Pickett Plots

Another very powerful tool for finding fracture perm and pay in a normal suite of logs (i.e. a porosity log and a deep resistivity log) is the use of Pickett plots. They are very accurate when counting total pay, estimating Rwa and Sw, defining rock types, and classifying reservoir quality. Herein we present an example of a Pickett plot from our own proprietary application package built specifically for this purpose, but you can do the same using a spreadsheet program, if you have the time and patience.

The log on the left of the image shows the clean Buda Lime at the bottom. The yellow window “hilites” the Eagleford section - both the “hot” lower Eagleford section and the Boquillas flagstones, which look very similar to the Lower Austin Chalk. The window can be moved in an animated fashion, gliding up and down the log.

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In the upper right of the image we have the actual Pickett plot where resistivity is plotted versus porosity on a Log-Log plot. It is the same data as is on the logs, but it is presented in a different way - independent of depth. In this way, a lot of details begin to emerge. In this case we have porosity derived from a cased-hole neutron tool, but it still gives us good usable data - if we do the right corrections and take care with the data.

For example, the first thing to try to establish revolves around the numbers that are wet - these always plot to the lower-left side. The red diagonal lines can be grabbed and “glide over the top” of the points, so you have control in picking what is the wet baseline. In the upper right side, you have your lowest Sw points, and best pay. Notice that the diagonal red lines represent lines of equal Sw, and we even see points align parallel along these lines, the slope of which helps us define the parameter “m” used in Archie’s equation.

The slope is one of the most diagnostic numbers for determining how much natural fracturing there is in the well. Wells with lower “m” have a higher degree of natural fracture perm. The typical “m” value used in the Archie equation is 2, because this is the average of all rocks; but, most rocks are almost never exactly 2. In our example we have a “m” of 1.8, and this well yielded about 36,000 BOE from the Lower Eagleford (natural completion with no frac). A collection of enough of these numbers helps to establish a nice data-base of what to expect in our future wells.

The Pickett plot also shows a point with one of the best pay values at 6760’. We have the ability to “probe” a given point and get the depth, Sw, etc. The program also allows us to use certain porosity and Sw cutoffs for counting pay. In this case we have a Phi cutoff of 0.06 (%) minimum, and a Sw cutoff of 0.50 (%). Given these rules, we calculate a total of 85.5 Net, out of a Gross interval (the area on the log in yellow) of 153 total gross feet of section.

In this case we later confirm our choice of Rwa and slope by looking at the Chalk section, and we see very similar numbers. It would not be rare if they were different, but when they are the same it tells us we are on the right track. The Rwa we used of 0.217 is also within the realm of what we have seen on other wells in the area.