The Stress Polygon: 20 Years Later, Still the Tool for Evaluating and Integrating the Scale of Stress Influence

Milton Enderli
The Gearhart Compant, Fort Worth, TX

“Structure”, as in structural geology, can be considered the response (fold, fracture, fault, and/or compact) of rocks to stress. Since structures can extend over a range of scales, from millimeters to kilometers, then it follows that extent of stresses’ influence has a similar range of scale. In 1987 (Zoback and Mastin) and again in 1990 (Moos and Zoback), the Stress Polygon method for visualizing the relationship of the magnitudes of overburden stress, maximum horizontal stress, and minimum horizontal stress was introduces and used. When used a “map overlay”, as printed on a clear plastic, the Stress Polygon provides valuable insight into the stress magnitudes and stress direction that were in place when the larger scale “structures”, depicted on the map, where created. But perhaps more importantly, integrating the Stress Polygon with smaller scale borehole information, such as breakouts and drilling induced tensile cracks, the Stress Polygon becomes an “extent of stress influence” scaling tool. This presentation will demonstrate the “map overlay” technique and how integrating the Stress Polygon with borehole information can be used to evaluate the scale of influence of a particular stress state.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas