Subsurface Laboratory Methods 327 



log is prepared from composite samples, which are obtained by mixing 

 proportional amounts of the samples from 200 feet of hole. If the fluores- 

 cence intensity of a composite sample runs above a given value, a possible 

 pay sand is indicated somewhere in the interval covered by the sample. 

 The depth and the true value of the showing are established by analyzing 

 each of the samples taken within the interval of showing. That part of a 

 log based on analyses of individual samples is referred to as "detailed." 

 The effectiveness of fluorologging for locating commercial pay horizons 

 can be appreciated only from personal experience with the results obtained. 



The over-all characteristics of the fluorolog of a well and the fluoro- 

 log of a dry hole are markedly different. A dry hole is characterized by 

 low values from the surface to the total depth of the hole. A producer 

 that cuts a fault between the surface and the producing horizon often 

 shows a fluorolog characteristic of a dry hole above the fault, but a fluoro- 

 log typical of a producing well below the fault. 



On a fluorolog of a producing well relatively high values are shown 

 at and for some distance below the surface, with a progressive increase 

 from the surface to the producing zone. Between pay horizons, in wells 

 having multiple producing beds, the curve shows a continuation of high 

 values, but it shows a decrease below each sand and a buildup as the next 

 producing formation is approached. 



Fluorologging is adapted to small samples, such as side-wall cores, 

 which are insufficient in size to be satisfactorily analyzed by core analysis. 

 Moreover, data are obtained on sections of a drill hole lost by stuck pipe, 

 a blowout, and similar difiiculties before an electric log is run. Fluorologs 

 supply information concerning the production possibilities of all strata 

 penetrated and afford an independent check on electric logs, core analyses, 

 and other data. 



Fluorographic Exploration 



Among the recent developments in the fluorochemical method of lo- 

 cating petroleum is the technique of fluorographic exploration, which de- 

 pends upon the response of soil samples to ultraviolet light; the samples 

 are collected over the area to be explored, and drilling is not required. 



Fluorographic exploration was originated several years prior to 1943 

 by Blau,^^ and recently procedural improvements have been directed to 

 the assay of soil samples.^^ ^^ 



In fluorographic reconnaissance field work, soil samples are taken at 

 stations a quarter of a mile apart on a regular grid pattern. A reconnais- 

 sance survey is adequate for most projects, as it establishes approximately 

 the outline of a favorable prospect and develops considerable detail. If 

 greater detail is desired, the samples may be taken at shorter intervals; a 



" Blau, W., U.S. Patent 2.337,443, Dec. 21, 1943. 



^ Squires, R. M., U.S. Patent 2,451,883. Oct. 19, 1948. 



^' Stevens, N. M., and Squires, R. M., U.S. Patent 2,451,885, Oct. 19, 1948. 



