VITRINITE PALAEOTHERMOMETRY OF OFFSHORE EXPLORATION WELLS, TASMANIA

Abstract

Over the last three decades organic metamorphism (coalification), as indicated by changes in vitrinite reflectivity, has been regarded as a function of both temperature and heating duration. This temperature- time concept of coalification has been developed into sophisticated computer programs to model the palaeo- geothermal history of sedimentary basins. However, several papers, published over the last six years, have presented evidence to support the view that, for heating times in excess of 0.001- 1 Ma, vitrinite reflectivity constitutes an absolute palaeogeothermometer. This proposition is broadly supported by a comparison between corrected bottom- hole temperature (BHT) and vitrinite reflectivity data from offshore petroleum exploration wells drilled in Tasmanian waters. Most of the corrected BHT/vitrinite reflectivity data pairs plot on, adjacent to or between two of the published vitrinite temperature/reflectance trends. Although these data indicate that some formations are at, or near, maximum palaeotemperature, there is clear evidence to suggest that many samples, in particular those from formations in the deeper well sections, have cooled significantly below maximum palaeotemperature. It appears that present- day geothermal gradients for some of the wells, based on corrected BHT data, are much less than maximum palaeogeothermal gradients inferred from the vitrinite depth/reflectance relationship.