石炭の炭質からみた石炭の石油根源岩能力について

Abstract

Recent interests for source rocks of oil are focused on the source rock potential of coal. On the van Krevelen diagram, coals which were collected from coal mines in Japan (Fig. 1) show higher H/C atomic ratio than that of the Type III (vitrinite) evolution path (Fig. 2). However petrographic compositions of main coal seam at the Taiheiyo Coal Mine show that they consist of more than 90% of vitrinite and a few percent of exinite and inertinite each (Fig. 3). Degradinite, which is a maceral of vitrinite group and used only in Japan, varies from 10 to 60% in content (Fig. 4) and has good correlation with H/C and O/C atomic ratios (Fig. 5, 6). This relation is also recognized at the Ikeshima Coal Mine (Fig. 7, 8). H/C and O/C ratios of pure degradinite are estimated from regression equations among the degradinite contents, H/C and O/C ratios within the Taiheiyo and Ikeshima coals. Estimations of the pure degradinite are plotted on the Type II kerogen evolution path (Fig. 9). Japanese Tertiary coals are rich in exinite group on the assumption that degradinite is included in exinite group.Based on ultimate analysis data for Cenozoic, Mesozoic and Paleozoic coals from Japan, China, Indonesia, Australia and USA, they are plotted several independent areas on the van Krevelen diagram (Fig. 10). Cenozoic coals from Japan, China and Indonesia suggest high H/C ratios than Mesozoic and Paleozoic coals of China and Australia. From the view point of oil generation, it is inferred that the Tertiary coals have higher potential than Mesozoic ones. Actually the geologic age of major oil fields where their source rocks are assumed to coal is limited to the Tertiary period.