# 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^{[1]} is a visual representation of the Archie equation^{[2]} 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.

- Plot points of matching porosity and true resistivity (R
_{t}) on log-log paper. - Plot R
_{w}point on the R_{t}scale. - Determine
*m*using the table of values. - Plot the 100% S
_{w}line. - Plot the lines representing lower values of S
_{w}.

## Step 1: Plot points

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

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

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

After plotting the 100% S_{w} line, plot the lines representing lower percentages of S_{w} using this procedure:

- Find the intercept of R
_{t}= 1 and the 100% S_{w}line (made in the last procedure). - From this intercept, draw a line parallel to the x-axis across the plot. Any point on this line has the same porosity.
- 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. - 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.