Characteristics and diagenesis of the kerogens associated with clay-size fractions from the sedimentary rocks, Nishiyama oil field, Japan

Abstract

Eight kerogen fractions obtained by the size-fractionation of the siltstone from the Haizume Formation of Pliocene age were examined by elemental analysis, and infrared and electron spin resonance (ESR) spectrometry. The kerogens from coarse fractions were relatively rich in aromatic structure, and they indicated characteristics smilar to fusinite maceral of coal, showing low g-value (2.0029), narrow line width (2.62G) and high spin density in the ESR properties. On the other hand, the kerogens from fine fractions were rich in functional groups and aliphatic structure, composing of amorphous kerogen and they showed similar patterns to humic acid from marine muds in the ESR spectrum. Kerogens associated with clay-size fractions obtained from thirteen core samples of UMEDA R-1, were investigated from the viewpoint of diagenetic change. As the results, a systematic change of those kerogen properties with burial was recognized, particularly in the ESR properties though the treated samples were immature in the degree of organic alteration. It is noteworthy that the variation of ESR line width from gradual increase to rapid decrease in the early stage of diagenesis was recognized in this study. This will be attributed to that clay-size fraction kerogens are composed of predominant ones in functional groups and aliphatic structure. Consequently, This may be regarded as being one of the special features in the diagenesis of kerogens associated with clay-size fraction.