The romantic days of early petroleum history when oil prospec tors invoked the aid of an "oil witch" or peach twig or other supernatural devices for "divining," before drilling, the presence of oil, have long since vanished. From the beginning of American oil development in 1859 to 1900, or even later, prospecting was carried out on a casual and random basis. Men drilled on oil and gas seepages, along the beds of streams, along trend lines, at some point where the topography resembled or had a fancied resem blance to some other area which they knew was oil-producing, or for no reason beyond a hunch. The demand for oil was not great ; drilling depths were shallow, and there were enough venturesome souls to find sufficient oil by the crude methods available to them, to meet, and often more than meet, the world's demand.
The old hit-or-miss methods of prospecting were no longer ade quate as the automobile and other modern agencies of oil con sumption came into being. Geology, which previously had been largely speculative as far as oil was concerned and only used spas modically, was accepted as a guide to prospecting. Well depths were still relatively shallow, and the beds in which oil was ex pected to be found were generally more or less parallel to those out-cropping at the surface. A surface dome or anticline could be accepted as indicating with a high degree of probability the existence of a similar dome or anticline in the oil sands. Thus, "surface geology" came to be applied. The earliest mapping of surface structures was done with clinometers, hand levels, and spirit levels. By 1913 the plane table had come into general use as being superior to the older methods, and most of the surface geology for the purposes of oil prospecting was mapped with the plane table.
The next geological aid to the feverish search for oil came in the early 1920s with a new technique of "subsurface geology" and core drilling. Structures actually penetrated by the drill were mapped and samples of the rock and sand strata studied. This was accomplished by using carefully kept well-logs and by drill ing special test wells of small diameter, enabling the geologist to obtain a core or cylindrical segment of the rock penetrated. Information obtainable from the driller's well-log was supple mented by microscopic examination of the samples of formations penetrated by the drill. Laboratories for sample studies were established, and micro-palaeontology became an integral part of the oil business in the United States. Revealed by this method were fossils and mineral grains so small that they could not be seen by the naked eye, and knowledge was gained not only of geo logic formations but of the origin, migration, and accumulation of oil by a combination of applied palaeontology and lithology.
As these methods have developed, in many cases drillers received instructions in distinguishing characteristics of formations; microscopes often were placed on derrick floors, and electrical logging came into vogue. A knowledge of subsurface structure in advance of drilling has resulted in the discovery of production at depths previously considered impossible. In less than 15 years producing well depth increased from 5,000ft. to 12,00oft., and the subsurface oil producing horizons of the future were no longer delimited.
The widespread use of core drilling was somewhat checked by the development of still another technique—that of mapping underground structure by the use of the reflection seismograph in the early 193os, a late development in geophysical prospecting. (See GEOPHYSICAL PROSPECTING.) Many of the existing producing areas of the United States would have remained undiscovered without the aid of geophysical instruments. Their use also has facilitated the delineation of subsurface structure conditions in producing areas and has proved the key to a veritable storehouse of previously unknown production.
These methods were first introduced in the Gulf Coast area in the 1920s, with the torsion balance and the refraction seismograph, the latter not to be confused with the reflection seismograph. The torsion balance was a delicate instrument used to measure the slight variations in• the gravitational force of the earth and thus to enable the prospector to deduce the shape of a concealed structure. The refraction seismograph was used to recognize dif ferent formations by different rates in the speed of transmission of artificially-induced sound waves. By the early 193os the whole area regarded as prospective territory had been explored— some of it five or six times by different companies. Some 50 or 6o salt domes, practically as many again as the number pre viously known to exist, were discovered by its use. The expense for geophysical work alone was estimated at from $20,000,000 to $30,000,000. At about 1931 the reflection seismograph, a further application of induced earth waves, came into use, and the entire Gulf Coast area began to undergo a resurvey utilizing this new technique. Other geophysical instruments used in oil prospecting were the magnetometer, an application of magnetic measure ments; the gravimeter, a further application of gravimetric meth ods, and devices for applying electric measurements still in the experimental stage in 1939.