Inversion, Detachment Folds, and Out-of-sequence Thrusts in the Papua New Guinea Fold Belt

Analysis of structural profiles through oil- and gas-fields of the Papuan Fold Belt indicates that the preexisting configuration of basement played a significant role in the compressional deformation. The Moran-Paua structures overlie upthrust basement and comprise small asymmetric detachment folds subsequently decapitated by out-of-sequence thrusts, causing shearing of the forelimb. Interpretation of reflection seismic data in the Agogo area suggests a 10° regional dip in basement and a thick syn-rift sequence in which a large detachment fold was formed, probably associated with early basement thrusting at Moran. The forelimb of the fold was then cut through by low-angle thrust faults creating oilfields in both the gentle hangingwall and steep footwall-forelimb. Along strike at Usano, in contrast, seismic interpretation suggests that the basement is shallower dipping and rift sediments appear to be thinner. There, the detachment folds were not developed and early basement thrusting/inversion resulted in a shear zone. Subsequent thrusting along Jurassic and Upper Cretaceous décollements created duplexes and cut through the shear zone such that it was oversteepened creating the Usano oilfield in the hangingwall. Understanding these relationships is important in defining new hydrocarbon plays.