Duties and Reports of the Subsurface Geologist 811 



verified by adding such material as corn, rice, or wheat to the circula- 

 tion system and then making a record of the time required for the 

 material to reappear in the surface overflow. From these data a graph 

 may be constructed for estimating the time lag for various depths. Com- 

 parison of lithologic breaks with the electrical profile and drill-time data 

 also permits correction of sample lag. 



Continuous coring does not necessarily require retainment of ditch 

 samples; it is advantageous, however, to collect them if core recovery 

 is inadequate. 



The quality and reliability of ditch samples decrease with depth 

 and penetration-rate increase. In soft formations where penetration is 

 rapid, sample contamination is excessive, and it is impossible to deter- 

 mine the actual depth position of the cuttings. When penetration is slow, 

 samples are more representative, and their stratigraphic positions can be 

 more accurately determined. 



Samples should be washed thoroughly of drilling fluid, dried, care- 

 fully examined by hand lens or microscope. The ratio of the various lith- 

 ologies is noted, described and plotted, tagged, and submitted to the sub- 

 surface laboratory for refined examination. 



Samples of the drilling fluid should be continuously examined while 

 fresh under the ultraviolet light at frequent intervals during penetration. 

 A gas detector is recommended to record gas shows in the drilling fluid. 



Drilling Rate 



A careful record of the penetration rate should be kept continuously. 

 Extreme care should be exercised to tabulate any abrupt increase or de- 

 crease in the penetration rate while drilling is in progress. These breaks 

 should then be evaluated in terms of the stratigraphy. 



Coring 



Coring programs vary with the problem involved and with the com- 

 pany policies. Several companies developing the same area may follow 

 entirely different avenues of coring procedure. There cannot be too many 

 cores from an exploration drill hole. 



Cores are generally extracted and washed on the derrick floor and 

 placed in core boxes or trays. It is important that the cores be carefully 

 marked so that correct depth sequence can be followed. 



The label of each core should include the interval cored and the 

 footage recovered. If more than one tray of the same core is involved, 

 a label should be placed on each tray; in addition, each tray should be 

 labeled as, for example, "Tray No. 1 of 3 trays." Labels should be 

 placed on the bottom end of the tray. Tray No. 1 is commonly assumed 

 to include the top three feet of the cored section. 



Cores should be taken at the discretion of the well-site geologist in 

 collaboration with the district geologist when important changes of for- 

 mations are noted. Coring is undertaken to determine the position of 



