Pickett plot construction

Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Predicting the occurrence of oil and gas traps
Chapter	Predicting reservoir system quality and performance
Author	Dan J. Hartmann, Edward A. Beaumont
Link	Web page
Store	AAPG Store

A Pickett plot lets us compare water saturations of different parts of a reservoir in one or many wells. The Pickett plot is a visual representation of the Archie equation and therefore is a powerful graphic technique for estimating S_w ranges within a reservoir. All that is needed to make a Pickett plot is a set of porosities and corresponding resistivities taken from well logs and 2×4 cycle log-log paper. The procedure for making a Pickett plot consists of five steps, detailed below.

1. Plot points of matching porosity and true resistivity (R_t ) on log-log paper.
2. Plot R_w point on the R_t scale.
3. Determine m using the table of values.
4. Plot the 100% S_w line.
5. Plot the lines representing lower values of S_w .

Step 1: Plot points

Figure 1 Plot points of matching porosity and true resistivity (R_t) values obtained from well logs on 2×4 cycle log-log paper.

Plot points of matching porosity and true resistivity (R_t) values obtained from well logs on 2×4 cycle log-log paper, as shown in Figure 1. Use the x-axis for the resistivity (R_t) scale and the y-axis for the porosity (Φ) scale.

Step 2: plot r_w point

Figure 2 Plot the R_w value (resistivity of formation water) by plotting the R_w point along the R_t scale on the x-axis at the top of the graph grid where porosity is 100%.

Plot the R_w value (resistivity of formation water) by plotting the R_w point along the R_t scale on the x-axis at the top of the graph grid where porosity is 100%, as shown in Figure 2. R_w values are published by logging companies, or we can calculate them from the SP log.

Step 3: Determine m

Estimate m (cementation factor) using the table below. Laboratory analysis is necessary for a precise determination of m. However, by knowing what the expected porosity type is, we can estimate the value. If you are unsure of the porosity type, use an m of 2.

Porosity type	Value for m
Sandstones with diagenetic or detrital clay in pores	1.7–1.8
Formations with clean, macro- to micro-sized pore throats (Archie rocks)	2
Formations with vuggy porosity (touching to non touching)	2.2–3.0

Step 4: Plot the 100% S_w line

Figure 3 How to plot an m of 2.

On a Pickett plot, the value of m determines the slope of the S_w lines. The first S_w line plotted on a Pickett plot is the 100% S_w line. To plot this line, draw a line with a negative slope equal to m that begins at the R_w point. Use a linear scale to measure the slope; for example, go down length::1 in. and over 2 in.

Figure 3 shows how to plot an m of 2.

Step 5: plot s_w lines

Figure 4 An example of plotting the lower percentages of S_w.

After plotting the 100% S_w line, plot the lines representing lower percentages of S_w using this procedure:

1. Find the intercept of R_t = 1 and the 100% S_w line (made in the last procedure).
2. From this intercept, draw a line parallel to the x-axis across the plot. Any point on this line has the same porosity.
3. Where this line passes through R_t of 2, 4, 6, 8, 14, and 20, draw a series of lines parallel to the 100% S_w line.
4. Points on these lines correspond to S_w of 71, 50, 41, 35, 27, and 22%. These percentages are calculated from the Archie equation using m = 2 and n = 2 at R_t of 2, 4, 6, 8, 14, and 20.

Figure 4 is an example of following this procedure.

See also

• Determining water saturation
• Archie equation
• Determining Rt
• Calculating Rw from SP logs

External links

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