Coal Petrographic Property and Formation Process of Coal Seam in the Jiiban Coal Field, Japan

Abstract

The calorific value of coal in the Yeban coal field changes from 6, 400 (d. a. f.) kcal/kg to 8, 190 (d. a. f.) kcal/kg. The general results on microlithotype of coal by petrographic analyses changes from 19.6‰ to 63.3‰ in vitrite, from 17.1 ‰ to 63.2‰ in durite, from 14.1‰ to 21.1‰ in clarite and from 0.1‰ to 3.6‰ in fusite (Fig. 6).Durite is richest, clarite is the second and vitrite is the third, in carbon (d. a. f.), hydrogene (d. a. f.), volatile matter (d. a. f.) and calorific value (d. a. f.) (Fig. 4, 5).The coal in the Northern Iwaki district is the richest in durite, calorific value (d. a. f.) and volatile matter (d. a. f.), and is the highest in rankham other districts. These values are decreasing in order of the Futaba, Taga and Southern Iwaki districts (Fig. 6).As the depth of coal seams in each district increase, the calorific value (d. a. f.), degree of coalification and content of durite increase, while the content of vitrite decreases (Fig. 1-3. 7). The causes of regional difference in the degree of coalification are as follows:(1) The coal in the Northern Iwaki district was affected by geothermal metamorphism (Fig. 8).(2) In the Taga district, coal was affected by the dynamic metamorphism (Fig. 1).(3) The geological age (Early Miocene) of coal in the Southern Iwaki district where the rank is lowest, is younger than that (Oligocene) in the other districts.While, it is presumed that the aquatic dendroid plants grew on shore in the plant accumulating basin, the aquatic herbs grew off-shore in the basin; and there was two types in the plant accumulating mechanism the subsidence and upheaval. In the case of subsidence, as the basement of basin subside, the coastal plain turns into the swamp where the vegetation grow, and, then the thickness and width of the layer of plant accumulate. In the case of upheaval, as the basement upheave, the marginal area of basin turns into the land, and then the thickness and width of layer decrease. Joban coal field belongs to the type of subsidence (Table 1, Fig. 9, 10).