Mechanism of Wiggly Compaction Band Formation in High-porosity Sandstone: Field Observation, Microscopic Analysis and Numerical Simulation

Abstract:

Field data and microscopic analysis are combined with numerical simulation to investigate the mechanism of wiggly compaction band formation in the high-porosity aeolian Aztec sandstone, Valley of Fire, Nevada. Field data show that the segments of wiggly compaction bands have similar orientations to preexisting shear-enhanced compaction bands, H1 and H2. The wiggly bands are inferred to propagate and periodically switch orientations between H1 and H2. The direction of greatest compression (σ₃) is interpreted as perpendicular to the overall strike of wiggly compaction bands, and the band segments that are perpendicular to σ₃ are pure compaction bands. Analysis of micropores shows that pure compaction bands have the greatest porosity, and may have a different failure mechanism.

In discrete element modelling, a particle is used to represent a pore structure surrounded by several grains. Similar to actual pore structure, the breakable particle is compacted when the resultant force acting on the particle exceeds the yielding cap determined by the failure force (F_f) and aspect ratio of the cap ellipse (k). The discrete element model, built up of many breakable particles, is compressed to simulate the formation of compaction bands. The direction of propagation of compacted zones is determined by the cap aspect ratio (k). When k=0.5, compacted zones tend to propagate perpendicular to σ₃, which corresponds to pure compaction bands. When k=2, two 45-degree directions are the predominant directions of compacted zones. The local stress state around a band tip is changed when it propagates, and the segment switches to the other direction when it reaches a critical length. As a result the wiggly compaction band takes on a wiggly pattern.