362 Subsurface Geologic Methods 



location, and initial production of the well should be shown in the heading 

 of the log strip. 



Using Sample Logs 



The first use of these sample logs is stratigraphic correlation by 

 laying the logs alongside each other and matching bed for bed as far as 

 possible. In doing this the geologist must keep in mind probable lateral 

 gradation, probable contamination of samples, and probable changes in 

 intervals from one horizon to another. Care should be taken not to 

 correlate over any greater distance than is necessary. For correlation 

 purposes one tries to select wells close together and extending as far as 

 possible along the lithologic strike. This information can be used for 

 construction of cross sections ^ and stereograms ^ to show stratigraphy. 



After these correlations are made and the stratigraphic section estab- 

 lished, zones can be found whose tops will make good index beds for 

 structural mapping. In selecting such horizons one must consider that 

 the depths of samples from v/hich the logs were made may not have been 

 corrected for the lag in coming to the surface. Therefore, especially in 

 zones far below the surface, the geologist must allow for this correction 

 or must correct his datum points by the drilling time, if available. 



Another important use of sample logs and descriptions is in the 

 analysis and evaluation of any pay zone, especially in limestone reservoirs. 

 This may be done by tabulating the pay sections in columns, as shown in 

 table 21. The first column shows the depth of each sample in the pay 

 section, the second column shows the net feet of pay which is the percent- 

 age of porosity and saturated material in the sample times the sample 

 interval, ai]^ the next columns give the quality of the pay which is the 

 geologist's estimate of the porosity and permeability in a qualitative 

 manner, such as slight, fair, medium, and good. The next column shows 

 the probable acre-foot recovery from the pay. This figure is derived from 

 core analyses in this field, if available, or from sample descriptions of pay 

 in fields where the ultimate recovery is reasonably well established from 

 past production. The next column in the analysis is the recovery per 

 acre, which is the acre-foot recovery times the net feet of pay. This gives 

 the recovery per acre for each sample interval. The total of this column 

 gives the recovery per acre for the well. Of course, this is a volumetric 

 estimate of the recoverable oil from any well, and since limestone reser- 

 voirs are commonly heterogeneous in their composition, this recovery 

 estimate falls within wide limits of error. Well spacing, of course, has a 

 considerable effect on ultimate recovery. Thus, if the acre-foot recovery 

 figures are derived from wells differing in spacing from the well under 

 analysis, due allowance must be made for this difference in spacing. 



' Hills, J. M., Rhythm of Permian Seas: Am. Assoc. Petroleum Geologists Bull., vol. 25, no. 2, 

 pp. 217-255, 1942. 



' Lewis, F. E., Position of San Andres Group, West Texas and New Mexico: ibid., voL 25, no. 1, 

 p. 73, footnote 1, 1941. 



