ABSTRACT

Since the discovery of the Luling Field, Caldwell County, Texas, in 1922, geophysics has contributed toward the successful development of the Edwards Trend in South Texas.

Sample gravity data, typical reflection records, and a cross section of a faulted zone in the Edwards illustrate the effectiveness of a geophysical program toward the common goal of both the geologist and the geophysicist -- to find more petroleum reserves.

After the confirmation of production in the Edwards formation at Luling in 1922, geophysics and geophysical methods have played an important integral part in the long range program of developing the Edwards Trend through South Texas. The recently revived interest in deeper Edwards production has stimulated the need for the assistance which geophysics can offer to the geologist and/or management in their decisions relative to exploratory programs. Geophysical methods are also being applied for more detailed information and further exploratory effort along the shallower portion of the Edwards Trend.

Some examples of gravity and reflection seismograph results along this trend will be presented in a number of illustrations that follow.

Gravity Work ... A regional gravity map of a portion of the Edwards Trend of South Texas is shown in Figure 1. Gravity data show the cumulative effects of both the basement rocks and the sedimentary deposits. The axis of maxima reveals that denser materials are nearer the surface along this axis than along the axis of minima that parallels it on the northwest. On a regional basis, the usual interpretation of the gravity data would result in a conclusion that basement rocks are closer to the surface along the line of maxima and deeper along the line of minima. This trend of maxima, incidentally, extends north and northeast along the Luling-Mexia-Talco fault trend.

A gravity profile across the Luling Field is representative of the effect of shallow density changes on the regional gravity picture (Figure 2).

The higher gravity readings are observed on the upthrown side of the fault system where the denser rocks are closer to the surface. The regional gravity gradient is represented by the average lines through the gravity readings on each side of the fault. The offset in the alignment of the average lines is a measure of the anomalous condition disclosed by gravity readings.

Seismic Work ... An index map showing the position of the gravity profile across the Luling Field, and the location of five representative reflection seismograph records, is shown in Figure 3. An outstanding reflection, believed to be from the top of the Georgetown-Edwards series, will, for the sake of brevity, be called the "Edwards" for the remainder of this paper.

Figure 4 is typical of the records obtained from southern Bexar County, with a strong Edwards reflection predominating. The Navarro, an Upper Cretaceous reflection event, and the Glen Rose are also identified. Depth to the Edwards is about 3300 feet.

A central Atascosa County reflection record discloses several strong reflection events in Figure 5. Here is shown the strong Edwards event, about 8000 feet in depth, and also a deep reflection unidentified but marked Jurassic (?). The time of 2.800 seconds on the Jurassic (?) event would be equivalent to approximately 15,000 feet in depth. One of these days in the future all of our efforts may be aimed at this level for our new reserves.

Figure 6 is a record taken in central Wilson County, with a strong Edwards event at 6100 feet. There is a marked similarity in the character of the reflected energy and the time intervals between events on this record and on the one for central Atascosa County (Figure 5). In addition to the Edwards, the Midway, Navarro, Upper Cretaceous, and Glen Rose are identified.

A reflection record in southern Atascosa County along the Deep Edwards Trend is presented in Figure 7. Note the strong Edwards event at depth of 11,500 feet on this record. In most areas, the Edwards is typically a strong reflection, and, in geophysical lingo, a "doubleton," that is, there will be two strong cycles shaped like the letter "W." Note also a deep reflection, unidentified but indicated as Jurassic (?), with a time of 3.300 seconds and with an estimated depth of 18,000 feet.

In southern McMullen County, the outstanding reflections are in the Wilcox zone, Figure 8. A deep pair of reflections are identified, one of which is believed to be near the top of the Edwards. Possibly a (?) mark should have been placed on the Edwards pick since no well has penetrated this formation for positive identification in this area. An estimated depth to the reflection picked as Edwards would be about 15,500 feet.

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Figure 9 shows the correlation of four seismic events across three records (B, C, and D) along the Edwards Trend where an up-to-the-coast major fault, cutting as high as the Navarro and with about 200 feet of throw on the Edwards, is disclosed. There is also a smaller, questionable, fault cutting no higher than the Edwards (also up-to-the-coast) and with a very small throw. This apparent smaller fault may result from distortion or erratics due to unusual ray paths from the shot through the major fault plane to reflectors below the major fault. This type of distortion is described by Miller Quarles, Jr., (1950) in his paper published in Geophysics.

A cross-section showing the same seismic data plotted in section on a one-to-one scale is given in Figure 10.

Geophysical costs for surveys along the Edwards Trend are about average. Much of the seismic exploration in today's programs is conducted along bulldozed lines in an effort to run the seismic lines at right angles to strike and at right angles to the trend of faulting. This policy, although partly in contrast to that followed in earlier work when roads were generally used, aids the seismologist in his interpretation.

Public relations is an important phase of the geophysical operation. Before any area is entered for a geophysical survey, the preliminary groundwork should be laid by a representative who can obtain the permit for the survey from the landowners, discuss the method of entry for the trucks, and explain the proposed plans for shotpoint locations, bulldozing, and the like. The costs of bulldozing and permitting, incidentally, add to the overall costs.

In many areas, two shothole drills are needed for maximum efficiency. Multiple detectors are accepted as an aid in obtaining optimum quality records for the seismologist whose responsibility it is to make the interpretation.

Magnetic recording is now the popular method of presenting seismic data. Many major oil companies have converted all their field crews to magnetic or tape recording and the trend is for more and more seismic crews to convert to this method. Seismic energy is recorded on tape and then played back through filtered channels. Weathering and elevation corrections and corrections for the normal stepout or moveout of reflection times are all removed. The final result is a cross-section, or record section, on which all of the seismic data for a number of consecutive shotpoints along a profile line are presented, totally corrected for all variables. Record sections have been accepted by geologist and geophysicist alike as probably the best way to present seismic data for evaluation.

The effectiveness of an exploratory program along the Edwards Trend, or in any area, is dependent upon the close co-operation, and the mutual understanding of the problems, between both the geologist and the geophysicist. Teamwork is essential and teamwork is paying off.