Precision in Pore Pressure Modeling, Part 1

The key to precision in pore pressure modeling is shale point selection. All shales in a single depositional sequence are not equal. The most common variance in the resistivity model is caused by varying carbonate content. The most common variance in the sonic model is caused by varying degrees of lithification. One must understand what the tool is measuring and the principle of how the tool makes the measurement.

The resistivity tool is a geochemical register, measuring the conductance of the combination of all compounds and elements that comprise the in-situ formation. The sonic tool measures the physical properties related to acoustic impedance of the in-situ formation. The density tool measures the electron density of the in-situ formation. The neutron tool counts the hydrogen atoms in the in-situ formation.

In this expose’ I will discuss the resistivity model.

A common excuse given for a lack of precision of a resistivity model is a baseless statement like “the resistivity model is noisy.” This is caused by using only the Gamma Ray log to select shale points for input into the model and ignoring the other logs, as well as a lack of understanding of the composition of materials deposited with the clay platelets.

Clay size clastics are deposited in a very low energy environment. Just like sand formation members have varying grain size distributions, shales likewise also have varying grain size distributions. The energy level at a depositional location determines the grain size distribution. The difference in energy levels of different grain size distributions is imperceptible to us, but significant to clay deposition. Within a basin a particular energy level will move allowing a different grain size distribution to be deposited. Through geologic time a specific energy level will again be in the same location. When drilling a vertical hole through the a shale formation member, each lamina identified by a grain size distribution, a particular grain size distribution will be encountered repeatedly. As burial continues inducing compaction, the rate of compaction will be related to the grain size distribution.

In addition to varying grain size distributions, carbonate tests of single cell organisms living in the marine environment are deposited. At times there is an abundance. Other times there is a paucity. Times of abundance contribute significant amounts of carbonate to shale deposits, and at times of paucity the shale has less carbonate content. Carbonate content of shale increases the resistivity. This causes the relatively small variance of resistivity through a shale formation member, which is why the resistivity log through a formation shale member is not a perfectly straight line. This relatively small variance has a significant effect on the calculated pore pressure of the resistivity model causing the calculated pore pressure to vary widely.

When broad strokes are used on only the Gamma Ray log without consideration of the above factors to select shale points for input into a resistivity model the resulting model will be unusable. Moving average filters are the most commonly used tool to avoid the tedious task of applying an understanding of the log and geology to pick shale points. However, moving average filters have a significant impact on the vertical resolution of the model, and very often are ineffective. Also, mathematically, a moving average reduces the precision of the model.

There is no short cut to picking shale points for a pore pressure model. It is a tedious task requiring log analysis and sedimentology skills. The objective is to be as precise as possible. The key to increasing the precision of a model is discrete shale point selection. The more consistent the shale picks, the more precise the model will be.