Subsurface Laboratory Methods 329 



minimum area of 25 square miles should be covered in a survey, so that 

 anomalies can be established. 



Equally satisfactory results have been obtained from samples taken 

 in all types of terrain from swamps to sand dunes in all parts of Texas 

 and in parts of Louisiana, Mississippi, Kansas, and New Mexico. Soil 

 samples taken at different times and under different conditions exhibit 

 the same fluorescent characteristics, so that a fluorographic survey can be 

 repeated with equivalent results. 



The Blau technique depends upon the fluorescence characteristic of 

 petroleum or its derivatives, the employment of soil standards of known 

 concentration, and the empirical correlation of soil samples with the 

 locality from which they were obtained. The measurement of fluorescence 

 intensity is made by photography, and the interpretation is performed by 

 an. experienced oil geologist. 



The novel feature of fluorographic exploration is that drilling is not 

 required. The soil samples are collected to a depth of several inches from 

 the surface. Plant material is carefully excluded, so as to avoid contamina- 

 tion. In a given region all samples are taken from uniform depths. 



The soil samples as obtained are placed under ultraviolet light and 

 the fluorescence intensity determined by comparison with standards pre- 

 pared from nonfluorescent soil containing ten, thirty, forty, fifty, and so 

 on parts of oil per million of soil. Visual examination is suitable for 

 samples carrying large amounts of oil. 



A fluorographic survey is interpreted by posting the* fluorescence- 

 intensity values on the survey grid according to the location from which 

 they were taken and by drawing contour lines, called "isofluors," to cor- 

 respond to the fluorescence values of the stations. A subsurface accumula- 

 tion of oil may show isofluoric enclosure. If structure is present, an iso- 

 fluor map may disclose the geologic pattern of that structure. The iso- 

 fluor map is effective for locating some faults and minor geologic features, 

 and the details may enable a geologist to differentiate between a structural 

 and a stratigraphic accumulation. 



The final interpretation or recommendation is empirical, and valid 

 data are obtained only after a study of a series of maps embracing both 

 the most detailed and the most generalized of information. 



SHALE DENSITY ANALYSIS 

 F. WALKER JOHNSON 

 Shale density is a criterion of formation evaluation which is some- 

 times applicable to subsurface geological problems. Shale compaction, a 

 major factor in density variation, has long been recognized by geologists 

 as being an important geological process.^'^ Compactio\of shale is the 

 result of pressure exerted by the weight of overlying sediments and, in 



^' Hedberg, Hollis D., Gravitational Compaction of Clays and Shales: Am. Journal of Science, Fifth 

 Series, vol. 31, no. 184, pp. 241-287, 1936. 



