A mechanical model for multiply-oriented conjugate deformation bands

A unique suite of three pairs of conjugate deformation band sets is present in Jurassic sandstones in the southeastern corner of the San Juan basin, northwestern New Mexico. In order of sequential development, these conjugate pairs are oriented to form each of the three principal conjugate attitudes: (1) upright Xʹs, (2) plan-view Xʹs, and (3) recumbent Xʹs. The symmetry axes of the three different X-geometries at this location are parallel, suggesting that the three systems are genetically related. A relatively simple stress history, with the horizontal stresses striking northeast and southeast and varying in magnitude but not in orientation, plausibly explains this suite of sequentially developed conjugate structures. In this model, the upright Xʹs formed with dip-slip, normal offset under the initial conditions where the overburden was the maximum stress and the northeast-striking horizontal stress was the intermediate stress. Plan-view Xʹs with strike-slip offset formed next, as the northeast-striking stress increased (due to northeastward Laramide translation of the Colorado Plateau and interaction with the local basement-cored Nacimiento Uplift) to become the maximum stress, leaving the overburden stress as the intermediate stress although unchanged in magnitude. As the northeast-directed horizontal compressive stress continued to increase, it eventually created a condition where the horizontal stresses equaled or exceeded the overburden stress, resulting in small-scale thrusting along the recumbent Xʹs. The proposed mechanical model shows that shear stress levels dropped temporarily after the formation of the upright Xʹs, providing a hiatus in deformation and making the deformation at the next stage distinct, without overlap. The three systems of intersecting conjugate deformation bands that resulted have severely compartmentalized potential reservoirs in this unit, and illustrate why similar high-porosity, deformation-banded units can have low hydrocarbon production rates despite otherwise good reservoir potential.