Apatite Fission-Track Analysis as a Paleotemperature Indicator for Hydrocarbon Exploration
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.
Fission-track data from the Otway basin contain more information than the simple decrease of age and length. The distributions of single grain ages show characteristic patterns, particularly above 90°C. The distribution of track lengths is also diagnostic of temperature. In particular, in samples at present temperatures between 102°C and 110°C, the distribution of lengths is almost flat, with tracks of all lengths from approximately 1 µm to 16 µm.
The temperature-sensitive fission-track parameters observed in the Otway basin may be applied in other basins to elucidate paleotemperature details. In cases of mixed provenance, individual grain ages may be identified using the external detector method. Fission-track lengths in apatites containing a significant track record at the time of deposition are generally characterized by one of two types of distributions, greatly simplifying interpretation of distributions of track lengths in samples showing significant down-hole annealing.
Presence of an inherited track component, or conversely of a total loss of tracks at some time since deposition, can be identified by a comparison of the stratigraphic age with the length-corrected fission-track age. Investigation of five fission-track parameters then allows semi-quantitative constraints to be placed on thermal history. Experiments are in progress to place this procedure on a more rigorous, quantitative basis.
AFTA offers numerous advantages over the other thermal history analysis techniques, including the ability to provide a chronology of events. The method is now established in hydrocarbon exploration as a quantitative maturation indicator and should find common application.
KeywordsThermal History Length Distribution Fission Track Track Length Track Density
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