278 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1916. 



A. NATURAL. FACTORS. 



1. Pressure. — The pressure exerted on the oil in its underground 

 reservoir may be hydrostatic, hydraulic, or gas; it may be coexten- 

 sive with the field or pool, in which case it is called " field pressure," 

 or it may be exceedingly local in extent, when it is called " local " or 

 " well pressure." Pressure in oil wells varies from to over 2,000 

 pounds per square inch, usually declining as the field or well gi'ows 

 old. Other things being equal, the production varies with the pres- 

 sure. 



2. Viscosity. — Production varies inversely with the viscosity, and 

 since the viscosity in general increases with the specific gravity (in- 

 creases inverse!}' with the Beaume degrees) it may be said that, other 

 conditions being equal, the production varies inversely with the spe- 

 cific gravity of the oil. Natural petroleums vary from substances as 

 fluid as water (low viscosity) to those having the consistency of "cold 

 molasses" (high viscosity), or even to those possessing the properties 

 of solids. 



3. Thickness and extent of 7-eservoir rocks. — The production varies 

 with the thickness and extent of the reservoir rock. The thickness of 

 the pay streaks may vary from 2 feet, as in some fields of the eastern 

 United States, to over 200 feet, as in some California fields. The 

 lateral extent of the layer or lens may be from a few feet to several 

 miles. 



4. Porosity of reseriwir rocks. — Production varies inversely with 

 the porosity within certain limits. In uniformly grained rocks the 

 coarser the grain of the reservoir the less is the actual porosity ; but 

 the larger the size of the interstices the less is the friction surface per 

 unit of oil. Therefore, coarse sediments are really less porous and 

 consequently hold less oil, but they give it up more readily than fine 

 sediments and usuall}^ give a greater ultimate yield per unit of 

 volume. Eeservoir rocks may be fine shale to the coarsest conglo- 

 merates, or porous or cavernous dolomites or limestones. Fracture 

 or fault zones also may act as reservoirs. The world's maximum 

 producers obtain their oil from cavernous limestones or dolomites; 

 the steadiest and longest-lived wells are in medium-grained sand. 



5. Structure of reservoir rocks. — Structure usually has a profound 

 influence on oil accumulation and production, the most advantageous 

 positions being in the crests of domes or anticlines, or on the flanks 

 of sealed or terraced monoclines. Lithology or other causes may 

 locally produce exceptions to all rules of accumulation. 



B. ARTIFICIAL FACTORS. 



1. Price of oil. — The price of oil is the dominant factor governing 

 the production of oil, especially as it relates to groups of wells, fields, 



