top of page

Stress Polygon Calculator


Estimation of the state of stress

The stress polygon app allows the estimation of the stress state for a given set of observations from image logs, pore pressure, vertical stress, minimum horizontal stress Shmin, and mechanical properties. It also calculates the possible range of horizontal stress anisotropy. The stress polygon concept was originally introduced by Prof. Mark Zoback from Stanford University. For more details, read his 2007 book "Reservoir Geomechanics" (p.137).

Input data/observations:

  • Vertical stress Sv (obtained from the integration of the density log, typically between 1.0 -1.1 psi/ft

  • Pore pressure (hydrostatic pressures are 0.433 psi/ft for fresh water and 0.465 psi/ft for brine)

  • Effective UCS (Unconfined Compressive Strength) of your rock (from tabulated values in the literature for sands, carbonates, mud stones, etc.) Use our Units Converter if needed to change the units of the tabulated values for consistency with the rest of the units in the calculator 

  • Coefficient of sliding friction (Mu, between 0.6 and 1, typically around 0.6). Lower for higher clay content

  • Delta P (mud weight – pore pressure): Additional pore pressure from drilling mud weight.

  • Observations from image logs regarding the presence (or absence) of tensile fractures and borehole breakouts (y -yes, n -no, NA - not available)

  • Measured minimum horizontal stress (Shmin)

  • Units: you can use any pressure or pressure gradient units, as long as you use the same for all parameters (use our Units Converter if needed), Units in the example are psi/ft. 

  • Depth: for conversion between pressure and pressure gradient units


Rock tensile strength is zero; thermal stresses are zero; width of borehole breakouts is zero (setting this value to 0 means that no wellbore breakouts are observed).​


  • Stress polygon for a given set of parameters

  • Possible horizontal stress anisotropy (as long as input data is consistent)

Meaning of different colored lines:

  • Tensile failure line (if drilling induce tensile fractures are observed, the stress state is above this line)

  • Compressive failure line (if borehole breakouts are observed, the stress state is above this line)

  • Measured vertical stress (Sv)

  • Measured minimum horizontal stress (Shmin)

  • Possible stress state for given input parameters: Normal faulting (N), strike slip (SS), or reverse faulting (R) (or combinations, depending on where the green line falls relatively to the areas of the polygon)

Stress polygon or "Zoback-o-gram"

Make sure the app reads "© 2023 SeisPetro Geosoftware". If not, try a different browser.


Learn how to integrate stress information and seismic data for optimal reservoir recovery here.

Learn more about our fast petrophysical modeling in complex lithologies here

Not designed to work on mobile devices.

bottom of page