OIL EXPLORATION. In the early days of petroleum prospecting in Texas most oil finds were the result of digging or drilling near known oil and gas seeps, as Lyne T. Barret did in 1866 at Oil Spring in Nacogdoches County, or of accidental finds while drilling for water, as with George Dullnig's 1886 strike in Bexar County or the discovery of oil in Corsicana in 1894. Because of abundant seeps, guesswork and good luck were sufficient for finding oil. Most prominent salines and salt domes had been recorded by the 1890 Geological Survey of Texas but did not necessarily become the focal point of oil exploration due to numerous unexploited seeps. Amateurs in geology, such as Pattillo Higgins, used geological hunches and knowledge of existing seeps to promote drilling for oil at Spindletop in 1901. At Batson in 1903, the Paraffin Oil Company, another group of amateurs, founded their venture on petroleum residue in soil samples collected from near a gas spring. This was the first time that "paraffin dirt" was used in prospecting for oil. Despite these finds, oil companies generally held the use of geology in low regard prior to the 1920s, when geophysical methods of exploration that enhanced the oil prospector's knowledge of subterranean strata began demonstrating an advantage for finding oil. Tools used by oil and gas explorers were fairly basic and depended on fundamental variables in the earth's physical condition: gravity change, magnetic field change, time change, and electrical resistance. The torsion balance was one of the earliest geophysical instruments used in the exploration for salt domes along the Texas Gulf Coast. The most common torsion balance employed in the early hunt for oil in Texas was designed by Baron von Eoetvoes in Hungary and was not available until after World War I for commercial use. Eoetvoes's torsion balance and instruments like it made use of the earth's gravitational field and the way the field varied according to differences in mass distribution near the earth's surface. Because the density of rocks varies, the gravitational force they exert necessarily varies. If very light rocks are found close to the surface, the gravitational force they exert will be less than those of very heavy rocks. With this in mind, geophysicists attempted to locate salt domes, which would be associated with minimum gravity, by using the torsion balance instrument. The first salt dome and oil-bearing structure that was discovered by any geophysical means was the Nash dome in Brazoria County in the spring of 1924, located with the use of the torsion balance. The pendulum method, another variation of the gravity method, also contributed to the discovery of oil in Texas. This method relied on the period of a pendulum's oscillation adjusted by variations in gravity due to changes in altitude and latitude. E. A. Eckhardt and R. D. Wycoff designed a new pendulum instrument in 1930 that led to the discovery of the Cleveland oilfield in Liberty County, the Tomball gravity anomaly, and a clear picture of the Conroe dome. The pendulum method was superseded by the gravity meter. Advances in gravity instrument technology afforded geophysicists better equipment with which to make more accurate determinations. The most common gravitational instrument in use today is the gravity meter or gravimeter, which measures variations in the earth's gravitational field by the gravitational pull on a mass balanced against some form of elastic force. Gravimeters were built as early as 1899 but did not prove effective until the mid-1930s when O. H. Truman used one to find the Tom O'Connor field in South Texas for the Humble Oil and Refining Company in 1934. Ship-borne gravity meters played a valuable role in marine exploration, and air-borne gravity meters received attention in research.
A second method of exploration is the Magnetic method. Most oil occurs in sedimentary rocks that are nonmagnetic. Igneous and metamorphic rock rarely contain oil and are highly magnetized. By conducting a magnetic survey over a given area, a prospector can determine where oil-bearing sedimentary rock is more likely to be found. Two types of magnetic instruments are used to measure the slight difference in magnetism in rocks, the field balance and the airborne magnetometer. The field balance is used on the earth's surface to measure magnetism in specific locations. The airborne magnetometer is used to measure the magnitude of the earth's total magnetic field over a large area. A magnetometer was used to define the serpentine plug on which the Yoast field in Bastrop County was discovered in 1927. A third method of exploration is the seismic method. The central physical property upon which seismic prospecting is established is the variation in speed of the transmission of elastic earth waves or sound waves through different geological structures measured by time. There are two principle seismic methods: refraction and reflection. Refraction prospecting consists of elastic earth waves, initiated by some concussive force, traveling down to a dense or high velocity bed, then being carried along that bed until they are rerefracted up to seismic detector locations on the surface some distance from the shot point. What is recorded is the time required for the sound wave to reach each detector location from the shot point. The speed of transmission of the waves through different geological structures is proportional to the density or compactness of the formation. Unconsolidated formations such as sands and shales transmit waves with a low velocity, weak sandstones and limestones with higher speeds, and massive crystalline rocks such as limestones, rock salt, schists, and various igneous rocks with very high speeds. The refraction method aided petroleum explorers in locating salt domes that transmitted elastic earth waves at high rates of speed. During World War I Ludger Mintrop, a German scientist, utilized a portable seismograph that he invented in order to locate Allied artillery firing positions. By setting up three seismographs opposite Allied guns, triangulations could be made from the instruments to the gun's firing position with adjustments in calculations for the variation of sound waves traveling through different types of geological formations. After the war Mintrop reversed the process. By measuring the distance from an explosion to the seismograph, Mintrop found that he could estimate subsurface geological formations based on the time it took the elastic earth wave to travel from the shot point, through a formation, to seismic detectors located about two miles distant. Variations in time were used to confirm the existence of salt domes that transmitted the elastic earth waves at higher rates of speed. On December 7, 1919, Mintrop filed for a German patent on his refraction profiling seismic method, but the patent was not confirmed until 1927 after Mintrop had already received a United States patent for it. Mintrop's instrument prompted the founding in April 1921 of Seismos Gesellschaft, which did seismic prospecting in East Texas and the Texas Gulf Coast from 1923 to 1925. Because of difficulty in determining breaks in the velocity of sound waves between different layers along the Gulf Coast, it was difficult to determine the depth of the layers. The maximum effective depth of refraction surveying by Seismos's crews was 2,500 feet. A fan pattern of deploying seismographs from the shot point was adopted and ultimately responsible for much of the success of the refraction method in finding salt domes. A Mintrop crew, employed by Gulf Oil, was responsible for the first seismic discovery of a salt dome along the Texas coast using the refraction method at the Orchard dome in Fort Bend County in 1924. This find may very well have been the first seismic discovery of a salt dome that produced oil in the world. Seismos dominated commercial work with the refraction method through 1925. Companies who were most active in refraction shooting in and around Texas from 1924–29 were Gulf, Humble, Roxana (Shell), and Pure and Louisiana Land and Exploration Company. Seismos and the Geophysical Research Corporation did most of the work for these companies except Humble and Shell. Other consulting companies included Petty Geophysical Engineering Company, Burton, McCollum and Frank Rieber. Advances in research during this time led to the development and commercial implementation of another seismic method. The reflection method of seismic exploration is based on the echo of sound waves off layers of varying density rock, which are reflected at a high angle back to the surface. The Geophysical Research Corporation began experimenting with the seismic reflection method in 1926 and by 1929 had seismic crews employing the method commercially throughout West Texas and the Gulf Coast. In 1931 Petty Geophysical Engineering Company of San Antonio invented and implemented the reverse profile method of reflection shooting that became the standard method of shooting throughout the industry. Now most seismologists, instead of using dynamite to make shock waves, use a machine called a thumper to produce elastic shock waves.
A final method of exploration is the study of stratigraphy. Stratigraphic exploration consists of establishing correlations between wells, matching fossils, strata, rock hardness or softness, and electrical and radioactivity data to determine the origin, composition, distribution, and succession of rock strata. Sample logs, driller's logs, time logs, electrical logs, radioactivity logs, and acoustic logs help geologists predict where oil bearing strata occur. Sample logs, compiled from well cuttings and cores, are used to identify key beds and lithologic sequences. A core is a narrow column of rock that is taken from the top to the bottom of a well and shows rock in sequential order as it appears in the ground. Core samples also provide information on porosity, permeability, and saturation of rock in the well. Cuttings are not a continuous record like core samples, but provide a means for identifying sections within larger thick layers through fossil and mineral deposits. The driller's log provides basic information to the stratigrapher concerning depth, type of rock, density, fluids, and other miscellaneous data. The driller's log keeps track of the time required to drill through various strata and the recognition of key beds he drills through. This data is correlated with other information to enhance the chance of finding oil. Early electrical methods of exploration in the 1920s tested electrical resistivity and electro-magnetic potential but proved to be more successful at locating metallic ores than oil and gas. Oil and gas have conductivity properties that differ from water, which conducts electricity more readily. Occurrences of oil and gas can be located by this difference in resistance. The most useful application of electric testing has been in the development and impact of well logging. Schlumberger electric well logging is now standard in the industry. These logs record the conductivity of interstitial water in rock, the movement of drilling mud into porous strata, and the movement of formation water into the well bore. Radioactivity Logs, which record both gamma-ray and neutron values, have been in use productively since 1941. Because radioactivity can be measured with precision it can be used to identify different layers within beds. Radioactivity logs give an indication of the type of rocks and fluids contained in those rocks. Acoustic or sonic logs are used to measure the porosity of a formation. This tool measures the speed at which an acoustic or sonic impulse is carried through a specified length of rock. The speed of sound through the rock gives an indication of the porosity and can be helpful in locating reservoirs. Maps, including contour, isopach, cross sections, and three dimensional computer images, also aid the petroleum explorer in locating oil and gas. Contour maps give details of subsurface structural features enabling geologists to visualize three dimensional structures. Contour maps include information about porosity, permeability, and structural arrangements such as faults, pinch-outs, salt domes, and old shorelines. Isopatch maps show variations in thickness of a given subsurface formation and are used in calculating the size of reservoirs and secondary recovery operations. A cross section map is a diagram of an imaginary vertical cut along a straight line that reveals subterranean features of a given area much like looking at a road cut. Three dimensional computer maps construct images of subterranean strata as deep as thirty miles.
Geophysical Case Histories (2 vols., Tulsa, Oklahoma: Society of Exploration Geophysicists, 1948, 1956). Mildred Gerding, ed., Fundamentals of Petroleum (Petroleum Extension Service, University of Texas at Austin, 1979). Edgar Wesley Owen, Trek of the Oil Finders: A History of Exploration for Petroleum (Tulsa: American Association for Petroleum Geologists, 1975). Olive Scott Petty, Seismic Reflections: Recollections of the Formative Years of the Geophysical Exploration Industry (Houston: Geosource, 1976). Charles Albert Warner, Texas Oil and Gas Since 1543 (Houston: Gulf, 1939).
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