東北日本新第三系泥質岩中でのメチルジベンゾチオフェン異性体変化

Abstract

Isomerizations of methyldibenzothiophene from Neogene mudstones in northeast Japan were examined as a maturity parameter of organic matter during the diagenesis to catagenesis stage．The relationship between methyldibenzothiophene（MDBT）ratio（MDR：4-MDBT/1-MDBT）and vitrinite reflectance（Ro）from the Tojo TS-1 well，which underwent subsidence at low heating rate，can be described by the equation：MDR=15.1 Ro-5.35（n=11，r=0.85，Ro=0.4-0.7%）．This equation is similar to that from Elmworth 6-28-68-13 W 6 M in Western Canada．New combination of the 1-MDBT，（2- ＋ 3-）MDBT and 4-MDBT isoomers are used to be a new maturity parameter which is useful in the high maturity stage，up to 3% Ro．The ratio（2- ＋ 3-）MDBT to（1- + 2- + 3- + 4-）MDBT increases from 0.2 to 0.6 over the range 0.4-3.0% Ro．The ratio（2- + 3-）MDBT to 4-MDBT was constant at about 0.5 in the range 0.4-1.5% Ro，and then increases from 0.5 to 4.5 in the range 1.5-2.9% Ro．Therefore，the（2- + 3-）/（1- + 2- + 3- + 4-）MDBT ratio is possibly on effective maturity indicator through the oil generation stage to higher maturity stages such as the dry gas zone．After the catagenesis stage，（2- ＋ 3-）/（4-）MDBT ratio and high Ro gradients could assist interpretation of scattered（2- ＋ 3-）/（1- + 2- + 3- + 4-）MDBT ratios with low gradient．