768 Subsurface Geologic Methods 



hydrocarbon-consuming bacteria will be at a maximum in the presence 

 of an abundant food supply. The well-known paraffin dirt of the Gulf 

 Coast is reputed to be the by-product of metabolic activity of certain 

 hydrocarbon-loving microorganisms. The process sometimes works in 

 the reverse direction also. The presence of sulfur or chlorine compounds 

 in abnormal abundance as a result of the secondary-mineralization effects 

 mentioned earlier, or perhaps the presence of sulfur compounds from 

 some crudes, creates a toxic condition that inhibits the growth and de- 

 velopment of certain microorganisms. 



The preparation of a geochemical well log consists in taking samples 

 of well cuttings and combining them into a composite sample. The ver- 

 tical extent of the resulting sample will be determined by the amount of 

 detail desired. It may vary from 5 to 100 feet. The usual interval is 

 about a 30-foot section. For most of the procedures which involve the 

 taking of soil samples or formation samples, a 100-gram sample is con- 

 sidered essential. Since the quantities being measured occur in parts per 

 million for the liquids and solids and parts per billion by weight in the 

 case of the gases, the larger the sample that is practical to collect the 

 better and more reliable the analytical routine will be. 



Correction Factors 



It can readily be seen that variations in the inherent characteristics 

 of various sediments and soils such as the crystal structure, the free-surface 

 energy, the interfacial tension, the surface work function, etc., may very 

 well influence the amount of adbsorbed, intrained and occluded gaseous, 

 liquid and solid hydrocarbons, as well as the fixation or stabilization of 

 certain inorganic ions present in the environment. Variations in the par- 

 ticle size and physical condition of the surface of the mineral grains plus 

 an almost infinite range in the combinations of the different minerals com- 

 posing the various soils and sediments complicates the problem further. 

 Superimposed upon these inherent characteristics are the transient effects of 

 changing barometric pressure, wind velocity and solar radiation, the an- 

 nual amount of precipitation and the seasonal distribution of that pre- 

 cipitation, infiltration of meteoric water, surface evaporation and move- 

 ment of the ground waters by capillarity, the slope of the land, the rate 

 of erosion, and the season of the year when the erosion is most intensive, 

 the length of time which the soil has been in its present position as it in- 

 fluences the establishment of equilibrium conditions, and the amount and 

 type of vegetational cover. 



Various means are used for minimizing these differences or evaluating 

 their effect so that subsequent measurements can be reduced to a common 

 base for relative comparisons. Without these corrections the geochemical 

 data usually has a rather restricted significance because the quantities in 

 question may vary only slightly above the background. 



