Speaker: Dr. Thomas Ewing

Subject: Tectonics and Subsidence in th West Texas (Permian) Basin, The Makings of a Hydrocarbon Superbasin”

Abstract: The West Texas (Permian) Basin is a complexly structured intracratonic (IC) basin with prolific oil and natural gas production. It began as a subsidence basin (‘Tobosa Basin’) from Middle Ordovician to Devonian time, a response to the Cambrian rifting that separated Gondwana and Laurentia. In the Pennsylvanian to early Permian, it formed part of the Ancestral Rocky Mountains (ARM) orogen. The Texas-New Mexico segment of the ARM contains small to medium basement-cored uplifts, folds, thrust faults and two trends of strike-slip faults, with a pattern that is consistent with SW-NE compression. The largest thrust fault known in the basin is SW-vergent, and faces the deepest part of the Delaware Basin. This direction of compression is similar to that observed in the southern Oklahoma part of the ARM, which shows NE-vergent thrusting and left-lateral faulting.

This SW-NE compressive stress is grossly inconsistent with the northwestward convergence of the Ouachita-Marathon thrust belt southeast of the ARM.  The ARM-generating stress may have originated either from the Pacific side (by flat subduction) or from strong continental collision in the Appalachian Orogen. Lines of weakness generated during the Proterozoic and/or Cambrian concentrated stress and created the complex structures.

The West Texas branch of the ARM is buried by over 2.5 km of post-deformational Permian strata — the Permian Basin. Subsidence began during ARM deformation, then increased in rate and continued to the end of the Permian. Permian subsidence resulted in the maintenance of isolated deep-water marine basins until Late Permian time. The Marathon orogen also subsided, and shed little clastic material into the basin. Despite Mesozoic basin-margin modifications, the Permian isopach pattern suggests a bowl-shaped subsidence centered on the Central Basin axis of uplift. The size and shape of the Permian Basin are similar to other IC basins (Illinois, Michigan, Williston). Similar to some IC basins, the central basin area hosts a 1100-Ma mafic complex, which was subjected to compression in Pennsylvanian time. Sinking of a mafic crust or its subjacent lithosphere, begun during compression, may have been a driving force for Permian subsidence.

Over most of the basin, later Permian subsidence was responsible for putting source rocks into the oil window. Further maturation to gas occurred within the deep basins generated by ARM deformation and Marathon thrust loading.

Bio: Dr. Thomas Ewing is a geoscientist with over 37 years of experience in hydrocarbon exploration and research. He is a Registered Professional Geoscientist in the State of Texas (#1320) and the State of Louisiana (#468) and an AAPG/DPA Certified Petroleum Geologist (#4538), and holds certification #1610 from SIPES.

He received a B.A. in Geology from the Colorado College (1975), an M.S. in Geochemistry from New Mexico Institute of Mining and Technology (1977), and a Ph.D. in Geological Sciences from the University of British Columbia (1981).

Dr. Ewing was a research geologist for four years at the Texas Bureau of Economic Geology in Austin, where he served as a co-author of the “Atlas of Texas Oil Reservoirs”, and compiled the Tectonic Map of Texas.  Since 1985 he has been an owner of Frontera Exploration Consultants, Inc., a San Antonio-based geoscience consulting company. He worked with Venus Oil and Venus Exploration from 1985 to 2005 as staff consultant and Senior Explorationist, playing a main role in its successful exploration in the Yegua Trend of the Gulf Coast Basin and elsewhere in Texas.

Dr. Ewing has served in many offices in AAPG and its Divisions, He served as Vice-President for Sections of AAPG (2012-14). He received Honorary Membership in the South Texas Geological Society in 2009, Honorary Membership in the GCAGS in 2010, AAPG Distinguished Service Award, and BEG Alumnus of the Year in 2011. Most recently he has completed service as President of the GCAGS (2016-2017). In 2018 he was awarded the “Don Boyd Medal for Excellence in Gulf Coast Geology” from the GCAGS (Gulf Coast Association of Geological Societies), their highest award.

Tom has spoken extensively at local, regional, and national geological meetings and published over 90 papers and abstracts. Among other awards, he has received the AAPG Levorsen Award three times, twice in Gulf Coast Section and once in Southwest Section. He has written articles on Gulf Coast geology and hydrocarbons, the geology and tectonics of Texas, and history and urban geology of the San Antonio area. He wrote the popular guidebook “Landscapes, Water and Man: Geology and Man in the San Antonio Area” published by the South Texas Geological Society in 2008. In 2016, Dr. Ewing completed “Texas Through Time”, an illustrated book and website on the geologic history and earth resources of Texas published by the Bureau of Economic Geology.

In his spare time, he directs a 60-voice German men’s chorus, the San Antonio Liederkranz.