18 
OTL AND GAS; OHIO, WEST VIRGINIA, PENNSYLVANIA. 
along the slope is saturated with water up to a certain level, with the 
oil immediatelv above. 
CONDITIONS NECESSARY TO DETERMINE SUBSURFACE STRUCTURE. 
The geologic structure of the oil-bearing stratum is an important 
factor in the location of accumulations of oil and gas. The statement 
has been previously made that under certain conditions this factor 
can be determined prior to the descent of the drill. It now becomes 
necessary to consider what these conditions are, the reasons for the 
same, and what steps are necessary for determining the structure of 
a deep-lying oil sand. 
The various rock formations which appear at the surface within 
the Appalachian oil fields, as well as the underlying strata that have 
been pierced by the drill, are of sedimentary origin—that is, they 
were laid down as sediments in a body of water. 
The Appalachian oil fields occupy an area which was an inland sea 
or gulf during the Devonian and Carboniferous periods. Into this 
sea was washed the disintegrated and dissolved material from the 
surrounding land, and this material was deposited on the sea bottom 
in layers more or less parallel. The size of the inland sea did not 
remain constant. From the geologic evidence it is plain that at some 
periods the sea was increasing in area and depth and that at other 
periods it was diminishing. This expansion and contraction of the 
water-covered area probably was repeated many times, and the sedi¬ 
ments laid down in this body of water varied according to the con- 
ditions of the sea. These deposits consist of shales, sandstones, lime¬ 
stones, and coals. 
I ine soft shale results from the erosion of a much weathered and 
deeply disintegrated land surface, and it is deposited in that portion 
of the sea where the currents are slight and no longer have power 
to transport fine particles. 
Sandstone represents a deposit made in moving currents or along 
shore where the motion of the water had power to wash out and 
carry along the finer particles of material, leaving the coarser grains 
to form sandstone. 
Limestone may be formed in different ways. The lime and mag¬ 
nesia of the soil are washed out and carried to the sea in solution. 
Myriads of animals living in the sea have formed their shells and 
bones from the lime and magnesia in sea water and, on dyinsr. left 
large deposits of these materials, which have been cemented together 
forming limestones. Some plants of the sea cause a deposit of lime 
about themselves. Limestone may be formed by precipitation from 
the sea water. In these ways the great beds of limestone may have 
been formed, Those limestones which carry marine forms, such as 
