Thermal maturation pattern in the southern Bowen, northern Gunnedah and Surat Basins, northern New South Wales, Australia

Abstract

Comprehensive maximum vitrinite (telocollinite) reflectance data have been obtained for more than 260 polished sections from 28 petroleum exploration wells in the southern Bowen and northern Gunnedah Basins, and the overlying Surat Basin, in an area from north of Boggabri to the New South Wales–Queensland border. The samples studied were coal and dispersed organic matter (DOM) taken from ditch cuttings and drill cores of the Permian, Triassic and Jurassic sedimentary successions. Vitrinite reflectance profiles show that the coals and DOM in the Early Permian Back Creek Group of the southern Bowen Basin have anomalously low (suppressed) reflectance values and a higher reflectance gradient (rate of increase in vitrinite reflectance with depth) relative to the overlying Late Permian, Triassic and Jurassic sequences. Suppression of vitrinite reflectance is recorded in the Early Permian Maules Creek and Goonbri Formations in the northernmost part of the Gunnedah Basin, relative to the overlying Triassic and Jurassic sequences. Dispersed organic matter in the marine sediments of the Watermark and Porcupine Formations of the Gunnedah Basin also shows suppression of vitrinite reflectance. The vitrinite in several wells of the study area, however, displays anomalously high reflectance in parts of the sequence due to the influence of igneous intrusions. After allowing for heat effects due to intrusions and anomalies due to marine influence, the reflectance gradient over equivalent intervals in the southern Bowen Basin is higher towards the west, where the Permian sequence pinches out and the Triassic sequence overlies the basement. Vitrinite reflectance also increases at a higher rate with depth close to the Gil Gil Ridge in the east and close to the Moree High in the south. This may reflect additional heat flow associated with the basement high features. The areas between the Gil Gil Ridge and the Goondiwindi Thrust in the east, and to the west of the Gil Gil Ridge, appear to have lower reflectance gradients, and equivalent or lower reflectance values despite the greater burial depths.