有機熟成(続成作用)評価の重要な指標であるビトリナイト反射率を色から推定する

摘要

Vitrinite reflectance (R_o) is one of the most confidential parameters for organic maturation. Although in case of concentrated organic matter such as coal it is easy to measure reflectance of vitrinite macerals, in case of dispersed organic materials in argillaceous sediments, it is not easy because those organic grains are often smaller than a spot of measurement under a microscope. We have measured both reflectance and colour (RGB intensities) of the same grains of vitrinite chemically isolated from 37 muddy rock samples of the Miocene from three boreholes in northern Japan. The reflectance of vitrinite grains are measured with the microscope of Axio Imager A2m connected to the microphotometer of MSP200 made by Carl Zeiss, and then images of the macerals under the reflecting microscope are taken with a digital camera. The colour of image of individual macerals are analyzed in a computer with image processing software. The condition on taking photos under the reflecting microscope is following: the light reflected from the standard of Gadolinium-Gallium-Garnet 11-87 (reflectance 1.719% in immersion oil) is set 20% of the maximum intensity of lighting of the microscope at colour temperature of 3200 K and exposure time of a camera is 31.0 ms. There is very strong correlation between R_o and each RGB intensity of vitrinite grains, whose correlation coefficients are all over 0.95. Three approximate expressions are gained with those data. R_o = 0.0000788R~2 + 0.000588R - 0.046 from R intensity of colour, _o = 0.00010G~2 - 0.0086G + 0.410 from G intensity of colour, R_o= 0.00020B~2 - 0.0213B + 0.920 from B intensity of colour Values of _o estimated from each RGB intensity of vitrinite grains are close to those of _o measured, and are plotted on or near an increasing line of measured _o on a diagram of _o vs depth of each three boreholes. Therefore, it is concluded that the organic maturity of sedimentary rocks which include only too small vitrinite grains to measure ref