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The stratigraphic distribution and diagenetic changes of various organic and inorganic materials present in the Middle Miocene marine shales and mudstones from the Shinjo Oil Field, northeastern Japan, were compared for possible diagenetic relationships. Vitrinite reflectance, bitumen and hydrocarbon ratios, Rock-Eval pyrolysis, elemental analysis of kerogen, CPI values of n-alkanes and kerogen-form fatty acids, δ13C of bitumen and kerogen, 1H-NMR T1 of the dioxane-insoluble organic matter, epimerization of steranes and triterpanes, aromatization of steranes, organic and ammonium nitrogen contents, surface texture of sponge spicules, and the transformation of silica, clay and zeolite minerals were investigated as diagenetic indices and correlation among them was examined. The transformation of smectite to illite/smectite mixed-layer minerals and of organic N to fixed NH4-N, and the disappearance of sponge spicules by corrosion exhibited a good correlation with major indices of organic diagenesis near the threshold of intense oil generation. It is suggested that the diagenesis of organic matter (particularly changes in the lipid materials, fixation of nitrogen, and carbon isotope fractionation trends) are caused by the thermocatalytic effect of clay minerals.

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