Subsurface Methods as Applied in Geophysics 1103 



were incomplete. Later geophysical work included a torsion-balance sur- 

 vey and several refraction and reflection surveys, which indicated the 

 presence of a piercement-type salt dome. The results of a dip survey 

 made in 1933 after three producing wells had been drilled are shown in 

 figure 581. This survey served to guide further drilling in the area. In 

 order to investigate the possibility of additional structural deformation 

 in the area favorable to the accumulation of oil, a detailed study was made 

 in 1942 using the McCollum profiling technique. This patented method 

 consists in lowering a detector in a well on the dome to a point at or near 

 the salt contact and observing travel times of seismic impulses generated 

 at shot points spaced along radical lines extending outward from the well. 

 A comparison of the salt outline determined by drilling with the results of 

 the refraction survey may be seen in the figure.^'' 



Lake St. John Field, Concordia and Tensas Parishes, Louisiana — 

 The Lake St. John field is located on a structure produced by a deep- 

 seated salt uplift. A reconnaissance gravity survey in this region first in- 

 dicated the presence of a possible structure. As may be seen in figure 582, 

 a closed gravity minimum of considerable extent and moderate intensity 

 lies in close proximity to the producing area. A dip-reflection survey 

 made to check the gravity anomaly showed the possibility of closure in 

 the area. A more detailed seismic survey in 1940 indicated the Lake St. 

 John structure to be a large anticline which trended northwest-southeast. 

 Maps made on several horizons showed increasing closure with depth. 

 Contours near the top of the Lower Cretaceous are shown in the figure. 

 On the basis of this work the first well was drilled to the Wilcox and 

 logged shows of oil. However, it was later found to be in a broad sub- 

 surface saddle between two highs. Additional detailed work led to the 

 location of the discovery well. Seventy-eight wells are now producing in 

 the area with 13,000 proved acres. ^^ 



Odem Area, San Patricio County, Texas — Although eight dry holes 

 had been drilled in the Odem area, San Patricio County, Texas, a re- 

 flection-seismograph survey located several structures that subsequent 

 drilling proved to be major oil and gas fields. As the early drilling had 

 shown the area to be regionally high, a reflection-seismograph survey was 

 begun in 1938. Owing to an unfavorable leasing situation in the Odem 

 area proper, the crew was shifted to the Riverside area to the south after 

 the discovery of abnormal dips. The Riverside structure was located after 

 very little work, and subsequent drilling confirmed the seismic structure 

 shown in figure 583. During exploration of the Riverside area, suflScient 

 lease had expired in the Odem area to warrant seismic investigations. 

 Continuous profiles shot across the structure revealed several hundred 

 feet of closure. Owing to the lack of continuous reflecting horizons, it 



^° McGuckin, G. M., History of the Geophysical Exploration of the Cameron Meadows Dome, 

 Cameron Parish, Louisiana: Geophysics, vol. 10, no. 1, pp. 1-16, Jan. 1945. 



^ Smith, N. J., and Gulmon, G. W., Geophysical History, Lake St. John Field, Concordia and 

 Tensas Parishes, Louisiana: Geophysics, vol. 7, no. 3, pp. 369-383, July 1947. 



