Sedimentological and stratigraphical studies on the porphyrin pigments in Neogene Tertiary rocks.

1. Yashima oil field, Akita Prefecture

Abstract

Nickel and vanadyl porphyrins in 54 petroleum source rocks from the Nadakayama, the Harigaoka and the Uguisugawa formations of Neogene Tertiary age in the Yashima oil field, Akita Prefecture, Japan were determined.
A profound significance of the results seems to be that there are remarkable difference in the distribution of the relative content of prophyrins among the above formations.
Nickel prophyrins were found in all samples examined with the higher contents in the lower and middle parts of the Nadakayama formation and the lower contents in the overlying Harigaoka and Uguisugawa formations.
Vanadyl porphyrins were classified into two types: A-type (normal type) having α non-soret peak at about 570 mμ and B-type (abnormal type) having α non-soret peak at about 587 mμ.
A-type is detected in 39 per cent of all samples, and generally in the lower and middle parts of the Nadakayama formation. On the other hand, B-type is more widely detected in 70 per cent of examined samples. The content of B-type of the Nadakayama formation shows very irregular distribution, while those of the Harigaoka and the Uguisugawa formations have uniform distribution and similar content in each sample. This difference of the stratigraphic distribution in A- and B-type vanadyl prophyrins will probably give a hopeful clue to the elucidation of the diagenetic mechanism of these compounds.
The Nickel prophyrin contents of the thin layers of the soft and black claystone in the Nadakayama formation which has a rhythmical stratification of Flysch type showed remarkable difference to those of hard or siliceous shale layers of the same formation and rather similar to those of the black mudstone composing the Harigaoka formation.
This fact seems to support the interpretation of Taguchi (1961) that the thin layer of black claystone will correspond to the autochthonous sediments at the time of deposition of the Nadakayama formation.