Halite Microstructures Reveal Deformation Mechanisms Occurring in Salt Detachments Underlying Resource-Rich Fold-Thrust Belts

Abstract

Fold-thrust belts, particularly in deepwater, play host to large amounts of the world's petroleum resources in their associated fold and thrust structures. The mechanics and geometry of these structures are controlled by underlying detachments; however, these are relatively unstudied. Detachments are often overpressured shale or salt, which are difficult to image with seismic tools; often demonstrating little or no features. The deformation of salt is understood at large (km–10m) scales. However, very little is known of the microstructural deformation mechanisms in salt detachments that control the large-scale deformation. We have used four samples from two salt detachments underlying petroleum producing basins in Australia and Pakistan to determine the microstructural mechanisms of deformation. Employing two techniques—γ-irradiation and acid etching—together revealed previously unobservable microstructures that demonstrate the micro-scale deformation. The γ-irradiation procedure induces the formation of colour centres within the halite matrix turning the salt mass blue, in thin section. Uncoloured areas are the result of crystal lattice deformation in the form of dislocations, grain, and sub-grain boundaries already present, thus, are unchanged during irradiation. The etching process polishes the notoriously difficult halite and highlights high and low angle disparities in crystallographic orientation unobtainable with conventional polishing techniques. Microstructures observed using these techniques indicate the type of deformation experienced and processes occurring. Preliminary results show vastly different deformation histories for the two detachments. Samples from Pakistan indicate multiphase deformation and recovery: dislocation creep and dynamic recrystallisation followed by solution-precipitation creep and grain boundary migration. Australian samples show simpler features with limited sub-grain dislocations yet greater grain scale microcracking indicating entirely recovered grains after significant deformation. Starkly different geological settings, Histories, and lithological heterogeneities contribute to the disparity structural geometries in the overriding fold-thrust belts, which are reflected in the observed microstructures. Linking the deformation at the micro-scale to the seismic or regional scale in these examples allows for a better understanding the factors controlling the mechanics and geometry of these major petroleum trapping structures.

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