812 Subsurface Geologic Methods 



marker beds, contacts, oil and gas shows and whenever required for 

 additional paleontologic or geologic information. The thickness of the 

 cored section should be reduced to a minimum yet be sufficient to satisfy 

 requirements. 



The interval between cores in exploratory holes should not exceed 

 50 to 100 feet within possible productive strata. Coring may begin at 

 the first bit change below the surface casing; thereafter, cores should 

 be taken at intervals of no more than approximately 500 feet down to 

 the top of a potentially productive zone. 



When drilling to an important stratigraphic marker, coring at 

 approximately 50-foot intervals should be started about 300 feet above 

 the anticipated depth. When establishing marker positions it is some- 

 times preferable to drill alternately with a wire-line core bit and a drilling 

 bit. 



If a decided change in the penetration rate indicates that a porous 

 phase has been encountered while a potentially productive formation is be- 

 ing drilled, two to five feet of section should be penetrated and drilling 

 suspended until circulation cuttings from the bottom have reached the 

 surface and have been checked lithologically and by the ultraviolet light 

 for oil showings. When oil or gas showings are encountered in this or 

 any other manner, coring should begin. If no indications of oil or gas 

 are obtained, drilling may be resumed through the porous body and re- 

 turns continuously analyzed by ultraviolet light. A core should be taken 

 regardless of shows after drilling 15 feet in any continuous porous phase 

 within a potentially productive part of the section. 



Ditch samples and cores frequently give oil shows from thin sand 

 laminae in shales that are not potential reservoirs. If the first core or 

 cores taken after encountering such shows should indicate that the for- 

 mation is not potentially productive, drilling may be resumed; but ex- 

 treme care must be taken to define drilling breaks in petroliferous sec- 

 tions, at which coring should again commence. 



If ditch samples should show an increase in sand content of a 

 petroliferous section being drilled according to the foregoing procedure, 

 indicating that a potential reservoir stratum is being penetrated, coring 

 should be started to determine the nature of the formation. 



If limestones or dolomites are being drilled, check cores should be 

 taken when the ditch-sample fragments show evidence of oil staining, 

 coarse crystallinity, vuggulation, or fracturing. 



On each exploratory well, core samples of prospective oil or gas 

 sands should be immediately preserved by the well-site geologist according 

 to laboratory instructions for general core analysis (porosity, perme- 

 ability, and fluid determinations) . 



On wells drilled subsequent to the first in a field, coring should be 

 reduced as a result of information already available, although care should 



