Analogue Modeling of Fold and Thrust Development Above Existing Basement Structures for the PNG Fold and Thrust Belt

Abstract

The island of New Guinea records a complex structural and tectonic evolution with large hydrocarbon reserves in the frontal section of the PNG Fold Belt. The jungle-covered mountains limit data acquisition so that the internal geometry and evolution of the large anticlines are poorly understood. It is postulated that the anticlines formed above normal faults in basement. This has been tested with 14 analogue sandbox experiments performed under a X-ray tomography device. These experiments provide new 3D images of structures similar to those in PNG testing different initial configurations. Each model was initially 0.4m wide and 0.7m long and ~5cm thick, with sediments overlying a step in basement to represent an old normal fault. Layers of pyrex, sand, silicone and sand/silicone mixes were used to represent the PNG stratigraphy of 2km of molasse, 1km of carbonate, 1km of mudstone, 500m of sandstone and shale reservoirs and 0.5–3km of syn-rift clastics. The thicknesses, strengths and velocities of deformation were all scaled appropriately and erosion/deposition was modelled by adding or removing ‘molasse’. The wood basement could be moved up and down to simulate inversion. Materials allowed the development of overturned folds cut by faults, as observed in PNG. It was found that the carbonate deformation was often decoupled from the underlying reservoirs and that the structural style was critically dependent upon the strength of the intervening mudstone. Structural style was also strongly correlated to deformation rate, with a shortening of 6cm/hour in the model giving optimum results, being similar in magnitude, when scaled, to tectonic convergence rates in PNG. Slower convergence rates yielded a single large fold, whereas optimum rates yielded detachment folds with development of forethrusts and backthrusts. A pre-existing normal fault in basement led to development of a detachment fold in the cover abutting the basement in the footwall of the fault. This required a weak layer providing a detachment above the basement, corresponding to syn-rift coals in PNG. The development of the overturned detachment fold was greatly enhanced when the basement fault was first partially inverted. These models have led to a significant improvement in the understanding of the geometry of PNG fold belt structures, in areas beneath the wells where seismic resolution is poor. Optimising this deep interpretation will hopefully yield new leads and better models of existing fields.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015