新生代炭酸塩ビルドアップの孔隙システムと近地表続成作用の影響

－沖縄県南大東島大東層の例－

抄録

Cenozoic carbonate buildup reservoirs having a vuggy pore system often show complicated reservoir quality and distribution due to near-surface diagenetic modification including subaerial exposure events. In this study, we evaluate the effects of near-surface diagenesis on pore systems of the Daito Formation carbonates on Minami-daito Island as an analog for carbonate buildup reservoirs. Based on lithological association in a framework of uplifted coral reefs, the outer rim zone and inner land area of the island correspond to the reef margin and platform interior, respectively.

Petrological study revealed that dolostones of the Daito Formation, in which dolomite typically occur as cements and fabric preserving replacements, have a similar pore system to limestones. Dolomitization in this formation is observed mainly on the outer rim zone and in the eastern part of inner land area. Dolomite distribution is controlled by reef topography rather than facies distribution.

Porosity - permeability of this formation is strongly influenced by dissolution pores such as moldic pores and vuggy pores. The carbonates are divided into 4 rock types （RTs） based on pore characteristics （RT 0; cemented rock, RT 1; isolated moldic pores dominant, RT 2; dissolution pores of corals dominant and RT 3; connected vuggy pores dominant）.

RT 0 and RT 1, which are composed of isolated pores, generally show permeability lower than 3 mD regardless of porosity values. Because RT 2 and RT 3 have a well-connected pore system, these are plotted on moderate to excellent porosity - permeability area.

Reservoir quality and its distribution are controlled by multiple near-surface diagenetic events related with the reef topography, facies and dolomitization. Calcite/dolomite cementations have been developed on the outer rim zone and formed the RT 0 and RT 1 rocks. In contrast, dissolution dominated in the inner land area and formed RT 2 and RT 3.