Characterization of organic matter in the Miocene turbidites and hemipelagic mudstones in the Niigata oil field, central Japan

The Teradomari Formation, a principal petroleum source rock in Japan, consists of turbidite (Et) and hemipelagic (Ep) mudstones of a Bouma sequence at the type section in Niigata Prefecture, central Japan. Maceral composition of immature kerogen shows characteristic differences between Et and Ep mudstones. Et kerogen consists mainly of vitrinite and non-fluorescent amorphous organic matter (NFA) both of terrestrial origin, while Ep kerogen mainly consists of weakly fluorescent amorphous organic matter (WFA) of marine plankton origin. Elemental analysis, Rock-Eval pyrolysis, FTIR spectra and stable carbon isotope ratios support the differences in maceral composition between Et and Ep kerogens. The carbon isotopic ratios of WFA from Ep kerogen, separated by centrifugation specific gravity method, were almost the same as for bulk Ep kerogen. This supports the case for their origin from marine plankton indicated by maceral analysis