新弥彦‐ｌ号井において観察されたアパタイトのフィッション・トラックの消失とその意味

Abstract

Fission track fading in apatite has been measured in samples from the Shin-Yahiko-1 well, which is situated in a very young sedimentary basin in Japan. Fission track densities were too low to detect confined fission tracks, and consequently semi-track lengths were measured directly instead.Mean lengths and frequency distribution of semi-tracks decrease clearly and progressively with increasing temperature and depth below 4, 400m, and the fading zone of semi-tracks spans an interval of less than 1, 500m, and 40°C in temperature.The smooth trend of fission track fading in the Shin Yahiko-1 well suggests a relatively simple subsidence burial history, which is concordant with the burial history roughly estimated from ages of planktonic foraminifers.From the Shin Yahiko-1 well data, fission track fading over a a very short effective heating time (0.05Ma) delineates a zone on the vitrinite reflectance nomograph.GC/MS analysis suggests that the oil produced from the Shin Yahiko-1 originated from the highly matured Teradomari Formation, which occurs at a depth greater than 4, 500m and roughly corresponds to the top of apatite fission track fading zone.Semi-track length measurement is useful as a supplementary thermal indicator even for very young basins, especially in igneous basement rocks where vitrinites are absent.