496 



Subsurface Geologic Methods 



been proved, and this chart is closely watched during the process of drilling 

 for any drilling breaks that might occur. A drilling break usually indi- 

 cates a porous zone which is a potential oil or gas reservoir. When a 

 drilling break is encountered, drilling is suspended, the cuttings are circu- 

 lated up from that zone, and a determination is made of any oil or gas 

 content. The suspension of drilling near the top of any oil or gas zone is 

 of great importance in the event of a relatively thin pay section. A re- 



Figure 232. Laboratory located at well where steel mud tanks were used above 

 ground. Note high flow line and connections to laboratory. 



liable drill-stem test cannot be made if the bit has penetrated the water 

 zone, if one is present, beneath the oil or gas. 



Routine tests are made each two feet of hole drilled from cuttings 

 collected at a predetermined interval. The length of time necessary for 

 cuttings to reach the surface from the bottom of the hole varies with the 

 depth and condition of the hole as well as the capacity of the pump. This 

 amount of sample lag is determined frequently to allow sampling for the 

 correct depth. The following method is a reliable means of determining 

 the lag: Some material that can be easily detected, preferably about the 

 size and density of the actual cuttings, is placed in the drill pipe when a 

 connection is being made. The length of time required for the material to 

 reappear at the surface is measured. The time required for the pump to 



