502 ^ Subsurface Geologic Methods 



be detected from any oil or gas showing. As the cuttings from the well 

 are used for all tests, any mud that might carry combustible gas due either 

 to chemical action or recirculation does not affect the results. 



The development of mechanical and electric well-logging instruments 

 has greatly eliminated the chance of human error or misjudgment. It is 

 of no less importance, however, that qualified personnel correlate and in- 

 terpret the data in order that the greatest benefit be obtained. 



Statistics show new oil fields progressively becoming harder to find. 

 Many new areas are being explored and new zones in old areas being 

 tested. It is becoming necessary to employ the best available man power 

 and methods in the search for new petroleum reserves. After a location 

 has been made, it is of the utmost importance that the well be accurately 

 evaluated for oil or gas, or the preliminary Avork will have been done in 

 vain. The cuttings-analysis-logging method should leave the operator 

 without question regarding the section logged. 



Questions 



1. How are cable-tool samples collected? 



2. Discuss contamination of ditch samples from rotary wells. 



3. What methods are used for obtaining true, ditch-sample depths? 



4. What is the general procedure in examining cuttings samples? 



5. Discuss the plotting of well samples. 



6. For what purposes are well-sample logs used? 



7. What two basic curves are obtained in electrical logging? 



8. What are the characteristics of the resistivity curve opposite fresh- 

 water and salt-water-bearing strata? 



9. What effect does mud resistivity have on the self-potential profile? 



10. What factors influence the resistivity of drilling fluids? 



11. Does hydrostatic pressure exerted by the mud column affect the char- 

 acter of the self -potential curve? 



12. How is the resistivity curve obtained and what does it actually rep- 

 resent? 



13. What applications does the electrical log have? 



14. What would be the characteristics of the self -potential and resistivity 

 curve opposite the following lithologies: dense limestone, sandy 

 shale, sandstone with fresh water, sandstone with salt water, anhydrite, 

 calcareous shales, quartzite. 



15. Is it advantageous to know the basic lithologies of a penetrated sec- 

 tion before making a lithic determination based only on the electrical 

 profile? 



16. May an electrical-log survey be made in cased holes? 



17. What is meant by "induction logging"? 



18. Under what conditions can the induction log be us^? 



19. Does the induction-logging system require any direct contact with the 

 mud or with the ground? 



