Integrated (U-Th)/He apatite and fission track thermochronology as combined thermal history tools for hydrocarbon exploration: an example from the Taranaki Basin

摘要

Earlier AFTA (apatite fission track analysis) results from four wells in southern parts of Taranaki Basin revealed major cooling as a result of Late Miocene inversion. Combined AFTA and vitrinite reflectance data from the Fresne-1 well suggest around 2.5 to 3 km of section was eroded from the Fresne-1 structure, which is highly consistent with estimates from reconstructions based on seismic sections. The well-constrained tectonic and thermal history framework for this well established by this earlier study provides an ideal test-bed for the novel thermochronological technique of (U-Th)/He dating of apatite, as part of a project to assess the utility of this new technique in sedimentary basins. Helium ages have been determined in apatites from the original AFTA study. New AFTA analyses have also been undertaken, using improved methods, and integrated with vitrinite reflectance data in order to provide a thermal history framework for understanding the He dating results. He ages decrease progressively from around 6 Ma at a depth of ～400 m to ～1 Ma at 2465 m. The oldest He ages (in the shallowest samples) are slightly younger than the youngest fission track ages (in the deepest samples), and are qualitatively consistent with Late Miocene cooling as suggested by the AFTA results. Results of quantitative modelling of (U-Th)/He ages, using the best-fit thermal histories from AFTA, are generally consistent with measured values, while in detail suggesting that cooling was protracted, involving accelerated cooling during the Pliocene. These results illustrate how integration of AFTA and (U-Th)/He dating can provide improved constraints on thermal histories in sedimentary basins, particularly on rates of cooling and/or resolution of multiple events. By defining the timing of peak maturation in relation to the formation of potential trapping structures, such information can provide invaluable information for assessing hydrocarbon prospectivity and can significantly reduce exploration risk.