The World Stress Map
Learning objectives
- Recognize that SHmax orientations are coherent over hundreds to thousands of kilometers
- Attribute the first-order stress pattern to plate-driving forces, not local geology
- Read a stress province: its SHmax azimuth and its dominant faulting regime
- Use the regional pattern as a prior for a new well before any local data arrive
Stress Is Organized at Continental Scale
A single well tells you the stress at one point. Compile tens of thousands of them, from breakouts, drilling-induced fractures, earthquake focal mechanisms, and stress measurements, and a startling order appears: the World Stress Map, maintained since the 1980s and now holding well over a hundred thousand data records. Its first lesson is coherence. The direction of maximum horizontal stress is not random from well to well; it holds a consistent azimuth across entire tectonic provinces, hundreds to thousands of kilometers wide. In eastern North America it points broadly northeast-southwest; in western Europe, northwest-southeast. Local geology, a fault here, a salt body there, perturbs the field in detail, but the regional orientation is set by something far larger.
Click the provinces in the figure. That something larger is plate tectonics: the same ridge-push and continental-collision forces that drive the plates set the first-order stress field within them. The trajectories in a plate's interior tend to align with its absolute motion, or with the push from its convergent boundary, which is why a mid-plate azimuth can stay coherent over a thousand kilometers of otherwise varied geology. The faulting regime varies more, extension near some margins, compression near collision fronts, but the orientation is remarkably steady. This is the deepest reason the stress field is predictable at all before drilling: it is not a local accident but a regional inheritance.
The Regional Prior
For the practicing geomechanicist the World Stress Map is a prior. Before a single meter of a new well is drilled, the regional map already gives the likely azimuth and the probable regime, which seeds the mud program, the preferred well trajectory, and the fault-slip screening, all before any local breakout has been logged. The local data, when they arrive, refine and occasionally overturn the prior, near a major fault the field can rotate sharply, but starting from the regional pattern is far better than starting from nothing. This completes the in-situ stress toolkit: Anderson's regime, friction's spread limit, the polygon that bounds the states, the leak-off that measures , the wellbore observations that box , and now the continental map that supplies the orientation and the first guess. Part 5 has assembled the full stress state; Part 6 takes it to the wellbore, where the Kirsch equations turn that state into the breakouts and fractures we have been invoking, and into the mud-weight window that keeps a well alive.
References
- Heidbach, O., et al. (2018). The World Stress Map database release 2016. Tectonophysics, 744, 484-498.
- Zoback, M. L. (1992). First- and second-order patterns of stress in the lithosphere: The World Stress Map project. Journal of Geophysical Research, 97(B8), 11703-11728.
- Zoback, M. D. (2007). Reservoir Geomechanics. Cambridge University Press.