ABSTRACT: Carbonate facies and diagenesis of the Pleistocene Ryukyu Group and their computer simulation

Matsuda, Fumiaki¹, Yoshihiro Tsuji², Michinori Saito², Ryotaro Iwahashi², and Hiroshi Oda²
(1) Japan National Oil Corporation, Chiba-shi, Chiba, Japan 
(2) Japan National Oil Corporation

Carbonate facies and diagenetic process of the Pleistocene Ryukyu Group exposed in the Irabu Island in the Ryukyus, Southwest Japan, were studied using cores and electrical logging data from ten wells drilled on the Island. The Ryukyu Group with a maximum thickness of about 110 meters uncomformably overlies the Late Miocene to Early Pliocene Shimajiri Group, composed of siliciclastic siltstones and sandstones. 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 the subaerial exposures. Vertical and lateral facies changes and presence of subaerial exposure surfaces are related to paleo-topography, basement subsidence and eustatic sea level changes.

An in-house computer simulation model Facies-3D carbonate describes three-dimensional carbonate facies based on water depth and current velocity in conjunction with estimated paleo-topography. The Facies-3D carbonate model also describes porosity changes in near-surface diagenetic environments. Output data are facies, thickness, and porosity for each grid at each step for the simulation period. Distribution of carbonate facies and porosity changes of the Ryukyu Group in the Irabu Island was simulated using the Facies-3D carbonate model. The results of the simulations show that the Facies-3D carbonate model approximated to a high degree the distribution of facies and porosity changes by near-surface diagenesis.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia