14 RECORD OF DEEP WELL DRILLING FOR 1904. [bull. 264. 
VALUE OF WELL RECORDS TO THE DRILLER FOR OIL AND GAS. 
The contractor or driller for oil or gas is often called upon to sink 
wells in regions wholly unfamiliar to him. Among the questions 
which will arise are the following: (1) What is the depth of the sup- 
posed oil- or gas-bearing rock? (2) What is the character of material 
to be penetrated before reaching it? (3) At what depths, in what 
amounts, and of what quality can water for the boilers be obtained? 
(4) What is the depth of water seams which must be cased off? (5) 
What are the character, depth, and thickness of rock which will 
require casing to prevent caving, if any? (6) At what depth and in 
what rock is it advisable to stop drilling in case of failure to strike oil 
or gas at the expected point? (7) At what depths will minor shows 
of oil and gas be encountered ? In case the contractor or driller is 
required to select the spot for drilling, another question will arise: 
(8) At what point is the structure most favorable for the occurrence 
of oil and gas? 
A certain amount of this information can be obtained, as has been 
pointed out, by observation of the character, dip, and strike of the 
rocks exposed at the surface. Other data, especially those relating to 
the occurrence of water, to caving rock, and to the depth at which it is 
advisable to stop drilling, are best derived from records of borings. 
These need not, however, be necessarily in the immediate vicinity, for 
a geologist or a driller with a practical working knowledge of the 
simpler principles of geology can combine the observations afforded 
by surface outcrops with records of borings at a distance, and intelli- 
gently predict the conditions in the intervening area. 
Depth of productive rock. — The depth to the productive rock is of 
great importance as affecting the success of a well. For a shallow well 
a relatively light outfit can be used, but for deeper borings heavy and 
more costly machinery and rigs are required. These, with increased 
cost of fuel, outfit, casing, and greater difficulty of manipulation, make 
the cost of a well increase rapidly with depth. The amount of oil or 
gas which would give a good profit on the investment required for a 
1,000-foot well might not be sufficient to warrant the sinking of a 
3,000-foot well. 
It is apparent, therefore, that as close an estimate of the depth as 
possible should be made before starting to drill. This may in some 
cases be determined more or less satisfactorily by observation of the 
distance of the well from the outcrop of the rock and determina- 
tion of the direction and amount of the dips at the outcrop and in the 
area intervening between it and the well, but in many instances the 
outcrop is several score, if not hundreds, of miles from the well, and 
there are many irregularities of dip and changes of thickness of the 
rocks in the intervening area, all of which serve to make computations 
from surface data difficult and unreliable. 
