penneman.] THE BOULDEE, COLO., OIL FIELD. 325 
Of faulting it can only be said that it is as yet undiscovered and 
probably undiscoverable unless it affects the strong sandstone ledge 
mentioned above. Faults abound, however, near by, as in the vicin- 
ity of Marshall, where the stronger Laramie preserves at the surface 
the records of displacements. The possibility of such dislocations 
within the oil field demands consideration both in the explanation of 
1 he occurrence of the oil and in the exploitation of the field. 
DRILLING IN THE PIERRE. 
With few exceptions the wells thus far put down have been drilled 
in the Pierre. The exceptions are among the scattered wells to the 
east, which are on the very similar Fox Hills or even on the Laramie. 
These also traverse the Pierre for the greater part of their depths. 
Rate of drilling and expense. — Drilling in this formation is com- 
paratively easy and rapid. It is not uncommon to make 100 feet in a 
day at considerable depths, and this is sometimes done without change 
of bits. On the other hand, slow progress is made in certain beds. 
The average expense of drilling a wed] under contract with respon- 
sible parties is, at the present time, about 11.65 per foot for the first 
2,000 feet. Below that depth the cost is greater. Under such con- 
tracts the owner of the well furnishes casing. 
Water. — Surface water is usually encountered at about 15 feet and 
may be found in the first 100 or even the first 200 feet. Below the 
surface the wells are commonly "dry;" that is to say, the seepage 
into the well from the dense shales is so slow as to be of no signifi- 
cance in drilling. In isolated cases deep water-bearing strata are 
encountered, and such water is sometimes salt. At Lafayette, 11 
miles east of the foothills, water is reported as spouting from 4 to 5 
feet between the 10-inch drive pipe and the 8^-inch casing. This 
water was struck at depths of from 400 to 700 feet and is still flowing 
after a period of four months. Hot water was reported found at 
nearly 2,800 feet about 12 miles north of Boulder. In one well the 
water from this depth was distinctly briny. Other wells have yielded 
salt water from much smaller depths. In one well now pumping the 
oil is mixed with about 10 per cent of salt water, which was encoun- 
tered just below the oil. A slight admixture is found in at least one 
other well. 
Many wells are cased only to the depth of the surface water. 
Others require 1,000 or even 2,000 feet of casing. In general deep 
casing is not for the purpose of shutting off water so much as to avoid 
caving, which is common though not general. It has necessitated the 
abandoning of several wells. 
Reports of materials passed through. — By far the most common item 
in reports is shale or "slate," occasionally varied as "clay" or "soap- 
stone." Sand or sand rock is often reported. With a showing of oil 
it is often called "oil sand," with no particular reference to correla- 
