Hydrocarbon generation potential and thermal maturity of coal and coaly mudstones from the Eocene Urahoro Group in the Kushiro Coalfield, eastern Hokkaido, Japan

Abstract

This study reveals the hydrocarbon generation potential and thermal maturity of coal and coaly mudstones from the Eocene Urahoro Group in the Kushiro coal basin (Kushiro Basin) of eastern Hokkaido (Japan). The coal is classified as subbituminous or high volatile C bituminous, and characterized by high hydrogen content (6.1 ± 0.4 wt%, 1σ, dry ash free basis). The hydrogen index (HI = 180–409 mg hydrocarbon/g total organic carbon) and random mean vitrinite reflectance (VRr = 0.42–0.56%) indicate that the coal and coaly mudstones in the Urahoro Group are gas- and oil-prone or oil-prone. Based on the HI and VRr values, the effective oil window starts at vitrinite reflectance ranges of 0.55 to 0.80%. The vitrinite reflectance of coal samples from the Kushiro Coal Mine is inversely proportional to the HI value, although each coal seam has the identical thermal history. This result suggests that the vitrinite reflectance of the samples in the Kushiro Coal Mine is suppressed. Based on the corrected vitrinite reflectance (non-suppressed vitrinite reflectance), the maturity of the coal in the Kushiro Basin increases towards the north. The non-suppressed vitrinite reflectance of the samples from the northernmost part of the basin (Hokuyo Area) was approximately 0.8%, which is within the start of the oil window. The maturity of the coal-bearing strata in the northern area of the basin could be affected by volcanic activity along the volcanic front. In the Hokuyo Area, the Urahoro Group overlies a sequence of Cretaceous–Eocene marine clastic rocks (Nemuro Group), which are about 3000–4000 m in total thickness, suggesting that the organic matter in the marine clastic rocks should reach higher maturity (VRr > 0.8%).