Four wells with complete log suites were selected for a comparative study. These four wells were analyzed using Pfeffer, QLA2, and Symbiolog (which was also used to analyze all the wells in the project), leading to various calculations of lithology and/or water saturation. The strengths and limitations of the various computer packages are demonstrated by this comparison (Figs. 11-17) .

Pfeffer

Pfeffer is a simple, inexpensive ($290) log-evaluation software add-in to Microsoft Excel, developed by University of Kansas researchers. It is extremely useful for quick-look analysis, and provides considerable insight into the petrophysical behavior of reservoir rocks. It is, however, also quite limited in its capabilities, and currently does not have a shaly-sand interpretation scheme. The four wells demonstrate a very fast data-load (from LAS) and an attractive and intuitive (to users of spreadsheets) interface. When data are loaded, and the proper curves identified, Pfeffer automatically creates an apparent resistivity curve, a water saturation curve, and a Bulk Volume Water curve. A "Super Pickett" plot can be constructed, which is basically a Pickett plot with additional lines superimposed for Bulk Volume Water, providing additional insight to possible water-cut problems during production. However, in the Pioneer data set, the formation is thinbedded and shaly, and a clean Archie-type analysis is insufficient. The Pfeffer program can be highly recommended for clean formations and for teaching purposes, but its lack of treatment of shaly sands limits its usefulness in the Monterey. Log displays are not necessarily the standard displays commonly used by petrophysicists, but can be modified by the Excel charting procedures to satisfy most users' needs.

Pfeffer Plotting Examples

Tenneco 62x-30 Los Lobos showing resistivity

Log Traces

Picket Plots

GeoGraphix' QLA2

QLA2 is an intermediate-level log-evaluation package suitable for general use by exploration and development geologists that is provided as an addition to the GeoGraphix system. Its interface is highly intuitive, and operation is fast and easy on medium-speed computers. The analysis of the four wells revealed a standard set of log displays (Fig. 14-17) , commonly used by petrophysicists, with modifications that are easy to make. Templates can be stored readily, for use or modification in later projects. The Archie-type analysis is not as straightforward as in Pfeffer, requiring the user to employ a formula, rather than a pre-packaged module. That formula, however, is included in the tutorial for the package, and is ready to implement. The results form the Archie analysis and the Pickett plot displays are, of course, identical to those obtained by Pfeffer.

There are dual-water shaly-sand interpretation modules built into the "model-maker" part of QLA2. These were applied to the four wells and the results compared with the clean Archie-type analysis. Considerable attention had to be paid to the determination of clay content in these extremely high-porosity Monterey rocks in order to obtain reasonable answers. A simple clay volume determined from gamma ray was useful, but improved responses resulted from more careful clay determinations obtained through other methods. These wells demonstrate the difficulty in shaly-sand interpretation in tight, high porosity rocks without core control.

The final analysis of formations such as the Monterey must include zoned studies, where the rocks, as their properties change with depth, age, etc., are treated differently in the computations. When one deals with one well at a time, this zonation is easily treated. It becomes more difficult when a large number of wells are being studied in the same field. For this reason, a more sophisticated treatment (described in the following Symbiolog section) was developed and applied by DPI.

GeoGraphix/QLA2 Plotting Examples

Los Lobos 62x-30	Los Lobos 82x-30C	KCL 44-375	Tenneco Muddy Creek 76-30