Abstract

The Permian Basin is famous throughout the world because its ancient sediments are exceedingly rich in hydrocarbons. The producing zones were deposited several hundred million years ago; and, in most basins this old, much of the oil and gas has escaped. The tremendous wealth of the basin is preserved because the Permian Basin has had a geological history that was ideal for the generation, migration, and preservation of oil and gas. Only now are we beginning to fully understand the grandeur of all we see here. The earliest geologists exploring our basin had no comprehension of the vastness of the reserves. Even today, many who should know better tend to downgrade the remaining economic potential of the Permian Basin.

If I had attempted to write this geological history twenty years ago, I would have found it very difficult, particularly if I were asked to write it for the general public. At that time, it all seemed too complex. Fortunately, we have undergone a tremendous revolution of geological concepts in the past two decades as a result of the development of modern plate tectonic theory, and this has allowed us to unravel the evolution of the Permian Basin. We now know that the structural history was very simple. That makes it a wonderful basin for exploring for oil and gas, and there is a great potential for additional discoveries.

Most people have heard or read about drifting continents, sea-floor spreading, etc., and it all sounds very complicated. What we see in the Permian Basin is the essence of this tectonic process—everything we see can be explained by simple crustal movements, up and down, of this part of the North American plate. The structures all were created by a process of thermal deformation. It is illustrated in the accompanying model.

Continents and oceans are two different things. When the earth was first formed, there was no such distinction; but over many billions of years, there has been a continuing segregation of the more soluble and less soluble minerals. The more durable minerals are the lighter-colored ones (quartz sand is a good example). Through many cycles of transportation, deposition, and remelting, the lighter minerals congregate to form granitic rocks of the continents, and the darker ones, the rocks of the ocean basins. Inasmuch as the lighter-colored minerals are also lighter in weight, the continents tend to lie above sea level, and the oceanic crust below. The earth’s outer crust is composed of brittle rock, but at a depth of 15 km or so, the mineral material becomes more plastic. The crust or lithosphere is really floating on a highly viscous substrate.

The earth is a spheroid, and any change in spatial relationships has to be explained using spherical or solid geometry. When geologists treat the earth as if it were flat (in Midland that is understandable), it causes a great deal of conceptual trouble in geology.

In the earliest stage, the planet was undeformed. The only forces available to cause deformation in the absence of any known external stress systems would be those caused by thermal activity. Concentrations of radio-active minerals will cause local areas on the planet to become hot. The surficial brittle rocks expand as they are heated. Increased thermal activity causes blisters or welts to form.

When these blisters or domes rise on the spheroid, they cause the rocks at the surface to dilate or extend in all directions. Granites and other continental-type basement rocks are very strong in compression, but very weak in tension, partly because they are always highly fractured. The dominant fracture system on the earth’s crust is vertical.

Let us examine the geometry involved in the dilation process. As an analog, if you were to push up on a Roman arch, the keystone block would drop through and hit you on the head. The same geometry applies on the surface of the fractured crust. If the crust is uplifted sufficiently, the vertical fractures will be pulled apart, and the crestal blocks will fall in. We call these downdropped blocks “grabens.”

Again, to point out the importance of spherical geometry as compared to the planar geometry most geologists use in constructing cross sections, the definition of vertical differs. On a planar section, vertical is parallel, but on a sphere, vertical converges at the center. This means that any type of uplift on a sphere stretches the arc and uncouples the fractures. The converse is also true; any downdropping effects convergence, or compression. Thus, when you uplift a portion of a sphere, you stretch the fractured basement by opening up the fractures. Once you uncouple a basement block, it loses its lateral support. The block is then free to drop downwards in response to gravity until it is recoupled along the vertical fractures. This is a type of deformation where the relative vertical relief is directly proportional to the intensity of the uplift. Furthermore, the blocks closest to the crest of the thermal dome will drop down the greatest distance.

The original basin of deposition, the Tobosa Basin, was a simple saucer basin that formed on the southern edge of the North American plate. This basin started as a swale between two thermal domes that helped cause the thermal breakup of our area in late pre-Cambrian time (500,000,000 + years ago). At that time, the North American plate broke apart and separated from the continental plate that contained what is now South America, Africa, and Europe.

When plates break apart like that, it is because the uplift too great to be accommodated by vertical block adjustments. The continental crust is dilated so much it actually spreads apart. When that happens, the underlying viscous substrate or mantle melts. The new molten rock rises up to its equilibrium position between the vertical fractures of the continental crust and cools and solidifies. These darker, heavier rocks became new oceanic crust. A new ocean basin developed between the North and South American (?) plates. This is what we mean by continental drift.

In this new ocean basin, deep-water sediments and volcanics were deposited throughout much of the Paleozoic Era. We call them the Ouachita and Marathon sections. Near the close of the Paleozoic, the plate movement reversed and this ocean basin closed again, and the plates came back together. The intervening thick sedimentary column was scraped off the ocean floor and squeezed, thrusted, and obducted back onto the continental plates, enabling us to study it in the outcrop. Some recent oil and gas discoveries have been made from this allochthonous stratigraphic section south of the Permian Basin near Marathon, Texas. The sediments are approximately the same age as was deposited on the North American plate, but the depositional setting was markedly different.

At the time of the initial late pre-Cambrian continental breakup, the whole southern and eastern edge of the North American plate was hot and high-standing, but as cooling occurred, the entire plate subsided below the sea. The rate of deposition was controlled by this cooling cycle. The early Paleozoic sedimentary section along the plate margin is quite similar all the way from the Tobosa Basin on the southwest to the Adirondack area on the north. The sedimentation was quite similar, but the hydrocarbon-producing ability is not. It was best in the Tobosa Basin of Texas and in the Anadarko Basin in Oklahoma. This is because both of those basins were subjected to a second period of thermal deformation during the upper Paleozoic Era. The second period of thermal activity assisted in the maturation and migration of the hydrocarbons in the Tobosa and Anadarko Basin sections. This second period of heating also provided the heat required for maturation of hydrocarbons in the sediments laid down subsequent to the second thermal event.

When the second thermal dome rose near the center of the Tobosa Basin in Pennsylvanian time, the least-work configuration of failure would be the formation of a three-armed graben system. This is what the plate tectonicists call a rift-rift-rift triple junction, and the grabens, roughly 120° apart, became the Delaware, Val Verde, and Marfa Basins. At the time of maximum uplift, the dome rose well above sea level. The upturned edges of the dome, adjacent to the deep basins or grabens, constituted the Central Basin, Diablo, and Elsinore Platforms.

Because of the high heat flow, there apparently was partial melting of the crustal rocks on this dome. In addition to the first-order doming that created the rift basins, there were fifty or more local second-order thermal welts raised. These vary from a few miles to a few tens of miles across, and some are quite linear. The second-order uplifts, obeying the same mechanical principles, are what created the discrete producing anticlinoria of the Permian Basin. They are most numerous near its crest, but evidence of these second-order thermal uplifts can be mapped almost everywhere in the Permian Basin. The individually producing structures are tilted fault blocks on the flanks of these anticlinoria. The geologist needs to understand that in every case, he is mapping a system of interrelated structures. There appears to have been a single phase of deformation.

As the dome rose, the axis of the old Tobosa Basin migrated eastward. That basin, a remnant of the original Tobosa Basin, is now called the Midland Basin. We in Midland live near the deepest part of it. In middle or late Pennsylvanian time, there were many oscillations of sea level, partly the result of changes of sea level related to a period of continental glaciation, but also because of the up and down motion related to rising of this thermal dome. Very thick productive Pennsylvanian reefs grew in different parts of the Permian Basin in response to these sea level changes. Reefs grow best during transgressions, and develop secondary prorosity during regressions.

To the east of the Midland Basin, and away from the uplift of the second thermal dome, the Eastern Shelf area continued to receive sediments intermittently from start to finish. This includes the vast oil-producing area around Abilene and San Angelo.

After the breakup of the Tobosa Basin into the lesser basin subdivisions, the deeper parts of the Delaware, Val Verde, Marfa, and Midland Basins continued to receive large amounts of sands and shales, whereas on the shelf edges there were vast areas of limestone and dolomite deposition. These shelf and basin reservoirs, both deep and shallow, became filled with hydrocarbons, but they would not have been nearly so prolific if the Permian Basin area had not been reheated in Pennsylvanian time.

Perhaps I should classify the term “Permian Basin.” I have discussed all the other geologic basins, but I have not specifically described the Permian Basin. The term “Permian Basin” was originally applied to the large basin in the central part of the U.S., occupied by the sea during the Permian Period at the close of Paleozoic time. That basin extended northward to Nebraska and beyond, and the term is really not geologically specific. Over time, the term “Permian Basin” became more of a geographic term applied to that part of the “Permian Basin” occupied by the Permian seas of West Texas and eastern New Mexico. We all know we live in the “Permian Basin,” but it is a non-specific word and difficult to use geologically.