石狩炭田南大ゆう張付近の古第三系のビトリナイト反射率と古地温の推定

Abstract

Vitrinite reflectivity measurements were made on core samples from eight boreholes which penetrated the Poronai Formation and the Ishikari Group of Paleogene age. The Ishikari Group, the productive coal measures, is divided into the Noborikawa, Horokabetsu, Yubari and Wakanabe Formations in ascending order, and is unconformably overlain by the Poronai Formation. The estimated maximum depth of burial is about 3300m at the basal part of the Poronai Formation. Vitrinite reflectivity changes gradually with stratigraphic position in the Poronai Formation, and increases rapidly in the Ishikari Group. The values of vitrinite reflectivity average 0.7% at the basal part of the Poronai Formation; 0.87% at the top of the Yubari Formation about 100m below the unconformity ; 0.94% at the bottom of the Yubari Formation about 250m below the unconformity. The paleotemperature estimated from vitrinite reflectivity of the basal part of the Poronai Formation is 130°C with a geothermal gradient of 3.5°C/100m, which is higher than the present gradient of 2.9°C/100m. The measured values of vitrinite reflectivity in the Ishikari Group are higher than values expected from the geothermal gradient for the Poronai Formation. These facts suggest that thermal effect of deep-seated igneous activity was superimposed on the normal burial coalification.