Constraints on Thermal Evolution of Sedimentary Basins from Apatite Fission-Track Analysis

P. F. Green, I. R. Duddy, A. J. W. Gleadow, K. A. Megarty, J. F. Lovering

Spontaneous fission tracks in apatite can reveal rigorous quantitative information on the thermal evolution of sedimentary basins in the temperature range of interest to the oil exploration industry. Over geologic time scales, fission tracks in apatite show readily measurable annealing effects in the temperature range of 20°-125°C, where the peak generation of liquid hydrocarbons also occurs.

Five fission-track parameters have been identified that are sensitive indicators of temperature. These parameters are the mean confined track length, the distribution of confined track lengths, the fission track age, the distribution of single grain ages, and the dependence of fission-track age with depth in borehole sequences.

The response of tracks to elevated temperature takes the form of a progressive shortening of individual tracks. This process is increasingly understood through detailed laboratory annealing experiments. The degree of shortening undergone by a single track depends primarily on the maximum temperature to which the track is exposed, with time being of only minor importance. This behavior, coupled with the continuous production of tracks through time, means that the final distribution of track lengths directly reflects the thermal history. Since the fission-track age is a reflection of the distribution of track lengths, the age also reflects the thermal history.

A quantitative understanding of these processes can be used to predict the fission-track parameters resulting from a postulated thermal history. Comparing such predictions with observed parameters allows rigorous constraints to be placed on viable thermal histories.

Apatite fission-track analysis offers significant advantages over other techniques in the presence of only a single reaction pathway, insensitivity to environmental factors other than temperature, clear distinction between reworked and primary material, and the potential for rigorous quantitative analysis.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.