318 ANNUAL REPORT 
That it is possible to have in rocks a large quantity of organic matter 
without direct evidence of its origin in the form of abundant fossil re- 
mains is well illustrated in these shales. Dr. Orton states: 
“The shales are for the most part poor in fossils, except in those of 
microscopic size. Banks representing a score or more of feet in vertical 
column often fail to reward a careful search with a single specimen of 
vertebrate, molluscan or articulate life, and as far as the unaided eye is 
concerned, they are almost equally barren of vegetable remains. Occas- 
ionally, however, fossiliferous bands are found, the contents of which serve 
to determine the geological age and equivalence of the portion of the series 
in which they occur.”! 
II. GEOLOGICAL CONDITIONS UNDER WHICH OIL AND 
| GAS ARE FOUND. 
There are three conditions which have been strongly advocated by 
geologists : 
1. A porous rock to contain the oil or gas. 
2. An impervious cover. 3 
3. Geological structure of the rocks which will permit the ac- 
cumulation of oil and gas from relatively large areas into 
smaller ones. 
All agree as to the necessity of a reservoir-rock. The one essential 
of this is porosity. Very fine grained rocks cannot, therefore, serve this 
purpose, and for this reason shales are not important oil or gas containing 
rocks. In Ohio three types are found—limestones, sandstones and con- 
glomerates. The limestones have been elaborately described by Orton. 
As has already been stated, they contain the immense repositories in the 
northwestern part of the state, but are not productive in other fields in 
Ohio. Analyses show the rocks to be strongly magnesian and Orton 
placed much weight on this feature, since limestones of this type are not- 
ably porous when compared with those of the ordinary or calcareous type. 
Recent developments in Texas, however, show that calcareous lime- 
stones may serve as a reservoir for oil. Thus according to an analysis 
made by Worrell, the oil rock in the great Beaumont field contains over 
97 per cent. carbonate of lime. When it is considered that this rock has 
yielded by far the greatest wells yet drilled in the United States, it is ap 
parent that ordinary limestone may serve as efficiently as that of the 
magnesian type. | 
Sandstone is the most common reservoir rock, and perhaps the Berea 
erit is the best illustration of this. The rock is composed of moderate 
sized grains of silica. However, it appears compact, and the quantity of 
oil which it contains often seems out of proportion to the porosity of the 
rock. In fact it is hard to believe that this formation ever contained the 
1Geol. Sur. of Ohio, Vol.'VII., p. 24. 
