Center of the Japan National Oil Corporation. The principle for the carbonate model and a case study for Pleistocene carbonates in the Ryukyus of Southwest Japan are presented.

The carbonate model consists of a Water Depth model (one- to two-dimensional) and a Water Depth + Current model (three-dimensional). The Water Depth + Current model describes carbonate facies based on water depth and current velocity. Water depth is calculated from sea level changes and basement movement. The accumulation rate of sediments is determined from facies and calculated water depth. Compaction and near-surface diagenesis are then predicted. In the Water Depth model, the facies are determined only from the calculated water depth. A case study for the Pleistocene Ryukyu Group in Irabu Island, where seven rock types were delineated by thirteen sets of core data, was carried out. The core data indicate that a coral limestone facies of shallow water is predominant in the eastern part of the island and a rhodolith limestone facies of deeper water occurs in the western and northwestern parts. Paleo-water depth was modeled using a facies-water depth relation of Recent sediments in the surrounding area with the Water Depth model. The three dimensional sedimentary process was simulated with the Water Depth + Current model.

Results of the simulation indicate that the simulation model closely approximates the sedimentary process. The next step for GEOSIM is in its application to petroleum reservoirs. A case study for Miocene carbonate reservoirs in Southeast Asia is being conducted at the present time and its feasibility as an exploration tool will be evaluated.