Pickett Plot Calculator
Plot log(Rt) vs log(φ) to determine formation water resistivity (Rw), cementation exponent (m), and tortuosity factor (a). Includes Sw iso-lines at 20%, 50%, and 100%.
Porosity & Resistivity Data
Enter your data points below. The Sw=100% line passes through 100% water-saturated points; select points you believe are wet to anchor the fit.
| # | Porosity (fraction) | Rt (ohm-m) |
|---|
Cementation Exponent (m)
--
Formation Water Rw (ohm-m)
--
Tortuosity Factor (a)
--
Pickett Plot — log(Rt) vs log(φ)
How this was calculated
Archie's Equation: Rt = a × Rw / (φm × Swn)
Taking log: log(Rt) = −m×log(φ) + log(a×Rw/Swn)
On a log-log plot of Rt vs φ, this is a straight line with slope = −m and intercept giving a×Rw (at Sw=100%).
Fit method: Ordinary least-squares regression on log-transformed data.
Sw iso-lines: At constant Sw, log(Rt) = −m×log(φ) + log(a×Rw/Swn). Lines are parallel, shifted vertically by −n×log(Sw).
Convention: a is computed assuming the Sw=100% line represents the best fit; a = 10intercept/Rw. If a is far from 1.0, check your data selection.
Related Tool
Permeability Estimator
Estimate permeability from porosity using 5 correlations.
Estimate Permeability →Related Tool
Neutron-Density Crossplot
Identify lithology from neutron-density data.
Open Crossplot →Need help with petrophysical analysis, water saturation modeling, or AI-driven log interpretation?
Book a free strategy call →Understanding the Pickett Plot
The Pickett plot is a graphical technique based on Archie's equation that allows the petrophysicist to simultaneously determine the cementation exponent (m), the tortuosity factor (a), and the formation water resistivity (Rw) from a crossplot of true resistivity (Rt) versus porosity on a log-log scale.
On this plot, zones with 100% water saturation (Sw=1.0) define a straight line whose slope equals -m and whose intercept at unit porosity gives the product a*Rw. Water saturation iso-lines run parallel to the Sw=100% line, shifted upward by a factor of 1/Sw^n. By plotting data from multiple zones and identifying the water-wet trend, the interpreter can extract Archie parameters without requiring separate Rw measurements.
The Pickett plot is most reliable when applied to clean (low-shale) formations with a range of porosities. In shaly sands, the data points deviate from the Archie model, and more advanced shaly-sand equations (Waxman-Smits, dual-water) may be needed.
All calculations run in your browser. Built by Groundwork Analytics. Get in touch or email info@petropt.com.