While several methods have been developed for assessing the magnitude of postdepositional heating of sedimentary rocks, apatite fission track analysis (AFTA ) can also define the time at which a sedimentary rock cooled from its maximum postdepositional temperature, up to ~110° C. This information is particularly important in hydrocarbon exploration, e.g., in defining the timing of hydrocarbon generation and identifying regions where the main phase of generation postdates formation of potential trapping structures.
Based on analysis of naturally occurring radiation damage features (fission tracks) in detrital apatite grains, the foundation of the technique is a detailed understanding of the kinetics of fission-track “annealing,incorporating observations from both laboratory and geological field conditions and making explicit allowance for apatite composition (chlorine content), which exerts a crucial influence over fission-track annealing kinetics. The thermal response of fission tracks is dominated by the maximum postdepositional paleotemperature, and this fundamental aspect of the technique imposes strict limitations on the information that can be obtained. In particular, no information can be obtained on the thermal history prior to the onset of cooling from the maximum postdepositional temperature. However, one or possibly two additional episodes of heating and cooling can often be resolved following the paleo–thermal maximum. Integration of AFTA data with paleotemperature estimates from other methods, in particular vitrinite reflectance (VR), provides additional support for thermal history interpretations and can often help to refine solutions from AFTA.
Most importantly, the combined use of AFTA and VR in a vertical sequence allows construction of profiles of paleotemperature with depth (or elevation), enabling quantitative determination of paleogeothermal gradient, which in turn allows identification of the mechanisms of heating and cooling. Heating due to deeper burial produces a linear paleotemperature profile with a similar gradient to the present temperature profile, whereas heating due primarily to increased basal heat flow will produce a profile with a higher gradient than the present temperature profile. Extrapolation of such profiles above the appropriate unconformity identified from AFTA to a suitable paleo–surface temperature allows determination of the magnitude of additional burial responsible for the observed heating. Estimations of additional burial in this way depend on assumptions concerning the lithology (i.e., thermal conductivity) of the eroded sequence and wherever possible should be combined with estimates of burial based on nonthermal processes, such as sonic velocity, in order to provide consistent constraints on the burial history. Nonlinear profiles are produced by contact heating around intrusive bodies and by passage of hot fluids within confined aquifer horizons. More complex situations that involve nonlinear profiles resulting from thick sequences with extreme thermal conductivities (e.g., coal or salt) can also be assessed by inspection of the variation of paleotemperature with depth.
AFTA has been applied to hydrocarbon exploration in a wide variety of settings. Some of the most important outcomes of AFTA analysis, in terms of events that affect regional hydrocarbon prospectivity, are: (1) the recognition of regional kilometer-scale exhumation (implying earlier deeper burial), often in areas that have traditionally been considered stable; (2) definition of major Phanerozoic paleo–thermal events in Proterozoic basins; and (3) revelation of the importance of hot fluids in transporting heat in sedimentary basins. In basins with complex histories, for example, exhumed basins or those in which heat flow was higher in the past, AFTA can provide unique constraints on the timing of hydrocarbon generation, which can significantly reduce exploration risk in such areas.