THERMOLUMINESCENCE GEOTHERMOMETRY — A CASE STUDY OF THE OTWAY BASIN

TL due to natural radiation (NTL) shows a first glow peak (which is commonly also the dominant peak) the glow temperature of which is a function of ambient radiation temperature and time. Progressive burial and increase of temperature results in a thermally 'cleaned' TL peak. Uplift, erosion or a decrease of temperature produces a new TL peak of lower temperature, with or without retention of the earlier formed TL peak. The glow temperature of those peaks can be evaluated in terms of time and ambient temperature of the basin. A comparison with palaeotemperatures estimated from vitrinite reflectance (VR) and apatite fission track analysis (AFTA) has shown very good agreement between NTL, VR and AFTA for those wells in the Otway Basin where present temperatures are at their maximum. In the case of uplift and a cooling trend the NTL temperatures reflect the most recent thermal regime, plus some effects from earlier thermal stages.

Artificial TL (ATL) shows at basin temperatures exceeding 50°C a gradual change of intrinsic TL parameters. The changes are probably due to a decrease in the number of certain defects. The ATL effects are cumulative and reflect the whole thermal flux. In this sense ATL is comparable with VR. Although absolute calibration of ATL is not yet possible, relative changes due to differential uplift, erosion and cooling can be interpreted once the prevailing thermal gradient is known from NTL data. In this way, ATL indicated an uplift differential of 500 m between certain wells in the Otway Basin. The same results are confirmed by VR and AFTA data.