--> Structural Style in the Eastern Papuan Fold Belt, From Wells, Seismic, Maps and Modelling

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Structural Style in the Eastern Papuan Fold Belt, From Wells, Seismic, Maps and Modelling

Abstract

Structural deformation in the PNG Fold belt involves reactivation of basement, detachment folds and out-of-sequence thin-skinned thrusts. In the Agogo area seismic data record a regional dip of ~10° to the NE in basement overlain by a thick syn-rift section. Immediately to the NE, reflection and earthquake seismic studies indicate that the Moran-Paua anticlines lie on a thrusted basement high. Geometric forward modelling, tied to wells, geology maps and poor seismic, suggests that they developed as detachment folds above a deep Jurassic décollement, but were decapitated and thrust along a shallow Jurassic décollement that cut-up through the forelimb of the folds. Each thrusted fold was oversteepened by the development of the next thrust, leaving the Moran oilfield with a 1 km hydrocarbon column. Thrusting of basement beneath Moran-Paua generated a large detachment fold at Agogo subsequently decapitated by shallower thrust faults, creating oilfields in both the gentle hangingwall and steep footwall-forelimb. The relative timing of basement thrusting and the overlying Moran-Paua deformation is uncertain. Along strike to the SE of Agogo, the Hedinia-Iagifu anticlines are similarly underlain by basement dipping at 5–10° and a thick-syn-rift section. Once again a large detachment fold was formed and decapitated on forethrusts and backthrusts creating the giant Kutubu oil and gas field in the hangingwall and a separate oil deposit in the steep footwall-forelimb. Analogue modelling suggests that the development of a large detachment fold at the site of an old normal fault is favoured by prior minor inversion of the fault. In contrast, further SE in the Usano area, seismic data indicate that basement dips shallowly NE in the foreland and may become sub-horizontal beneath the mountains. The Mesozoic stratigraphy appears to be thinner than at Agogo and Hedinia. At Usano, no large detachment fold structure is developed and the footwall is sheared out. Modelling suggests early inversion or thrusting of basement followed by shortening along Jurassic and top Cretaceous décollements. The latter facilitated the development of a duplex in the Mesozoic clastic section, including the main reservoir, and a separate duplex in the overlying Miocene carbonates. Thrusting along the Jurassic décollement decapitated the inversion fault which ‘snowploughed’ the adjacent Cretaceous mudstones causing oversteepening of the fault which created the Usano oilfield in the hangingwall.