ANALYSIS OF PRE-1960 LOGS CAN PINPOINT PAY IN ILLINOIS' AUX VASES, OTHER ZONES, REGIONS

Hannes E. Leetaru
Illinois State Geological Survey
Champaign, Ill.

Old (pre-1960) electric logs are still a valuable source of information for the oil industry to use for improved and enhanced oil recovery.

Old electric logs can be used effectively to estimate porosity, water saturation, and in local areas, permeability. Results of the analysis can be applied to identifying passed over pay in older wells and as input into reservoir models.

The following discussion of old electric log techniques is a synopsis of a more detailed study at the Illinois State Geological Survey and has been published as Illinois Petroleum 134.

The study is based on old electrical log data calibrated with cores from sandstones of the Upper Valmeyeran (Mississippian) Aux Vases formation (Fig. 1).

The sandstones of the Aux Vases formation in southern Illinois commonly are fine to medium grained, moderately to well sorted, and contain 81-98% quartz and as much as 13% feldspar.

The described techniques can also be applied in other geographical areas; however the methods are empirically derived and should not be used with another formation or lithology without careful calibration of the logs to measured core data.

Old electric logs are wire line logs that combine the spontaneous potential (SP) and the normal and lateral resistivity curves. By 1956, the induction log began replacing the electric log as the primary resistivity measurement tool (Hilchie 1979), although in the Illinois basin, electric logs continued to be run during the early 1960s.

Since many of the Aux Vases oil fields were discovered before 1960, an understanding of old electric logs is important for detailed reservoir analysis.

Logging tools for measuring porosity were rarely used. For example, in a typical field such as King field, Jefferson County, Ill., fewer than five neutron or micrologs were run out of the 163 wells drilled.

One suite of modern logs was run, but the reservoir facies was not present in this well.

DATA ANALYSIS, METHODOLOGY

The study area includes Franklin, Hamilton, Jefferson, and Wayne counties in south central Illinois.

The distribution of the 70 wells from which both Aux Vases core data and electric log data were collected is shown in Fig. 2. Within each well, the log response of the Aux Vases was subdivided into zones of similar electrical properties that were calibrated with the core analyses,

In total, 73 zones (or data points) were used in this study. Three methods for using old electric logs to estimate the porosity of sandstones in the Aux Vases formation were compared.

The results of this comparison showed that the empirical normalized SP (NSP) method was significantly better than were the short normal (Pirson 1957 and Hilchie 1979) and Rocky Mountain (Tixier 1949) methods for estimating porosity in the sandstones of the Aux Vases formation.

NORMALIZED SP METHOD

Griffiths (1952) showed an inverse relationship between the amount of clay in the rock and the magnitude of the SP deflection.

The SP-clay relationship is the basis for the technique of estimating porosity presented in this report. Although the SP response is an indication of the amount of clay in a rock, not of porosity or permeability, an increase in clay implies a corresponding decrease in porosity and permeability,

Because the actual value of SP on the log is not an absolute number, SP values cannot be directly compared among different wells.

Normalizing SP values against an internal standard can compensate for changes in the scale of the log, the mud resistivity, and the size of the bore hole, thus allowing direct comparisons of SP values between different drill holes.

The first step in normalizing the SP measurements is to establish a shale baseline (SPmin) through the average SP curve in a thick shale.

Similarly, a clean sand baseline (SPmax) is established.

The millivolt values for the shale baseline and the clean sand baseline established for each log record were input with the SP for the zone of interest in the normalizing equation. The NSP values are unitless and range from 0 to 100.

NSP -SPlog - SPmin

----------------- x 100

SPmax - SPmin

where

SPmax = average maximum SP reading (mV) for a clean Cypress sandstone

SPmin = average SP (mV) at the shale baseline

SPlog = SP value (mV) for the zone of interest

In the area of investigation, each well was standardized using the value for the cleanest thick Cypress sand encountered in the well (SPmax) as a reference.

The Cypress sandstone occurs some 200 ft above the Aux Vases and consists of multiple sandstone bodies that can each be more than 50 ft thick.

Analyses of numerous Cypress cores reveal that porosity values are relatively consistent. Therefore, the Cypress is especially suitable as a standard for normalizing SP.

The relation between NSP and measured core porosity in 73 zones from 70 wells has a Pearson correlation coefficient (r) of 0.83 (Fig. 3).

The data exhibit considerable scatter or deviation from the least squares regression line.

More than a 5% porosity difference can occur between the core analysis and the estimated porosity calculated using the best fit regression line. However, most wells show a significantly smaller amount of error. The calculated standard error of estimate is 2.6% porosity.

PERMEABILITY

A logarithmic relationship is observed between average core porosity and core permeability (Fig. 4).

A direct correlation also exists between the NSP and log of the permeability but the relationship is not linear (Fig. 5).

Kolodzie (1980) found a general relationship between permeability and NSP and estimated permeability by using the NSP. Measured permeability values for the Aux Vases plotted against NSP show a wide range.

Therefore, this method cannot be used in general reservoir studies to predict permeabilities. For example, in Fig. 5 for an NSP value in the mid-60s, measured permeability ranges from < 10 md to 100 md.

Aux Vases sandstone reservoirs with permeabilities < 10 md are not commercial, whereas those at 100 md can be prolific producers. In restricted areas such as portions of Jefferson County, the NSP-permeability method may be useful (Fig. 6).

The NSP cross plots may work here because all of the Aux Vases was formed in a similar diagenetic and lithologic facies. Subsequent work may document a regionally distributed relationship between NSP and permeability, which would allow semiquantitative estimations of this reservoir characteristic.

WATER SATURATION

A Pickett plot analysis was used to determine the water saturation of King field, which has produced more than 4 million bbl of oil from the Aux Vases sand. All of the wells that had usable logs were plotted on the Pickett plot (Fig. 7). The porosity was calculated using the NSP method; resistivity was measured from the long normal.

The cementation exponent (m) is the most difficult of the variables in the Archie equation (Archie 1942) to determine.

The value of m is dependent on pore geometry and equals 2 in sandstones that contain no clay matrix. A common method of compensating for the effects of clay on old electric logs was to vary the cementation exponent (Hilchie, 1979).

The Aux Vases at King field has clay in its rock matrix. For this reason, the cementation exponent of the Aux Vases at King field was assigned a value of 1.7.

Log values of porosity and resistivity from sands that have Sw = 100% and that display a constant m plot should ideally plot along a straight line on log-log graph paper (Lang 1973).

At King field, the oil-water contact is not well defined, and some wells that have been interpreted as wet may contain oil. All wells drilled into the postulated water zone plot below Sw 50%.

The data are more scattered than on modern logs. This scatter probably resulted from error in using estimated porosity from the SP.

Note that the Pickett plot will work only if m stays constant throughout the study area and the resistivity tool has the same depth of investigation.

Only long normal values (AM 64) were used in this study.

Resistivity values from different types of tools cannot be combined on a Pickett plot.

For example, values of resistivity from the induction tool cannot be used together with values from a long normal tool.

SUMMARY

In the Aux Vases sandstones of this study area the empirical normalized spontaneous potential method was significantly better in predicting porosity than was either the short normal or Rocky Mountain methods.

Normalizing spontaneous potential values to an internal standard can compensate for changes in the scale of the log, the mud resistivity, and the size of the borehole and allows for direct comparison of spontaneous potential values between drill holes.

The NSP cannot be used to accurately estimate permeability.

Although calculated values commonly are in the correct order of magnitude, they usually are not accurate enough for detailed reservoir analysis.

In local areas with similar lithologic facies the correlation can be strong enough to allow for semiquantitative estimations of permeability. Water saturations can be estimated by using Pickett plot analysis.

The major advantage of Pickett plots over the basic Archie equation is that Pickett plot is a pattern recognition tool and anomalies in the data are easily seen.

ACKNOWLEDGMENTS

This research was done under U.S. Department of Energy Grant DE-FG22-89BC14250 and the State of Illinois through Department of Energy and Natural Resources Grant AE-45.

BIBLIOGRAPHY

Archie, G.E., The electrical resistivity log as an aid in determining some reservoir characteristics: Transactions of the American Institute of Mechanical Engineers, Vol. 146, 1942, pp. 54-62,

Griffiths, J.C., Grain-size distribution and reservoir-rock characteristics: AAPG Bull., Vol. 36, No. 2, 1952, pp. 205-229.

Hilchie, D.W., Old Electric Log Interpretation: Institute for Energy Development, Tulsa, Okla., 1979, 161 p.

Kolodzie. S., Analysis of pore throat size and use of the Waxman-Smits equation to determine OOIP in Spindle field, Colo.: presented at SPE meeting, Dallas, September 1980, SPE paper 9382.

Lang, W.H., Porosity-resistivity cross-plotting: The Log Analyst, Vol. 14. No. 1, January-February 1973, pp. 16-20.

Pirson. S.J., Formation evaluation by log interpretation: World Oil, April, May, June 1957.

Tixier, M.P., Electric log analysis in the Rocky Mountains: OGJ, June 23, 1949, pp. 143-147, 217-219.

Copyright 1991 Oil & Gas Journal. All Rights Reserved.