FISSION TRACK ANALYSIS: A NEW TOOL FOR THE EVALUATION OF THERMAL HISTORIES AND HYDROCARBON POTENTIAL

Abstract

Fission track dating is a new approach to the interpretation and quantitative modelling of thermal histories of sedimentary basins for hydrocarbon resource evaluation. This technique depends on the observation that annealing of fission tracks in minerals, like the generation and maturation of hydrocarbons, is a function of temperature and time. The temperature interval over which track annealing occurs in the mineral apatite, a common detrital mineral in sedimentary rocks, is virtually identical (60° to 125°C) with that required for the maximum generation of liquid hydrocarbons. Fission tracks in apatite separated from a rock sample thus contain a record of its heating in the oil generation window. The pattern of apatite fission track ages, together with detailed analyses of the distributions of track lengths, will yield information on thermal history unobtainable by other methods. The unique advantage of the fission track method is that it can give information not only on maximum palaeotemperatures, but also their variation through time.

Fission tracks in detrital zircon and sphene are stable to higher temperatures (200° - 300°C) than in apatite enabling limits to be placed on maximum temperatures reached in sedimentary basins, as well as giving important information on sedimentary provenance.

In the Otway and Gippsland Basins fission track ages and lengths determined on apatites, and ages determined on sphenes and zircons, have been used to reconstruct the thermal histories of different areas. Ages and track lengths of apatites from deep wells in the Otway Basin show the expected down hole decrease reaching zero apparent age where present well temperatures are about 125°C. The shape of the track length distribution is characteristic of the position of a sample within the track annealing zone and hence the oil generation window. Flaxmans-1 in the Otway Basin has an age profile indicating that present temperatures are at or very near the maximum experienced. In Eumeralla-1 sediments that lie above the present-day track annealing zone show clear evidence of track annealing in the past, indicating that temperatures have decreased. This is consistent with the relatively small amount of post-Early Cretaceous sedimentation observed in this well compared to Flaxmans-1.

Fission track analysis thus has the potential of giving a new, quantitative perspective on the temperature history of rocks, which should have an important impact on techniques of petroleum exploration.