An evaluation of organic maturation by pyrolysis of the kerogens from the Upper Cretaceous to Miocene formations in northern Hokkaido, Japan

Abstract

Organic maturation of kerogen was discussed from the yielded n-alkanes and weight losses by the pyrolysis of 67 kerogens separated from the Upper Cretaceous to Miocene muddy rocks in the northern part of Hokkaido. The results are as follows; 1) Ratios of n-C₄H₁₀/CH₄ and n-C₆H₁₄/CH₄ of the pyrolyzed products increase with burial depth in the Miocene core samples of the MITI-Hamayuchi Borehole, but decrease in the Paleogene and Cretaceous core samples of the MITI-Sorachi Borehole. 2) n-Alkane composition generated from kerogens by pyrolysis at 200℃ depend on their geologic ages, though independent at 400℃. Ratios of C₁₆〜C₂₀/C₁₀〜C₁₅ generated at the former condition decrease from the Neogene kerogens to the Cretaceous through the Paleogene. 3) There is the distinct correlation of the Carbon Preference Index of n-alkanes (C₁₃〜C₂₀) between the pyrolyzed products of each kerogen at 200℃ and 400℃, the former being smaller than the latter, though they do not show any relation with burial depth.