112 CLASSIFICATION OF THE PUBLIC LANDS. 



Oil and gas are composed for the most part of carbon and hydro- 

 gen, but they vary greatly in the proportions of these elements and 

 in the way in which they are combined. The petrolemn oils range 

 from low-grade heavy oils containing much asphalt to high-grade 

 light oils which contain a large percentage of paraffin and volatile 

 constituents and little or no asphalt. The heaviest oil is chiefly valu- 

 able as fuel, for which it is used in its crude state. Progressively 

 lighter oils are used less and less for fuel and more and irfore for the 

 other products that are obtained from them. There is a general 

 opinion that weight for weight the fuel value of light and heavy oils 

 is about equal, but because of the value of the distillates from the 

 lighter oils it may be stated in general that the lighter the oil the 

 greater its value. 



In classifying lands as to their probable content of oil and gas it 

 must be borne in mind that oil and gas are mobile substances and 

 that, owing to their mobility and to the resulting increased impor- 

 tance of gravitation, temperature, hydrostatic pressure, and capil- 

 larity, it is necessary to make certain variations from the type of 

 procedure employed in classifying lands containing coal, phosphate, 

 or other stable minerals. The mobility of oil and gas has, in many 

 regions, permitted their migration through varying thicknesses of 

 pervious strata to their present places of accumulation, so that the 

 problem is not to discover where the hydrocarbons originated but 

 rather where they have accumulated. 



The present position of these accumulations depends mainly on 

 the character of the strata, the attitude of the strata (commonly 

 spoken of as the rock structure) , the presence or absence of water, and 

 the character and specific gravity of the oil. The fluid hydrocarbons 

 do not, as is supposed by some, occupy underground lakes or reser- 

 voirs surrounded by walls of rock. Instead they saturate porous 

 rocks in places where the geologic structure, the conditions with 

 regard to underground water, and the succession of strata are such 

 that the accumulations are sealed by relatively impervious beds. 

 Thus, although the accumulations of oil or gas are called " pools," 

 they are not to be confused with such, pools as are formed by the 

 collection of liquids upon the surface of the ground. The porous 

 stratum in which the hydrocarbon collects is often spoken of as an 

 oil or gas "sand," although it may in reality be sandstone, gravel, 

 limestone, or a zone of fractured rock. Into this " sand " the hydro- 

 carbon comes from' one or another* source, but if there is to be an 

 accumulation of importance the migration of the oil or gas along 

 this pervious bed must be stopped by a change in dip or by some 

 other obstacle to continued progress, and, in addition, the reservoir 

 thus formed must be sealed by strata that are relatively impenriQUS, 



