Duties and Reports of the Subsurface Geologist 



815 



Fossils should be wrapped separately to prevent breakage during 

 shipment. 



Core sections to be analyzed for magnetic polarity should be care- 

 fully selected, labeled, and packed. Immediately after removing the core 

 from the inner barrel, the geologist should clearly mark the top and bottom 

 of the core. The greatest source of error in this method is the incorrect 

 labeling of the tops and bottoms. Core sections should be at least eight 

 inches in length and should exhibit sufficient bedding to provide dip data. 

 Dips exceeding 10° give the most satisfactory results. Interbedded sand- 

 stone and shale cores are most desirable. Rarely do massive limestones 

 and dolomites give favorable orientation results. When core samples 

 are sent for analysis, the following information should accompany them: 

 the type of material, the age and stratigraphic position, and the inclina- 

 tion and direction of the hole at the point at which the sample was taken. 

 All cores should be removed from stray magnetic fields, wrapped wet in 

 cellophane, and sent as soon as possible to the laboratory. 



• • • • • • 



• • • • 



A 



Bottom 



^ < ^ < Y ^ 



• • « » • • • • • • • %^ 



B 



Figure 414. Correct (A) and incorrect (B) manner of sampling a well core for 

 analysis. Spot samples (•) may be taken at any desired interval and com- 

 posited (Zi, Xi, etc.). Groove sampling (Fi) is sometimes followed. Samples 

 composited as shown by the Z series should be used only for general purposes, 

 since such samples represent mixed lithologies. 



Formation Testing 



Information given on a drill-stem or formation test depends on the 

 nature of the equipment used. Some of the points that should be con- 

 sidered are the name and type of the tester, the depth of the hole, the 

 depth at which the packer it set, length of pipe below the packer (whether 

 blank or perforated), and choke data. The time the packer is set should 

 be noted. The hole should be full of mud, and the geologist or engineer 

 should note at the time of packer setting whether the fluid level in the hole 

 drops, and, if so, the rate of the drop. After the tester is opened, results 

 should be carefully observed and tabulated. Pressure data should be 

 recorded at regular intervals and all variations recorded. The time the 

 tester is released should be recorded. When the drill pipe is extracted, the 



