ABSTRACT

The subject of this paper, the "Sam Fordyce-Vanderbilt Fault System", necessarily includes the great "Turn-over" or anticlinal feature that is found roughly paralleling the fault system throughout its definable limits since it is the turn-over, and not the faults, that is of the greatest interest to the oil operators. This important anomaly extends across South Texas for a distance of about 240 miles with the area of influence varying from one-half mile to 10 miles in width. The "turn-over" is present in some form throughout most of the 240 miles, although of course various saddles and branch faults break it into separate segments. Figure 1 indicates the relative location of the fault, the trend of production, normal regional dip, reverse dip into the fault and the locations of known or suspected deep seated salt domes. Along this "golden lane", there are approximately 115 distinct oil or gas fields which have produced to date over 1 billion barrels of oil and contain possibly 2 billion barrels of recoverable oil in reserve. Accurate data on gas production and reserves are not available but the present rate of gas production is very high with hundreds of known gas sands remaining untested.

The fault, or fault system, is downthrown to the coast, with the displacement varying from "questionably there at all" to over 3000 feet. Naturally, there are several different theories of origin of the feature, but the generally accepted theory is that of faulting being contemporaneous with sedimentation, a theory principally based on the greater thicknesses of formations on the downthrown block as compared to applicable formations on the upthrown side. At this time, this theory will suffice but will be discussed later in the paper.

Affected by the "turn-over" are the prolific Miocene and Oligocene sands adequately separated by shales from a depth of 2000 to below 10,000 feet. It is from this trend that the so-called non-Marine Frio sands gained their importance in the ranks of oil and gas producers. Nomenclature of the stratigraphy may very considerably between individuals and companies, but in this paper, the non-Marine Frio is intended as that section found below the Anahuac formation where present and above the Vicksburg as designated by the Textularia warreni. The non-Marine Frio along this trend is composed of fine to medium grained porous, permeable unconsolidated sands separated by frequent shales and containing no fossils of any kind. This section has accounted for over 80% of the total production along this trend and is from 3000 to 8000 feet thick. The Fleming group of the Miocene and the Anahuac produces to some extent in the central-northeast portion of the anomaly while the Vicksburg produces to the south.

Minor faulting, formation wedging forming stratigraphic traps, erratic sand lensing, and various structural adjustments forming saddles and undulations of beds all combine to render this vast regional feature the ideal hunting ground for the oil operator. Development wells are constantly being drilled in proven fields and wildcats are continually probing the flanks and undrilled areas, searching for one or more of the various types of traps common to the trend.

Figure 1. SAM FORDYCE - VANDERBILT FAULT

FOOTNOTE 1. Prepared primarily for oral presentation to the management of The Author's company. Also read before the South Texas Geological Society in April, 1956 and the Dallas Geological Society in May, 1956.

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Figure 2 indicates a slightly "pencil-engineered" cross-section across Agua Dulce-Stratton Field from east to west, and including the major fault. This section is not intended to be entirely accurate but merely to illustrate the typical reverse dip into the fault, the slight thickening of formations near the fault, greater sand deposits near the crest, and the positions and thicknesses of the sands and shales. It is important to note that in this field, as in many others, there is a greater percentage of sand on the crest than near the fault even though the actual formations thicken to some extent toward the fault. It is admitted that this cross-section would not be typical of a similar one in the southern portion of the turn-over where increased sand deposits are noted near the fault.

Figure 3 includes the common types of traps of the region, and, is not meant to refer to any one specific area. The normal anticline or gently folded sand is the most important type of reservoir, but millions of barrels have been produced from the stratigraphic traps, "shoe-string" sand lenses, fault traps, and the less common "double-lobed" anticline, a feature which may be the result of compaction forces.

As in most regions of exploration, there are many problems to master before new discoveries are made. In this region, without doubt the biggest problem is acquiring leases. There just "aren't any" open tracts. However, in the event that a drilling block can be "scrounged" out of some independent or major, such factors as unsuspected faulting and unpredicted "saddles" or "lows" may show up and create problems. Correlation of electric logs is not too difficult, but seismic reflections generally must come from "Phantom" horizons which could result in the overlooking of very small scale faulting or formation warping. The wedging of beds updip along the east flanks of the anomaly, especially shale sections, frequently cause false fault detection. Figure 4 illustrates this worrisome wedging to some extent. The well updip does not penetrate all beds found by the downdip test; thus a fault is suspected. Subsequent wells will of course clarify the situation.

For some reason, geologists always seem to want to know why something takes place regardless of whether or not the solution is of economic value. Consequently it can be assumed that many theories as to the origin of this major anomaly including both fault and "turn-over" have been presented. As stated previously, the most widely accepted theory is that the faulting was contemporaneous with deposition. Other well known theories are the Hinge line theory, the Slip Plane theory, and the Salt Ridge theory. When investigated closely, all of the theories seem to have definite merit and appear to fit at some place along the fault trend. Actually, it is my opinion after a relatively thorough study of all available information that the anomaly is a result of the cumulative effect of all the theories thus far proposed, and that in some locations, a specific theory may stand out as being the major factor. At this time, I would like to add another theory

Figure 2. Cross-section of Stratton field.

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that has rarely if ever been considered as a major factor in our South Texas geology. I do not believe it is the sole reason of origin of this feature, nor even the greatest contributor to the cause, but I definitely believe it to be a very major factor overall, and in specific areas may be the prime factor. I firmly believe that differential compaction contemporaneous with deposition figured strongly in the formation of this anomaly and a large number of other structures throughout the Gulf Coast region. By this theory, I mean that as the sediments were deposited, particle by particle, inch by inch, foot by

Figure 3. Type traps.

Figure 4. Wedge effects.

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foot, the shales, or oozes at the time of deposition, were compacted more so than were the equivalent aged sands. With progression of a geologic time in which the shorelines or areas of sand depositions remained fairly constant, or with little fluctuations, this small scale differential compaction would be increased to large proportions sufficient to cause faulting and turn-over or both. At the same time, it is apparent that as the magnitude of the process increased, other factors would become involved until such time that all proposed theories would be working together on such an anomaly as we have here. There are several places along the trend under discussion which appear to bear out the compaction idea, and certainly there are many local features that must surely have been caused by differential compaction. Small faults, undulations, and saddles of anticlines could easily be the result of compaction. Figure 5 is not intended exactly as a cross-section, but more of a section of key correlation points taken across the La Rosa Field area of Refugio County. The lower correlation band denotes the base of a sand body that decreases in thickness from east to west more than 1000 over a distance of 6300 Apparently the sand completely shales out before reaching the next point of control. It is more than coincidence that the correlation band across the top of the sand dips sharply to the west in the area having a paucity of sand. Compaction apparently could not absorb all pressures and movements; thus the small fault originated. It is noted that exclusive of the fault displacement, the top band does not reflect the double-lobed anticline as does the lower band, indicating that at shallower depths, the overload is less and compaction has not progressed to the point of flexing the more recent beds. And with this in mind, the answer is found to the question often arising as to how a fault con be of small displacement at shallow depths, greatest displacement at medium depths and diminish at greater depths. Differential compaction starts it, other factors such as rate of deposition, type of deposits, transgression and regression limits of shorelines, and resistance of underlying material including basement complex get into the act and aid compaction until a point or time is reached at which the overload is not sufficient to require any more relief and the fault disappears.

At any rate, the varied structure found along this trend certainly indicates that a variety of forces have been at work, some exceeding others in certain areas, but all of great importance and any specific area should be closely studied before making a guess as to "how did this happen like this?" At one time, a certain geologist had a black sheep in a flock of white ones pointed out to him by a traveling companion. The geologist replied that "at least he is black on one side." The moral, in geology -- always analyze something from all sides before making a statement.

FIG. 5. ROCHE - LA ROSA CROSS-SECTION

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