ground water during their upward migration. That such an area of induration 

 exists may be shown by an inspection of drilling records made by fish-tail bits 

 and light equipment in the Gulf Coast salt-dome area. The rate of penetration 

 was lower over the domes than in the surrounding area. Others have postulated 

 that cementation of strata above the structure could be effected by the reduction 

 of calcium bicarbonate to insoluble calcium carbonate. The reduction was caused 

 by the greater solubility of CO2 in oil than in water. That sulfates were reduced 

 to sulfides by bacteria has also been suggested. 



GEOCHEMICAL LOGGING The chemical analysis of well cuttings for 



various hydrocarbons was undertaken by 

 Horvitz (1949) to prove that surface soil anomalies were caused by the vertical 

 migration of gaseous hydrocarbons from a deep source. Though continuous 

 cores generally were not available, analysis of samples from many wells did sug- 

 gest that such a relationship could exist. In the course of the investigation, it be- 

 came apparent that evidences of oil and gas accumulations could be detected 

 as much as 1000 feet above the source. These analytical results, which become 

 available as the drilling progresses, serve to forewarn the driller of approaching 

 potentially productive sands. Premature abandonment of drilling also is mini- 

 mized. 



Horvitz's scheme of sample collection consists of removing cuttings from the 

 circulating drilling fluid at the end of each 30 feet of section drilled. After the 

 samples are washed free of the fluid, they are sealed in jars and sent to the 

 laboratory, where composite samples for each 90 feet of section are prepared 

 for chemical analysis. When detailed information is required, a greater sampling 

 density is used. 



The chemical analysis method is the same as Horvitz uses for soil, except 

 that a separate determination of the moisture content of the sample is made for 

 the final calculations. The hydrocarbon data is plotted on a log to a scale of 

 1 inch = 1000 feet. Figure 28-5 shows the Danzig no. 1 well, in Rosenburg 

 Field, Texas, which produced high-gravity oil (59° Be) from a sand at 7736 

 to 7743 feet. Values for hydrogen and total hydrocarbons are shown in the first 

 two columns. Methane, ethane, propane, butane, pentane, and heavier hydro- 

 carbons do not show a decided increase until about 6000 feet, when approximately 

 5000 parts-per-billion is reached. Just above the producing sand a value of 

 about 28,000 parts-per-billion is attained. The pressure of such a distribution 

 of the heavier hydrocarbons is usually indicative of an accumulation of gas 

 and distillate. Samples of nonproducing wells show neglible quantities of hydro- 

 carbons. It has been noted, too, that longer sections of higher hydrocarbon 

 content are obtained from wells located at the margins of petroleum accumula- 

 tions than are obtained from wells in the center. 



628 



