SUMMARYThe underthrust fold belt of the Andes contains some very large gas fields in which the Lower Palaeozoic Sandstone reservoirs are buried to depths of over 4-6km. The foreland depositional environment received high amounts of metamorphic and igneous rock fragments and deep burial was accompanied by substantial tectonic folding and fracturing. The combined effects of Temperature, Pressure and time on these labile sediments have reduced typical porosities <7% and matrix permeabilities <1mD so that these reservoirs are ultra-tight and effectively unconventional. Nevertheless, the fields contain very large recoverable amounts of gas with minor liquids per well. They provide an intriguing case study that contrasts with the typical concept of a conventional petroleum sandstone prospect and pose the question how many more large gas fields fit this model?These complex structural fields required modelling by 3D-GEO using fractured and folded simulation models. The micro-porosity and micro-permeability has been investigated by Curtin University using nano-scale special core analysis in an attempt to identify where the hydrocarbons reside in these rocks, how they migrate out on production, and how best to estimate and optimize ultimate recovery?High technology characterization included TIMA-SEM to map mineralogy and texture at the nano-scale; X-ray Micro-CT analysis of 3D microstructure; NMR, ultra-low Helium porosity and permeability, Hg injection for capillary pressure; Elastic and Electrical Properties to tie the seismic and log data for the modelling. Tri-axial tests helped understand the structural and tectonic history and its relation to the burial history of the reservoirs.