114 CLASSIFICATION OF THE PUBLIC LAUDS. 



horizontal, as first laid down, and where a series of beds was de- 

 posited one above another, there being no earth movenaent during 

 the deposition, the several beds were parallel. After the beds of 

 sand, mud, and marl were deposited and hardened into the resulting 

 sandstone, shale, and limestone, they were in certain areas bent by 

 earth movements into folds of various shapes, and it is about these 

 folds that the accumulations of oil are found. The attitude in which 

 the rocks lie, the shape of the folds, and the presence of faults or 

 breaks in the strata constitute the rock structure. 



In any consideration of the factors which control the accumula- 

 • tion of oil or gas the importance of the part played by the structure 

 can hardly be overestimated. The fluid contents of porous beds obey 

 the laws of gravitation and capillarity, separating and distributing 

 themselves in the main in accordance with their specific gravities. 

 If water, petroleum, and gas are, as is usual, present in petroliferous 

 beds, the gas would as much as possible disengage itself from the 

 fluid and rise to the highest point in the fold, while the water would 

 endeavor to displace the petroleum and find a resting place as low 

 down as possible. If the bed of rock is inclined and the water is 

 under artesian pressure, it will be forced upward along the bed, the 

 oil remaining above the water because of the difference in specific 

 gravity. If the porous bed is continuous in dip to the outcrop, the 

 gas and oil are likely to exhaust themselves at the outcrop in the 

 form of seeps. If, however, the progress of the hydrocarbons up 

 the dip is stopped by a fold in the bed, or by a fault which seals in- 

 stead of opening the stratum, or by saturation of the bed with water, 

 an accumulation takes place, the oil and gas remaining between the 

 water down the dip and whatever has impeded their progress up the 

 dip. This theory, which is known as the anticlinal theory, is in some 

 form now accepted by practically all geologists, not as indicating 

 absolutely the limitations of the occurrence of oil and gas but as ex- 

 pressing the general relations of their occurrence to geologic struc- 

 ture, subject to various modifying conditions. Other factors less well 

 understood enter into the problem, such as the difference in the capil- 

 lary attraction exerted between water and the rock particles and be- 

 tween oil and the rock particles and the differences in friction experi- 

 enced by the two fluids in passing through the rock. There is much 

 to be learned concerning the whole problem, but enough is known to 

 make the study of any oil field of economic as well as scientific value. 



If the rock containing the oil does not also carry water there is no 

 force to impel the oil into the upfold or anticline. On the contrary, 

 gravity tends to pull it downward and it collects in the adjoining 

 downf old or syncline. This condition is found in some of the Penn- 

 sylvania fields. 



