Many geologists have earned their field trip stripes in the Guadalupe Mountains in West Texas, but fewer have heard – and braved the dangers – of hiking the mysterious and mineral-rich Franklin Mountains north of El Paso. The Franklin Mountain region is a west-tilting series of horst blocks that became 5,000-7,200-foot tall mountains as a result of the Laramide orogeny. The sheer eastern escarpment exposes rocks dating from the Pennsylvanian down to 1.25-billion-year-old basement granite. It was a “field trip through time” focused on the Cambrian, Ordovician, Silurian and Devonian formations, representing more than 100 million years of geologic history.
These rocks are equivalent to prolific oil source, reservoir and wastewater disposal rocks under the West Texas Delaware and Midland basins.
The magical nature of minerals is an incentive to explore backroad places. Mines of the Franklin Mountains have yielded copper, tin, tungsten and iron ores not found elsewhere in Texas, and you can shove a few collectible, colored minerals like blue fluorite, calcite, malachite and blue azurite into your pocket. A nagging mystery that such a rich area has relatively limited commercial geology activity today.
Geologists have to be in good physical shape to explore the stratigraphic section of the Franklin Mountains. The AAPG Field Trip Committee, headed by Andrew Roark of Chevron, took up the challenge to lead a trip to the Franklin Mountains from April 28- May 1, immediately ahead of the May 2-4 Convention in Midland. Roark warned potential field trip hikers of the strenuous itinerary by stating on the registration page, “Participants must be physically capable of hiking up to six miles over trails with uneven footing, and be able to carry sufficient food and water for the duration of the hike.” Yikes!
Ten geoscientists, young and old, men and women, agreed to the adventure and traveled from Midland on a three-day excursion to the Franklin, Organ and Sacramento mountains. The stratigraphic sections investigated were nearly identical in thickness and facies to the age-equivalent Woodford, Wristen, Fusselman, Montoya and Ellenburger zones in the Permian Basin. The route traced depositional sequences and bounding surfaces along a nearly 80-mile distal-to-proximal axis. Examining the Percha shale (equivalent to Woodford shale) offered a rare window into lateral facies relationships in unconventional reservoir systems. Participants also observed the large seismic-scale karst collapse features at McKelligon Canyon.
“The trip organized by West Texas Geological Society was a great opportunity to see field examples of Permian Basin geology from the billion-year-old Grenville-age formations to the Devonian Woodford-equivalent Percha shale,” noted field trip attendee Sean Romito of ConocoPhillips. “These rocks are not only important as conventional reservoirs and deep injection zones but they’re also being explored as emergent horizontal drilling targets.”
“Trip leader Andy Roark guided us on a field-based masterclass to study these deep reservoirs with special focus on the stratigraphy and porosity/permeability, while Ben Davis supplemented this effort with his knowledge of the Grenville orogeny and its impacts on deep induced seismicity,” he added.
Field trip adventurers also included Russell Berry, Alena Grechishnikova, Casey Mitchell, Craig Davis, Denton O’Neal, Grant Sha, and Larry Wollschlager, representing multiple companies including Chevron, ConocoPhillips, Endeavour, EOG, Suttles Logging, Wolf Energy and Ring.
Direct applications of field trip observations include identifying horizontal drilling targets, characterizing pay in the challenging deep zones, and mapping regional changes in reservoir properties to high-grade acreage.
On the first day of the field trip, leaders simulated what it would be like to see inside a Permian Basin borehole. The Franklin Mountain trail passed through the formations from youngest to oldest, replicating the order encountered when drilling a well. Participants tracked the likely wireline response to each of the different rock units, effectively creating a synthetic log of a hypothetical Permian Basin exploration well. Roark noted the fictional well would have to be a dry hole because of the extremely high thermal maturities of the region.
The second day concentrated on Bishop’s Cap, the northern extension of the Franklin Mountains fault block in southern New Mexico. The group explored abandoned fluorite mineral mines, observing 3-dimensional cross-cutting fault and fracture networks. Roark noted that the fracture sets are critical to understanding the tectonic history of West Texas and the present-day stress regime of the area. Mineralization continued from 33 million to at least 5 million years ago due to heat from the adjacent Organ batholith, with dominant species including fluorite, barite, dogtooth calcite and cryptocrystalline quartz.
On the third day, the group hiked into Alamo Canyon in the Sacramento Mountains to study a significantly thinned stratigraphic section, noting evidence for facies changes and post-depositional erosion. Igneous dikes in this area resemble sills that penetrate some areas of the Permian Basin. A scenic drive up the western escarpment of the Sacramentos revealed age-equivalent formations to the major early Permian unconventional, and the day ended with a visit to beautiful Cloudcroft.
Congratulations to the West Texas Geological Society and the AAPG on successfully organizing this historic field trip to the Franklin Mountains in West Texas. We hope they collected some good mineral samples!