filler] IMPORTANCE OF ACCURATE WELL RECORDS. 17 
sunk into such rocks in search of oil, but the uselessness of this is 
apparent to those who have noted the records of similar wells else- 
where, as well as to geologists and others who understand the nature 
of the rocks. The value of accurate records in indicating- the depth at 
which it is advisable to stop drilling will be manifest in those cases 
where altered or igneous rocks, as described above, are encountered, 
and in general the logs of the deeper borings are of great value in 
showing the conditions to be expected at great depths. 
Qil and gas shows. — In drilling, especially where the well is being 
sunk to a known producing sand, small shows of oil or a little gas are 
often encountered and passed through without much attention being 
paid to them. Probably many instances have occurred where valuable 
flows have been cased off, bat one or two examples will suffice. In 
the region southwest of Olean, N. Y., on the outskirts of the Bradford 
oil field, the early wells were all sunk to the Bradford sand, at a depth 
of 1,000 feet or so below the valleys. In some instances shows of oil 
were noted at a certain depth, but were cased off without it being sus- 
pected that a producing sand was present above the Bradford. Later, 
when the Bradford supplies ran low, it was shown that the sand at the 
upper oil show could be developed into a producer, now known as the 
Chipmunk sand. Where accurate records had been preserved, the 
casing of the old wells was locally removed and both sands drawn upon; 
but where because of lack of records this could not be done, new wells 
had to be sunk, at an aggregate cost of man} 7 thousands of dollars. 
Similarly, in the Bellevernon and other gas fields in southwestern 
Pennsylvania gas flows which would now be of great value were cased 
off in the old days of high pressure. Where accurate records have 
been kept, these can be utilized by removing the casing in the old wells, 
but otherwise new wells must be drilled. 
Structure. — It has been many } r ears since the anticlinal theory of oil 
and gas was proposed. This theory postulated the axis of the anticline 
or crest of the rock arch as the most favorable point for the occurrence 
of gas, while oil and salt water should be successivel} 7 encountered in 
going outward from the crest. While all anticlines do not afford oil 
or gas, and while oil is often found under somewhat different condi- 
tions, as at the point just above where a steep dip changes to flat (at 
the top of the rock step, as it were), the general principle involved in 
the theory has not been affected. On the contrary, almost every dis- 
trict, when closely investigated, is shown to agree with these princi- 
ples, so that at the present time the problems of rock structure are as 
important as they ever have been in the history of drilling for oil and 
gas. In no way can the structure be determined so satisfactorily and 
with such accuracy as by wells or boring records. 
Bull, 264—05 2 
