During the period from 1926 to 1930, the percentage of major oil pools in the United States discovered by geophysical methods was 9.3%. During the period 1931 to 1935, this had increased to 55%. In 1938 it was estimated that about 75% of the new major oil fields in the United States were being discovered by geophysical methods.
In these advances in oil exploration and discovery methods are found the explanation of the United States' continued leadership in oil production, and of the fact that year after year the coun try has been able to meet its own and much of the world's oil re quirements. It is to be noted that as known pools or producing horizons have been depleted or exhausted, new pools or producing horizons have been found, not alone to take up the slack but to provide the ever greater requirements. Similarly, as a prospect ing technique has come to the aid of the producer, has been used and has exhausted its usefulness, another has arrived in time. An even new technique may be some form of soil-gas analysis to supplement geophysical prospecting.
While the cost of actual field operations has increased enor mously, the oil per unit field found has increased substantially and the hazard of prospecting also has decreased considerably.
It is estimated that with good practice, the cost of finding oil in the early 192os was 20 to 25? per barrel, while in 1938, on volume operation, it was from 10 to 12? per barrel. However, not every newly discovered field is directly attributable to scientific advancement. Although, by and large, the drilling of wells is based upon geological data, the "wildcatter," drilling at ran dom, still is a great factor. Not only new pools, but occasion ally, new regions and new types of pools, have been found through the drilling of a rank wildcat well. The great East Texas field developed in the early 193os is a case in point and indicated that stratigraphic, rather than structural, conditions were the controlling factor in the accumulation of the oil. This opened up studies of the order and relative position of the strata of the earth's crust.
In 1937 one out of every eight wildcat tests drilled in the United States was successful. It is estimated that only 12% of wildcatting was carried on without some scientific basis, and ex ploration based upon technical data was shown to be almost three times as effective as random effort.
Advances in Production Methods and Drilling.—Great advances in production technology have come with the increase of knowledge concerning the conditions governing the occurrence of oil. The fanciful ideas prevalent in the early days that oil was
found in underground lakes and streams have been replaced by fundamental knowledge of the character of the reservoir rocks. It is known that the rocks or sands in which oil is stored vary greatly in texture and porosity. Frequently oil, itself charged with gas, occupies a zone in the rock overlain by free gas and underlain by water. Gas, oil, and water alike are confined under great pressure. When wells penetrate the oil-bearing portion of the reservoir, this pressure tends to drive the oil out of the reservoir to the surface. The researches that eventually made-over pro duction methods in the United States had their start with efforts to use natural gas and air in rejuvenating old oil fields early in the loth century. By 1923 the air-gas lift system was installed and operated practically in all new fields in the Mid-Continent, on the Gulf Coast, and in California, principally to increase the oil yield of wells. Production engineering, as a new art, accom panied this development. The old practice had been to let wells produce virtually wide open, the oil being driven up by and with the gas, and the gas allowed to escape in the air. The production engineer applied himself to making better use of the gas as an efficient agent in extracting oil from the ground by preventing its escape and back-pressuring it into the oil sand again. Thus some degree of conservation of natural gas, long having been wasted in the air, was accomplished, as well as prolongation of the life of the oil pool.
In 1924 the theory was advanced that oil in an undisturbed pool was different in character and behaviour from the same oil when raised to the surface of the earth, and that gas dissolved in the oil caused a marked reduction in its viscosity and surface tension. This theory started exhaustive studies, and it was de termined that holding as much as possible of the gas in solution in the oil would keep the oil more fluid and, through lowered sur face tension, reduce the resistance of the oil to flowing to the well. Thus a function of gas in the reservoir, not before known, was discovered, and in the next 15 years an entirely new system of oil production came into existence. It was furthered by con servation laws in various States and by proration control in the United States.