Occurrence and content of inorganic and/or organic carbon in sedimentary rocks

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The presence, genesis, and mode of occurrence of inorganic and/or organic carbon in clastic sedimentary rocks were investigated in the present study. More than 80 samples were collected from coal-bearing or oil showing Tertiary, Cretaceous and Triassic strata that crop out in Southwest and Northeast Japan.The inorganic and organic carbon in samples was determined by transmitted and reflected light, X-ray diffraction, and CHN analysis.Inorganic carbon content was found to range from 0.1 to 9% for 38 Tertiary samples (present carbonates such as calsite, siderite, and ankerite), 0.1-13% for 35 Cretaceous samples (present mainly as carbonates such as calcite and siderite).and 0.1-10% for 9 Triassic samples (present mainly as carbonates and partly as amorphous carbon). Organic carbon content varies from 0.0 (ND) to 72% for Tertiary samples, 0.0 (ND)-70% for Cretacous samples.and 0.1-81% for Triassic samples. So-called "black shale" generally contains abundant organic carbon, although its color partly derived from iron sulfide. Organic carbon compounds in some Cretaceous and Triassic sedimentary rocks are charcterized by low hydrogen and oxygen content relative to carbon content, which corresponds to a semi-graphite state as assessed by X-ray diffraction.The various states of carbon in sedimentary rocks have been revealed in this study and the relationship between inorganic and organic carbon in sedimentary rocks requires further detailed study.