In order to identify the source rocks for the Japanese Tertiary oils, about 250 shale and mudstone samples were selected from oil fields in the Akita and Yamagata Prefectures, in which the eight Miocene and Pliocene stratigraphic units, shown in Fig. 1, have been established. In addition, 19 crude oil samples were collected from a number of different fields and producing horizons for oil to rock correlation studies.
Scope of the study for the source rock identification has covered essentially the examinations from the following points :
comparison of the analytical results with accepted "source rock criteria" suggested AKITA TERRACE OEPOSITS jg KATANISHI F.
WAKIMOTO F. 0 KITAURA F.
FUNAKAWA F u O NISHIOGA G.
LITHOLOGY SAND SILTSTONE SILTSTONE SANDSTONE GRAV MUDSTONE BLACK MUDSTONE HARD BANDED SHALE SANDSTONE SILTSTONE I d CONGLOMERATE 8 F> WODUCI NG ZONES t i Fig. 1-Tertiary stratigraphic units and oil-producing horizons in Akita Basin, Japan.
CARBON PREFERENCE INDEX (CPI) The mature n-alkanes extracted from shales of the Onnagawa formation are characterised by petroleumlike CPI values which range from 0.9 to 1.5. By contrast, the CPI values of extracts from formations younger than the Onnagawa formation are almost all greater than 1.5, indicating that their contained hydrocarbons are less mature than crude oils.
POLYCYCLIC AROMATICS WITH FIVE-RING PERYLENE STRUCTURE Six types of polycyclic aromatics among those from the samples studied have been distinguished, based on their spectral characteristics. The samples from the Onnagawa formation showed markedly fewer positive indications and significantly lower concentrations of those polycyclic aromatics (except the samples from the marginal parts of the sedimentary basin), than the samples from formations younger than the Onnagawa. Samples from the younger formations showed positive indications of polycyclic aromatics ranging from 0.001 to 13-218 ppm. On the contrary, no polycyclic aromatics have been detected in 19 crude oils collected from different oil fields and producing horizons.
Ni/V PORPHYRIN RATIO by prior studies, (2) consideration of the influence of The ratio of nickel to vanadyl porphyrins in 19 postdepositional history on source bed development samples of Japanese crude oil ranges from 0.21 to 6.85. and (3) organic geochemical correlations between the About 60 % of the 250 rock samples studied had Ni/V produced crude oils and their presumed source rocks. porphyrin ratios within this range and all those came Here, however, only some properties found to link the from Miocene formations. All porphyrin ratios from crude oils and their presumed source rocks are samples in the Wakimoto formation and younger presented, as shown in Fig. 2. Pliocene formations fell outside the crude oil range.
The percentages of rock sample
