Log Analysis Background

Log Analysis Background:

It ones regret to inform that due to the quality of the logs that was done in the later 1940’s to early 1950’s.  This resulted in logs that are primitive and difficult to read.  These early logs did not give a great depiction of the field compared to present day logs.  Also, it should be noted that the older logs have been known to report values that are inaccurate and false.  Therefore, when looking at older logs one must remember to correct the readings represented on the logs.

        The mentality of the day was to find a suitable perforation zone was done by finding a steady spontaneous log reading that has a reading of -23 millivolts.  Also the perforation zone had to have a huge fluctuation change between the normal and lateral logs.  It is noted that the fluctuation zones should be present above and below the proposed perforation zones.  The higher reading from the Spontaneous Potential logs indicate that there is a great possibility that there are areas that oil is there is the formation.

There are discrepancies in reading of the older logs.  The example of the discrepancies between the interpolated values of the older logs can be found in the analysis of normal and lateral logs.  In present day logs the interpolation of normal logs are used to find electric resistivity of a formation.  The electric currents will penetrate the bore hole and into the formation.  But, for the most part the older normal log values are not to be trusted and should be corrected. The corrections are done by first locating the perforation zones on the log print out and one should notice the fluctuations of the normal and lateral logs.  The solid lines are usually the normal logs and the dashed line is the lateral log values.  Then the values of the normal log values are to be corrected by using the chart--------------- correcting for Rm to find Ro.  This Ro can then be used to calculate oil satutation by the equation So= Ro/Rt.

 But, after correcting the normal logs it is still hard to do any analysis of the logs without any Resistivity Logs reading to justify much the calculations. The values of resistivity logs are depicted as Rt in the equation above. The Resistivity log readings are the key to all log analysis.  With out a Resistivity log reading it is hard to obtain any of the reservoir fluid properties. The fluid properties include water and oil saturations as well as the bulk volume water.

In ones analysis of the logs that were available for the Ringwald field it was found that the majority of the logs were of the normal, lateral and spontaneous potential log types.  This is a common trend in the early 1950’s.  The logs had a common strata that included an abundance of correction were needed to be make in the correction of the normal log values and no indications that a resistivity log was ran.  This proposed a great problem to persons wanting to analysis this field for the reservoir characteristics and properties.  There is not much that the logs can give and the time needed to adjust each value and correct for the normal logs is not reasonable or will be very time consuming.