Petrolog is a multi-featured, sophisticated log analysis system that is very powerful yet is easy to use. It was used to perform all log data capture, editing, depth shifting, and environmental corrections. Much of the log graphics work for this project was also done using Petrolog's very capable output modules. The analytical work was performed using Symbiolog's advanced clastics models, user programming, and multi-facies zone summation modules. Symbiolog is a multi-well/multi-zone analysis system which has recently been introduced to the log analysis software market. Symbiolog reads and writes log data using the digital database for the system that the user already owns, thus not requiring any data duplication. It provides capabilities that are lacking at least to some degree in every commercial log interpretation system on the market today, while letting the user keep the log input/output, editing, and graphics tools with which he/she is familiar.

The main zone of interest in this study, the Monterey Formation, consists primarily of diatomite and its diagenetic equivalents. The Etchegoin sands are shaly and feldspathic, and the formation waters are brackish in both formations. These rocks present log analysis problems very typical for California, but for which no available system today supplies adequate tools to obtain accurate reservoir values. Lithology and porosity are very important in any reservoir, but in diatomaceous intervals diagenetic grade is also a critical factor.

Clay volume is the most important log-derived reservoir parameter, and the one for which currently available analysis packages have the least diagnostic tools. Symbiolog incorporates a rule-based, geologically-driven clay analysis model that was crucial in calculating clay volumes in the lithologically diverse rocks of the Pioneer area. For the wells with modern logs (full-suite wells), a clay volume was computed from both gamma ray and SP using Symbiolog's unique three-slope transform. A parameter set was developed for the SP log which gave a clay volume closely matched to the core data and to the clay volume from the gamma ray. These parameters were then applied with confidence to the SP logs from the E-log only wells in the area. A few wells had poor quality SP's, and for these a clay volume was derived from the shallow resistivity curve and compared for verification with the results from nearby wells in the same formations.

In the same way, total and effective porosity were computed for the full-suite wells, calibrating the results to the available core data. A rule-based total porosity transform was developed as a function of lithology and depth, using comparisons to the computed porosity from the full-suite wells. Through iteration, this transform was made to be predictive of porosity over all the lithologies and depth ranges in the study area. It was then applied to the older E-log only wells. With a clay volume and total porosity, it was possible to compute an effective porosity using the same crossplot techniques that had been used for the full-suite wells (Fig. 4), Water saturation was not computed for the e-log only wells, since the deep-reading lateral resistivity curves are asymmetrical in their response to lithology changes and varying bed thickness, and therefore are invalid for calculation of a foot-by-foot saturation.

Zone summations have always been important output from a petrophysical study, but with the increasing use of reservoir modeling it is becoming necessary to generate more accurate summation values. As more attention is focused on the oil from low-resistivity pay zones, shaly sands, and thin-bedded reservoirs, it is clear that the old methods of using single-valued cutoffs for summations of porosity, clay volume, and water saturation will either under-count intervals that do not meet the criteria for clean intervals, or will overcount non-pay that is included when the criteria are widened to include less conventional reservoirs. Symbiolog features a new and unique solution to this problem: multi-facies zone summations. To use this program option, the user first computes a facies curve using any lithology model desired. For the simplest of model, a facies type (expressed as a number) is assigned to each depth level depending on the value of the clay volume parameter. More sophisticated facies computations that could be used include probabilistic or neural network models, or predictions from geostatistics. All that is needed for the Symbiosum program is an input curve with integers for each depth level representative of the chosen facies type. For this study, a rule-based deterministic model was used based on values of clay volume and porosity.

Within Symbiosum, a set of cutoffs for the summation can be defined individually by facies for clay, porosity, shale, water saturation, and permeability. The program output, which is stored in a database, includes net thickness and average value for each input curve by facies and by geologic zone.