830 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 2ld 
would prove beneficial to one well would be ruinous to another. On 
this account, if on no other, the texture and composition of the oil 
strata should b3 carefully studied by methods far more discriminat- 
ing than the superficial ones now used. 
A second reason for injurious effects from shooting lies in uncer- 
tainty about the exact depth of the sand which it is intended to shat- 
ter. Measurements of depths by steel tape are indeed becoming more 
common, but in a considerable number of wells the depths of all for- 
mations are known only by cable measurement. Even in wells but 
recently sunk, it is not uncommon that the stated depths of impor- 
tant sands are thus liable to errors of 25 to 50 feet. 
The possible injuries from a shot at the wrong place may be readily 
seen from the following considerations: Given a porous rock satu- 
rated with oil which is under a certain pressure. This rock is now 
pierced by the drill. The oil soon fills the hole and is pressed upward 
for the sole reason that it has no outlet in any other direction, being 
surrounded (as in this field) by impervious rocks. This well is now 
shot in such a way as to rupture the impervious rocks which have 
surrounded the oil sand. The oil may now leave the sand by other 
openings beside the well and may thus be dissipated in other porous 
beds and the well may be ruined. Such an effect may be produced 
even by shooting at the proper depth if the charge employed be too 
heavy. In one instance a well was shot at 740 feet with 500 pounds 
of dynamite, 00 per cent nitroglycerine. The formation above the 
sand was a uniform dense shale. A good quality of sandstone was 
blown from the hole in chunks reaching a maximum of 14 pounds* 
The shale was ruptured to the surface. Open cracks of an inch or 
more extended for some rods from the well. Presumably also, cracks 
reached a considerable depth below the sand which was to be shattered* 
It can not be too carefully borne in mind that the one object in 
shooting is to shatter the rock which carries the oil and that only. 
With this object in view, it is plain that intelligent and discriminat- 
ing shooting must depend upon information which the following 
questions may suggest: Is the texture of the oil stratum such as to 
give promise that it will be shattered rather than compacted? What 
is the exact depth of its top (and bottom if drilled through)? How 
much of a shot will the overlying rocks bear without giving other 
outlets to the oil? This last question is one of great importance in 
this field. It is needless to say that such questions can be answered 
only by a carefully kept log and close study of samples, not only of 
oil sands but of all strata in order to properly forecast their behavior 
under the influence of a shot. 
Sources of the oil. — The source of the oil is not yet determined 
within narrow limits. Much of the lower part of the Pierre is black 
with disseminated carbonaceous matter. The Niobrara below is simi- 
larly bituminous, yielding a strong odor from its more fossiliferous 
