頸城(潟町)ガス•油田における原油の性状と地質との関係について(その2)

抄録

We published a paper "Some Relationships between Crude Oil Properties and Geology in the Kubiki (Katamachi) Gas and Oil Field, Japan (Part 1)", which appeared in the Journal of the Japanese Association of Petroleum Technologists, Vol. 35, No. 1. In the paper we pointed out the existence of the correlation between specific gravity of crude oils and reservoir conditions. This paper now presented here is a sequel of the "Part 1", dealing with the results of gaschromatographic analysis of the crude oils from the Katamachi field.
We sampled the crude oils of the le reservoir from a number of producing wells in the field, and made a gas-chromatographic analysis of lighter cut (lower than 105°C) of the crude oils. The result is shown in Table 2 of the "Part 1". The purposes of the analysis are (1) to determine the degree of evolution of the crude oils, (2) to find out any measure that characterizes the Ie crude, and (3) to presume the direction of lateral migration of the oils. As for the subject (1) we already described in the "Part 1". With regard to the subjects (2) and (3) we adopted Silverman's idea (1965) as working hypotheses. Silverman showed that the ratio of amount of some liquid hydrocarbons to that of other hydrocarbon (excluding benzene) is kept almost constant during secondary migration and accumulation of oil, if the differences in boiling points of these hydrocarbons are small. He also suggested that a trend of decrese or increase of the relative amount of benzene may imply the direction of secondary migration, which is mainly due to the selective adsorption effect of rock-forming minerals.
At first we discuss the subject (2). The ratios of amount of hydrocarbons that Silverman demonstrated are as follows:
(cylcopentane+2 methylpentane+23 dimethylbutane)/3 methylpentane
(23 dimethylpentane+cyclohexane)/n-heptane
The ratio that characterizes the crude oil and differs from others should at least have smaller variance and bear no correlation to any other factors, i.e. specific gravity of crude oil, reservoir conditions and so on. We tested the above ratios for the crudes from the Katamachi Ie reservoir, but no satisfactory results were obtained. We further tested many other ratios that Silverman had not shown, but we could not find out suitable one to the crude oil. In the course of this study we found out that some ratios have correlationship with the specific gravity of the crude oils (Figures 10 and 11). We cannot explain the reason, but we presume the alteration of crude oils by formation water which is of low salinity and has a considerable amount of sulfate ion. Furthermore, we can recognize the tendency that a number of ratios in the east side of d-d' fault are different from those in the west side of the d-d' fault (Figures 13 and 14).