Time of uplift and thermal history of the Papuan Fold-belt -implications for hydrocarbon potential

Apatite fission track analysis of 35 Mesozoic sandstone and basement samples from outcrop, core and cuttings from the Papuan Fold-Belt(PFB) has demonstrated that the rocks throughout the fold-belt were uplifted close to 4.0±0.5 Ma. With increasing temperature, fission tracks in apatite crystals are progressively annealed, becoming shorter and less abundant, therefore giving a reduced apparent age. At temperatures of 100 deg.C. - 130 deg.C. the track damage is repaired (complete annealing). A typical partial annealing zone is illustrated. By comparing the annealing curves of the various stratigraphic sections with the idealized partial annealing zone curve, it is possible to determine the thermal maturity of each section, shown by the relative depths of burial of the Toro sandstone, the main hydrocarbon reservoir. Determining depth of burial assumes a consistent temperature gradient throughout the PFB, but increased thermal maturity could also be caused by higher local heat flow. From this analysis it is inferred that in the western PFB the rocks were more deeply buried, so would have generated gas-condensate, whilst shallower burial to the east allowed oil generation. This concurs with the gas-condensate at Juha, in the west, and oil at Iagifu, in the east. 4 refs