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Abstract
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Chapter 2
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Early  Diagenesis
of Pleistocene Carbonates from a Hydrogeochemical Point of View, Irabu
Island, Ryukyu Islands: Porosity Changes Related to Early Carbonate DiagenesisHiroki Matsuda
Yoshihiro Tsuji
Japan National Oil Corporation
Chiba, Japan
Nobuyuki Honda
United Petroleum Development
Co. Ltd.
Tokyo, Japan
Jun-ichi Saotome
Japan Oil Development
Co. Ltd.
Tokyo, Japan
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ABSTRACT
To elucidate the early diagenesis
of carbonates, a hydrogeochemical study was carried out on groundwater
in shallow-marine Pleistocene limestones on Irabu Island, southwestern
Japan. Carbonate diagenetic processes and porosity changes within vadose,
freshwater phreatic, and mixing zones were determined on the basis of hydrogeochemical
analyses. Net dissolution of limestone occurs mainly in the vadose and
upper mixing zones where dissolution results in an increase in porosity
of 1.98% per 10 k.y. and 2.10% per 10 k.y., respectively. Calcite precipitation
occurs in the upper 1.5 m of the freshwater phreatic zone, and causes a
decrease in porosity of 5.36% per 10 k.y. Significant dissolution and precipitation
apparently do not occur in the middle to lower part of the freshwater phreatic
zone, even though that zone has active groundwater flow. The main factor
controlling diagenetic reactions on Irabu Island is considered to be CO2fluxes
into the groundwater system. Differences in solubility of carbonate minerals
are not significant for the diagenesis on Irabu Island because the island
is composed almost entirely of low-Mg calcite. |
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