Duties and Reports of the Subsurface Geologist 819 



until the porous zone is penetrated. A core should be taken regardless of shows 

 after drilling 30 feet in any continuous sand body or other porous zone. 



Core-Analysis Samples — In each exploratory well, core samples of prospec- 

 tive oil and/or gas sands should be carefully selected and properly preserved 

 according to laboratory instructions immediately after extraction for general 

 core analysis, particularly for porosity and permeability, and also for fluid 

 saturation whenever occasion arises. Representative samples from among these 

 should be forwarded to the main office in the event that local facilities for 

 analysis are inadequate. 



Step-Out Wells — On wells subsequent to the first on a given structure, cor- 

 ing should be reduced as a result of information already at hand. 



Side-Wall Cores— Side-wall cores should be taken in sands not cored orig- 

 inally, which look promising from electric or radioactive log data and at points 

 where samples may be required for geologic information. All sand samples from 

 side-wall cores should be tested in ultraviolet light immediately after extraction. 



Core Recovery — Coring operations must be continuous until satisfactory 

 recovery is obtained or until the objective of coring has been satisfied. On cer- 

 tain occasions, a small percentage recovery may be considered as "satisfactory 

 recovery," as in many limestone sections in which experience has proved that 

 normal recovery is low. 



Formation Testing. 



When depth and other drilling conditions make it practical, formation 

 tests in a wildcat well should be made on any sand or other permeable zone 

 where productive possibilities are indicated. Ordinarily, when a productive 

 zone is encountered, formation testing should be continuous in stages up to 

 100 feet throughout the vertical extent of the productive material to locate gas- 

 oil and oil-water interfaces, if present, as well as to evaluate productivity within 

 reasonable limits. Overlap intervals between tests should be minimized to pre- 

 clude obtaining misrepresentative information. On well subsequent to the 

 first on a given structure, zones to be tested can be more closely planned from 

 results of the first well. Provision must be made to record volumetric measure- 

 ments of all oil, gas, and/or water recovered. 



Mud and Water Tests 



Salinity Tests — Drilling-fluid salinity and water-loss tests should be made 

 periodically, preferably not in excess of 24 hours, and should be reported on 

 daily drilling reports. In case water is recovered from formation tests, a salinity 

 test should be made and a properly marked sample retained for possible sub- 

 sequent geochemical analysis. 



Mud Tests, General — Other mud tests and analyses conforming to accepted 

 current practice must be made and reported as a routine procedure. Special 

 mud tests should be made as called for. 



Electric and Mechanical Logging 



Electric Logs — Electric logs should be taken prior to landing all casing, 

 except surface casing, on completion of hole to final depth, and also at other 

 points as required by special circumstances in which specific information 

 is required regarding a portion of open hole during the process of drilling. 

 Recognition of any zone, the possibilities of which have not been satisfactorily 

 indicated by other means, should constitute such a special circumstance. Elec- 

 tric-log data should not be relied upon as final evidence regarding productivity 

 in the early stages of development, but should be carefully checked with other 

 evidence obtained during the drilling of the well. 



