# Pickett plot

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A Pickett plot is a method used in petrophysical analysis to evaluate formation characteristics of conventional, granular reservoirs. It was developed by professor George Pickett. The method provides a graphical solution to Archie’s equation to determine water saturation within a reservoir by plotting resistivity versus porosity on a log-log scale.[1]

## Background

### Archie's Equation [2]

Archie’s Equation describes the empirical relationship between the formation factor F, porosity ϕ, water saturation Sw, and resistivities R, in granular rocks.

Where:

a = proportionality constant varying from 0.6 to 1.5

m = cementation factor that varies between 1.3 and 3

n = saturation exponent, often assumed to be 2

Ro = resistivity of the formation when 100% saturated with formation water

Rw = resistivity of the formation water

Rt = true resistivity of the formation

## Construction

### Step 1: Plot Reservoir Data Points

Select reservoirs for evaluation from the well log. Plot the points on a 2x4 cycle log-log graph by using the true resistivity, Rt, on the x-axis and porosity, ϕ, on the y-axis.

### Step 2: Plot Rw

Plot the point for the resistivity of the formation water, Rw. This point should be plotted at 100% porosity since it is accounting for only the resistivity of the water and not the resistivity of the reservoir rock. The value of the resistivity can be obtained from logging reports or calculated from the spontaneous potential.

### Step 3: Plot 100% Sw Line

As seen in the manipulated Archie’s equation, the slope of the Sw lines comes from the cementation factor, m. If the porosity type is known then m can be obtained from a table of empirical values. If not, then a value of m = 2 for clean sandstone is commonly used. To plot the 100% saturation line begin at the Rw point and draw a line with a slope of -1/m using a linear scale.

### Step 4: Plot Remaining Sw lines

Mark the intercept of the 100% Sw line and Rt = 1. Draw a line from this point towards the right, parallel to the x-axis. Mark where this line passes through the Rt values of 2, 4, 6, 8, 14, and 20. Draw lines through these points parallel to the 100% Sw line. These lines represent Sw percentages of 71, 50, 41, 35, 27, and 22.

## References

1. [ Pickett, G., R., 1973, Pattern recognition as a means of formation evaluation: The Log Analyst, vol. 14, no. 4, p. 3–11.]
2. [ Archie, G. E., 1942, The electrical resistivity log as an aid in determining some reservoir characteristics: Petroleum Transactions of AIME 146: 54–62.]