Pinedale Anticline

The Pinedale Anticline is vitally important for the nation’s energy needs. Looking out at the sagebrush landscape when driving south of Pinedale, it is hard to believe that this geologic anticline includes one of the largest gas fields in the world. This giant gas field was discovered in the late 1990s and is expected to produce 39 trillion cubic ft of natural gas. Partially due to the Pinedale Anticline, Wyoming is one of the top producers of natural gas in the United States. More than half of U.S. homes are powered by natural gas. The Pinedale Anticline in Wyoming helps power U.S. homes.

Trapper’s Point

Trapper’s Point was named for its location near the 19th century fur trappers’ rendezvous sites where mountain men gathered to trade their goods. The narrow ridge on which the overlook is situated also forms a bottleneck on the route of the longest large mammal migration corridor in the lower 48 states. Archaeological evidence indicates pronghorn antelope have used this route for more than 7,000 years. Today, near Pinedale, the route includes a wildlife corridor or bridge over the highway, a project designed and constructed by the Wyoming Department of Transportation.

Edge of the Pinedale Anticline
Edge of the Pinedale Anticline [Credit: World Resources Institute].

Pinedale Anticline

Pinedale field is the largest gas field in Wyoming and the sixth largest in the United States. This giant field is named after the Pinedale Anticline, a northwest-southeast trending folded structure about 5 mi (8 km) wide and 35 mi (56 km) long.

The Pinedale Anticline was first delineated by detailed geologic mapping in the 1920s. In 1939, the first well drilled to explore for oil on this structure was California Company’s Government #1, to a depth of 10,002 ft (3,049 m). There were no oil shows in this well, and although there were gas shows, there was not a market for natural gas, and the well was eventually plugged and abandoned.

Pinedale Anticline - Wyoming's Cultural Geology Guide

As the demand for natural gas grew over the ensuing decades, numerous exploration efforts attempted to unlock the hydrocarbons from the reservoir rocks. Production in the Pinedale field is from the “Lance Pool” — nearly 6,000 ft (1,800 m) of fluvial and alluvial sedimentary rocks that are Upper Cretaceous through Tertiary in age (approximately 83–60 million years old), and include the upper Mesaverde Group, Lance Formation, and lower Wagon Wheel Formation. The reservoirs on the Pinedale Anticline are “tight,” meaning they have very low porosity (average 7 percent) and permeability (micro-Darcy), and that producing the hydrocarbons requires fracturing the reservoirs to create flow pathways. In 1969, drilling began on the Wagon Wheel #1 well that was completed in cooperation with the U.S. Atomic Energy Commission for the purpose of evaluating nuclear devices to fracture tight reservoirs and stimulate hydrocarbon production. However, the program was abandoned before these tests could be carried out.

The first significant production from the Pinedale field came in 1983 when natural gas was transported via truck from the Mesa Unit #1 and #2 wells. Modern production began in 1998 with the Mesa 15-8 well, the first multi-stage fracture-stimulated well in the Pinedale field. Success from the Mesa 15-8 well paved the way for the production boom that occurred in the late 1990s and early 2000s. Current estimates suggest approximately 39 trillion cubic ft of natural gas is expected to be recovered from the Pinedale field.

Pinedale field
The Pinedale field, Sublette County, Wyo [Credit: SkyTruth].

Exploration, technology, and environmental concerns have played major roles in the development of this giant natural gas field. Although active exploration no longer occurs on the crest of the anticline, infill drilling is common and operators are beginning to drill horizontal wells along the flank of the anticline. The Bureau of Land Management’s 2008 Supplemental Environmental Impact Statement Record of Decision (SEIS ROD) planned for 4,399 additional wells with an estimated 40 years of production. Operators currently follow a plan of concentrated development, where drilling is confined to a small part of the anticline in an effort to leave large parts of the area undisturbed for wildlife. Technological improvements have also minimized surface disturbance. Up to 50 wells can now be drilled from a single well pad with a shared production facility, and drill time per well has been reduced from more than 60 days in the early 2000s to an average of 12 days in 2013. Further environmental concerns have been addressed through the process of adaptive management. This is one of the first oil and gas development areas in the country that provided a way to propose changes to the SEIS ROD based on scientific results studied in the field. Adaptive management has led to more stringent requirements on air pollution, resulting in completions without flaring and the elimination of non-essential operations on high ozone days.

Development of the Pinedale field is a good example of operators, private landowners, and state and federal agencies working together in an attempt to strike a balance between hydrocarbon exploration and production for current human needs while preserving cultural and environmental resources for the future.

Ancient Discoveries

By Julie Francis

Some wildlife biologists have suggested that migration of pronghorn from the Green River Basin to Jackson Hole dates to modern times, but archaeological evidence clearly shows that the Trappers Point corridor has been used for at least 7,000 years. The narrow ridge between the Green and New Fork rivers provides an ideal setting for intercept hunting, especially during the migration seasons, and Native Americans took full advantage of the bottleneck, as revealed by abundant remains of the butchering and processing of pronghorn along the Trappers Point ridge system.

Pronghorn migration
Pronghorn migration, Wyoming. [Credit: National Park Service].

A rare combination of geologic factors has lead to the preservation of this evidence. Most of the upland surfaces along the Green River have been severely deflated by the strong western and southwesterly winds which often rage across the basin. As these winds hit the north-south Trappers Point ridge, windblown sands accumulate on the leeward side. Formation of this sand shadow began as early as 9,000 years ago and continual accumulation of sand has buried the archaeological remains. During the early 1990s, excavations in advance of WYDOT construction at Trappers Point uncovered broken and battered pronghorn bone, hearths used for marrow and grease extraction, and points, tools, and lithic debris, all left at the site between 7,800 to about 3,000 years ago. Pronghorn fetal remains found in one of the levels indicate that the animals were killed in the spring, coinciding with the modern migration period. Projectile points found in the bone bed and near hearths exhibit a variety of styles typical of the southwestern Wyoming, Utah and the western slope of Colorado. The stone from which these tools were made also came from a variety of sources around the margins of the Green River Basin, suggesting that ancient people gathered in this area for spring hunting for thousands of years. Though no specific evidence of a trap or corral was found, the steep slope of the leeward side of the ridge, loose sand, and snow drifts could have been used to trap animals as they moved along the slope during their migration.

Other sites investigated prior to highway construction paint a much different picture of human usage of the upper Green River Basin. Prior to modern agriculture, the wet meadows of the floodplains would have supported significant quantities of roots and tubers such as wild onion, biscuit root, yampa and death camas. Thousands of deflated hearths and scatters of firecracked rock on the upper terraces give testament to intensive usage of these plants. Once baked, the roots are easily dried and stored for future use. One extremely well-preserved hearth, dated to about 4,000 years old, contained large amounts of charred roots. Assuming biscuit root was baked in this huge pit, one use could have provided about 12% of the calories for a family of four for a year. All told, these sites point to the extremely important role the resources of the upper Green River Basin held for the ancient hunter-gatherers of the region.

Journey of the Pronghorn

By Julie Francis

Every year, hundreds of pronghorn antelope journey from winter range, as far south as I-80 and Green River, to summer range in Jackson Hole, a distance of about 170 miles. The Mesa’s broad, sagebrush-covered surface, with excellent visibility, is important pronghorn habitat throughout the winter. As animals move north in the spring, they reach the narrow northern tip of the Mesa and are funneled into a bottleneck at Trappers Point. From this point, they cross U.S. 191 and continue northward up the Green River valley through another bottleneck, over the crest of the mountains, and then down the Gros Ventre River into Jackson Hole. The odyssey is repeated in reverse during the fall months.

Ancient Migrations - Wyoming Cultural Geology Guide

Owing to modern agriculture and expanding residential development, other migration routes into Jackson Hole have been cut off. The Trappers Point – Green River route is last remaining corridor. The dense concentration of game, coupled with increased traffic due to rapid expansion of the oil and gas industry on the Mesa and growth in and around Pinedale, has led to extraordinarily high numbers of animal-vehicle collisions on this stretch of U.S. 191, with significant loss of property and life (both human and pronghorn).

With glacial melting at the end of the Ice Age, the Green and New Fork rivers cut wide, deep valleys into the floor of the basin. This formed a wide plateau, known today as the Mesa, between the two rivers. This landform plays a critical role for the longest big game migration corridor in the lower 48 states.

Trappers Point overpass
Trappers Point overpass [Credit: WyDOT].

After trying several warning systems, the Wyoming Department of Transportation built an overpass for pronghorn over U.S. 191 at Trappers Point. This project was done in cooperation with several state and federal agencies and local ranchers and residents. It includes a network of underpasses and one other overpass across U.S. 191 between Pinedale and a point about 15 miles north of Daniel Jct., along with a network of wildlife fencing and gates to channel animals to specific crossing points.

The Trappers Point overpass basically re-connects the northern end of the Mesa with the uplands around Cora Butte, and to pronghorns, it appears identical to the surrounding landscape. It has been an unqualified success. Even before construction was complete, animals were using the overpass. Monitoring studies are still underway, but current information shows that over 36,000 pronghorn and deer have utilized all the crossings in just the two years since its completion.