Monday, 17 November, 2025/Edition 85

This week we will visit Texas — the land of friends, for Texas is a Spanish mispronunciation (“Tejas”) of a native Caddo word “taysha,” meaning friends. I don’t know how it was named, but I know many geoscientists, including myself, have friends in Texas because the Lone Star State is also a land of great geology — both surface rock record and subsurface reservoirs. Let’s take a look.

Rasoul Sorkhabi

Editor, Core Elements

Thermal History of Delaware Basin

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Two recent papers, one presented at this year’s URTeC conference and the other at the SPE Convention in October, address the thermal history of the Delaware Basin.

Geological setting:

• The Delaware Basin in west Texas and southeastern New Mexico is the western part of the Permian Superbasin, the most prolific shale basin in the world. The Delaware Basin is separated from the Midland Basin to the east by the Central Basin Platform.

• The Permian Superbasin formed in the Paleozoic as part of the much larger Tobosa Basin along the continental margin of Laurentia continent.

Study #1: Burial to Exhumation

Researchers at Texas A&M University utilized a data set of 1,185 vitrinite reflectance (VR) measurements from 276 wells in the Delaware Basin, plus apatite fission-track analysis and the back-stripping method to calculate tectonic subsidence.

What they found: Results revealed the following thermal and exhumation history…

For the Delaware Basin:

• Main basin subsidence from 485 to 359 Ma with tectonic subsidence reaching 1.5 kilometers

• An uplift event from 310–290 with the Pennsylvanian-Permian disconformity related to collision of Laurentia and Gondwana

• Cretaceous sedimentation associated with the Western Interior Seaway

• An uplift event from 75–45 Ma due to Laramide orogeny

For the Central Basin Platform: The presence of immature to early oil window (VR less than 0.6 percent) at depths less than 8,000 feet suggest that this structural high was established in the Paleozoic and has persisted since.

For the Western Delaware Basin: A shallow gas window (samples with VR greater than 1.35 at depths of less than 10,000 feet) and oil window (samples with VR greater than 0.65 percent for depths of less than 2,500 feet) indicate the removal of overburden strata after the Permian and possibly extra heat input from the Neogene Basin-and-Range extension and igneous activity.

Study #2. Common Equation of State (CEOS) Model

Equation of State (EOS) is a thermodynamic equation that quantifies the state of matter under physical conditions such as pressure, volume, temperature and internal energy.

What researchers did:

• Researchers at Occidental applied CEOS modeling to capturing reservoir fluid pressure-volume-temperature (PVT) properties and regional variations in the Wolfcamp Formation—the main producing reservoir in the Delaware Basin. 

• The researchers used VR equivalent values and 1D thermal maturity modeling and the CEOS models were fine-tuned and validated by matching laboratory PVT data.

What they found:

• The CEOS model can forecast gas-to-oil (GOR) variation regionally and through production time based on VR, total organic carbon, and composition of PVT fluids (carbon-1 to carbon-7 mol percentage).

• Regional PVT systems impact well performance and capture the regional thermal maturity of the Wolfcamp across the Delaware Basin, from volatile oil in eastern part, through near-critical fluid in the middle part, to rich gas condensate in the western part.

Why it matters:

• In 2024, the Permian Superbasin produced 6.3 million barrels of oil per day or 48 percent of the total U.S. crude production.

• Understanding a detailed thermal history of the basin helps improve oil exploration and development in the region.

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Tectonic Shifts in Southern Rio Grande Basin

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A study published in Geosphere presents an evolutionary sequence for the formation of southern Rio Grande rift in Trans-Pecos, Texas.

Rio Grande rift tectonics: The Rio Grande rift spans from central Colorado through northern Chihuahua, Mexico and formed after the Laramide orogeny.

The Rio Grande rift has two main segments:

1. The Northern segment, which trends N-S with crustal extension ranging from 8 to 12 percent in the San Luis Basin, to 30 to 33 percent in the southern Albuquerque Basin.

2. The Southern segment, which trends NW-SE, with crustal extension of 4–10 percent over distances of 60–125 kilometers. The new study focuses on the southern segment.

What researchers did: Jesse Kelsch and colleagues collected 308 measurements of fault outcrops from six localities in the Big Bend area and dated four samples of calcite slicken lines by the U-Pb method, which gave ages of 30.7 Ma, 34.6 Ma, 21.8 Ma, and 13.7 Ma.

What they found: The researchers interpret the tectonic evolution of the southern Rio Grande rift in three stages:

1. 32–30 Ma or onset of the NW-SW extension

2. 10 Ma: Rotation of the crustal extension to E-W and NE-SE direction

3. Youngest stage (timing uncertain): Rotation to NNE-SSW

These rotations were in response to the evolving Pacific-North American Plate Boundary.

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Roadside Geology of Texas

The Geological Society of Texas has released the third edition of Roadside Geology of Texas, first published in 1979 and then republished in 1991 by the Mountain Press. The third edition is written by Paul Brandes and Darwin Spearing.

How to use the book: Roadside Geology of Texas is a helpful companion for geology teachers and students as well as geologic time travelers driving on the highways and backroad of Texas. The book is very well-illustrated with maps and photographs.

Geological division of Texas:

The book follows the traditional classification of Texas into five geological provinces, from east to west:

1. Gulf Coastal Plains

2. Central Plains

3. Llano Uplift

4. High Plains

5. West Texas or the Basin-and-Range province

Highlights of the Llano Uplift:

• Of these, only the Llano Uplift in central Texas is entirely Texan. The other provinces extend into the neighboring states of Louisiana, Oklahoma, or New Mexico.

• A recent GSA Today article describes the Llano Uplift as a geoheritage site, and a chapter in Geological Society of America Field Guide 74 give a field trip description of Llano Uplift in five stops.

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