Apatite Fission-Track Analysis as a Paleotemperature Indicator for Hydrocarbon Exploration

Abstract

Apatite Fission-Track Analysis (AFTA) is emerging as an important new tool for thermal history analysis in sedimentary basins. At temperatures between approximately 20°C and 150°C over times of the order of 1 to 100 my, fission tracks in apatite are annealed. This is due to a rearrangement of the damage present in unetched tracks, with the result that less of a track is etchable than in fresh, newly created tracks. Because of this, the length of an etched fission track reduces with increasing annealing, and in turn, the track density (and hence the fission-track age) is also decreased. In selected boreholes in the Otway basin, southeastern Australia, apatites from the Otway Group show reduction in confined fission-track length and apparent fission-track age, in a fashion characteristic of a simple thermal history in which samples are at or near their maximum temperatures at the present day. Track lengths show a steady decrease from lengths of approximately 15 µm in outcrop or near surface samples, to zero at about 125°C. Fission-track ages, however, show little or no decrease in age until temperatures exceed about 70°C. Above this temperature, ages rapidly reduce to zero at about 125°C.