MICROSTRUCTURAL AND PETROPHYSICAL EFFECTS OF OVERTHRUSTING ON THE AZTEC SANDSTONE, BUFFINGTON WINDOW, SE NEVADA
Field observations, thin section observations and field permeability measurements were done to evaluate the microstructural and petrophysical variations from the thrust contact and down into the sandstone. The results show a low-permeable zone immediately below the thrust contact (≤50 mD), in which the sandstone is very compacted and the dominant deformation mechanism is pressure solution, limited to a few decimeters from the fault zone. Grain crushing is also an important deformation mechanism that, together with episodic and localized fluid flow along the thrust plane may have triggered pressure solution along the thrust.
The transition into high porosity/permeability sandstone is marked by an increase in permeability of up to 2 orders of magnitude (reaching 4 Darcy) and porosity from ≤3% to 40%. In the highly porous sandstone the deformation is expressed in the form of deformation bands: i) sporadic near-vertical pure compaction bands (PCB) with broadly N-S strike; ii) populations of cm-thick, reverse-sense, shear-enhanced compaction bands (SECB) oriented NNE-SSW, dipping steeply towards east (~80°), locally associated with occasional conjugate zones of gently W-dipping SECB, and iii) thin reverse-sense cataclastic bands (CB) with various orientations that consistently postdate SECB, showing a change in the dominant deformation mechanism from grain reorganization/compaction to grain crushing (cataclasis) as thrusting and lithification progressed.
This study shows that the intense deformation related to the thrusting is confined to a ca. 10-50 m low-permeable zone in the sandstone, in which pressure solution and cataclasis are the dominating deformation mechanisms, whereas highly porous and permeable sandstone with deformation band formation extends for more than 140 meters below the thrust, which can have important implications in reservoir settings.