818 Subsurface Geologic Methods 



Electric Logging 



Electric logs should be periodically run in order to check the pene- 

 trated section. Anomalies should be evaluated as soon as possible by side- 

 wall cores, by re-examination of cuttings or by formation testing. All 

 lithologic adjustments should be made on the progress log. 



Radioactive Logging and Caliper Logging 



Radioactive logging should be used to determine porous intervals in 

 limestone and as an aid to accurate selective perforating. Before the 

 casing is set, a caliper profile should be obtained in order that proper 

 cement computations can be made. 



Through the courtesy of the Richmond Exploration Company the 

 following procedure for sampling, logging, and testing of wildcat wells 

 in Venezuela is given. 



Ditch Samples 



Cuttings — These must be taken at ten-foot intervals down to the vicinity 

 of the first important marker and at five-foot intervals below that point to total 

 depth, always including intervals cored. Ditch samples must be examined by 

 ultraviolet light during drilling through a possible producing section, as soon 

 as possible after the cuttings reach the surface. Samples showing fluorescence 

 must be tested for "cut." Samples must be preserved in sacks and tags properly 

 marked for identification. Ditch samples should be used for general logging 

 purposes. 



Drilling Fluid — Samples of drilling fluid must be examined while fresh 

 under ultraviolet light at frequent intervals during drilling through all possible 

 producing sections. 



Coring 



General — Cores should, in general, be taken when changes of formation are 

 noted, and to determine location of marker beds, at contacts, at oil shows and 

 also whenever required for essential paleontologic or geologic information. 

 Thickness of section cored should be kept at a minimum to satisfy requirements. 



Maximum Interval — The maximum interval between cores in any explora- 

 tory hole should not exceed 500 feet. This interval may vary according to de- 

 sired results. 



Interval — Important Markers: In any case where drilling depth to an im- 

 portant stratigraphic marker is not predictable within fifty feet with a reason- 

 able degree of certainty, spot coring at 50-foot intervals should begin 300 

 feet above the anticipated depth to the marker. 



Porous Zones — When a decided change in penetration rate indicates that 

 a porous sand or other porous zone may have been encountered, two feet to 

 five feet of penetration should be taken into this zone and drilling suspended 

 until circulation samples from bottom have reached the surface and have been 

 checked for oil showings by ultraviolet light. When oil or gas showings are 

 encountered in this or any other manner, the full vertical extent of the zone 

 should be ascertained by continuous coring; this coring should be continued to 

 a minimum of 20 feet below the last-observed oil show. If no indications of 

 oil or gas are obtained as first noted above, drilling should be resumed through 

 the porous zone and continuous analysis of returns by ultraviolet light made 



