280 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1916. 



transportation facilities are usually quickly developed to their maxi- 

 mum capacity, while fields farther away are often left for years 

 without even being adequately prospected. 



7. iTTi'proveTiients in development and recovery methods. — New 

 methods of drilling and increasingly efficient methods of recovery are 

 favorably affecting production in many fields. The most important 

 advance in recent years has been along the line of increased use of 

 compressed air in the recovery of oil, especially in California and 

 Pennsylvania. 



8. 'Water comjMcations. — "Water troubles" may be either natural 

 or a combination of natural conditions and human carelessness or 

 ignorance. Water causes the final ruin of practically all oil fields; it 

 is the omnipresent and greatest menace of the producing fields. In 

 most cases w^ater troubles are inexcusable. Their results almost 

 always are negative and sometimes irremediable. 



Oil in most fields of the United States and, in fact, throughout the 

 world, occurs in inclined or sloping beds of sand or other porous 

 rock, and these oil zones usually are overlain and underlain by water 

 sands or zones which are separated from the oil zones by impervious 

 clay, shale, or other strata. In these two cases the water is extraneous 

 to the oil sands. These waters are called " top " and " bottom " 

 waters, in accordance with their occurrence, respectively, above or 

 below the oil zones. In a properly finished well the " top " water is 

 cased off or cemented off before the well is drilled into the oil sand. 

 The " bottom " water never is drilled into except by accident, in which 

 event it is plugged off. With the " top " water shut off and the " bot- 

 tom" water untouched, the oil is produced practically free from 

 water. Water, being heavier than oil and often also imder a greater 

 hydrostatic pressure, will replace part or all of the oil at the point 

 of ingress into the well if it is allowed to reach the oil sand. In this 

 way it replaces the oil, in whole or in part, and thus lessens the 

 amount of oil produced and increases its cost of recovery. Water 

 also occurs indigenous to the oil sands in certain fields, but in this 

 case it does not at first occupy the same part of the stratum as that 

 occupied by the oil, but lies in the lower or " down-slope " portion of 

 the sand, and the line marking the junction of the oil in the "up- 

 slope " part of the bed and the water in the " down slope " part de- 

 termines the limits of the productive territory. The water under 

 these conditions is called " edge " water. Upon exhaustion of the oil 

 by flowing or pumping, the " edge " water, through hydrostatic or 

 other pressure, usually " follows up " and replaces the oil. The ap- 

 pearance of the originally extraneous " top " water or " bottom " 

 water in a well indicates a failure to exclude the water properly by the 

 manipulation of casing, cement, or plugs. Such a condition usually 

 can be remedied and the offending fluid kept out of the oil sand, 



