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Analog Modelling of the Hides Anticline, PNG: Structure of a Giant Gasfield


The Hides Anticline lies in the jungle-covered mountains of Papua New Guinea and was first recorded by Jack Hides in in 1947, mapped in 1986 and drilled in 1987. Gas was discovered in Neocomian Toro sandstone reservoirs overlain by ~1.8 km of Cretaceous Shale and ~1.5 km of Miocene limestone. The overall Hides structure is ‘double-humped’ and the gasfield was encountered in the NE anticline. The shallow structure is reasonably well constrained by the 17 wells and sidetracks, but the deeper structure is poorly imaged on seismic data and relies largely on structural interpretations. Early interpretations, included ramp anticlines and basement thrusts, but concluded that it was mainly a broad fold above an inverted normal fault with a minor shallow detachment in the Cretaceous sequence. Recent interpretations have included decapitation by a thrust system detached in the Jurassic and an inferred triangle zone along the leading edge of the thrust faults. Importantly, none of the models include the tight crestal fold at Toro level, locally with an overturned NE limb and a relatively long SW-dipping limb demonstrated by recent wells. In order to constrain the deep structure and understanding of the structural evolution, two methods were applied. First, we attempted forward geometrical modelling and structural balancing. Second, we carried out a number of detailed 3D analogue model experiments made under an X-ray tomography device. Several experiments were carried out with pure compression applying different velocities that scaled to reasonable tectonic rates. It was found that increasing the rate led to common thrust-fault displacement, particularly in the competent beds, but did not generate the fold structures observed in the Hides Anticline. Oblique compression was also found to have a poor match to the observed geometries. The final experiments, that gave the best fit to the observed geometry and the balanced sections, involved compression across a pre-existing large normal fault that had several detachment horizons in the thick, hangingwall syn-rift section with less on the footwall. The experiment was deformed slowly, which allowed a large detachment fold to develop above the old normal fault that was decapitated with displacement passing into a series of triangle zone faults in the cover. Locally, backthrusts nucleated off the crest of the normal fault as observed in the Hides Anticline. Total shortening was <20%.