User:Adsivils/Railroad Valley, Nevada (Petroleum Geology)

Railroad Valley, Nevada lies within the Basin and Range Province which is characterized by it’s north-south block faults and intervening valleys covering much of the inland western US that lies east of the uplifted Sierra Nevada Mountain Range. The valley has produced a number of producible oil fields since the 1950's.

Article Draft
Geology of Railroad Valley Nevada

Railroad Valley lies within the Basin and Range Province which is characterized by it’s north-south block faults and intervening valleys covering much of the inland western US that lies east of the uplifted Sierra Nevada Mountain Range. The Basin and Range Province began to form as the Farallon-Pacific plate subsided, triggering the San Andreas transform fault, causing the Pacific Plate to move Northwest relative to the North American Plate, beginning in the late Oligocene-early Miocene. Normal faulting has continued at least until the end of the Tertiary. Lithosphere base is around 60-70 km and crustal thickness only 30-35 km.

The Grant range, Pancake range, and the pre-Miocene basement flooring of Railroad Valley, all consist of Cambrian-Pennsylvanian carbonate and siliciclastic strata that have either been locally intruded by cretaceous-tertiary granite stocks, concealed beneath Eocene aged Sheep pass formation, or have been covered by caldera-related ignimbrite of the Garrett Ranch Group. This valley can be characterized by it’s high geothermal gradient and active geothermal systems which account for it’s hydrocarbon maturity and lithium-bearing brines.

 Petroleum Systems 

Source Rocks

Sheep Pass Formation (Eocene)- Local non-marine sedimentary rock sequence including lacustrine limestones, conglomerate, sandstones, and shales. This formation acts as a reservoir and source rock for oil produced out of Currant and Eagle Springs oil fields. The Sheep Pass formation is one of the most significant sources besides the Chainman formation for oil production.

Chainman Formation (Mississippian)- Source rock from marine black shales deposited in deep waters that developed organic and phosphate rich accumulations due to oceanic-upwelling.

 Reservoirs 

Volcanic ignimbrite (Oligocene)- Mostly welded and unwelded rhyolitic tufts that overly unconformably overlie the Sheeps Pass formation. These volcanic tufts are part of an extensive ignimbrite sequence that once covered much of Nevada and western Utah. The average thickness within Railroad Valley and it’s adjacent basins is up to 3000 ft. They make up the Garrett Ranch Group and are oil reservoirs for Eagle Springs, Sans Springs, Sand Dune, and Trap Springs oil fields.

Guilmette Formation (Devonian)- A vuggy, brecciated dolostone. Porosity in the Guilmette falls around 3-4%, and permeability locally, over 1000 md. Thermal brines may have contributed to the abundance of vugs.

 Seals 

Valley Fill (Miocene to Modern)- Fluvial, lacustrine, and alluvial fan sediments, over 9000 ft. in the deepest part of the basin. An unconformity separates the Valley Fill from the older underlying Oligocene volcanics. The valley fill acts as a seal for the underlying oil reservoir rocks.

 Trapping Mechanism 

Faulting, folding, truncation, and overlap are all trapping mechanisms that can be found in Railroad Valley.

 Thermal Evolution 

The association of oil fields and hydrothermal activity in Railroad Valley suggests that oil migration in this case is part of a larger convection system where oil and brine are carried upward together but are separated at the top of the convection system due to their different densities. The high-temperature gradient characteristic of this area probably is related to Recent volcanic outpourings, which cover parts of western Railroad Valley and the Pancake Range. Hydrocarbons in valley have either reached maturity or are currently reaching maturity. Current reservoir temperatures indicate that there are ongoing geothermal systems in the valley, some of which are associated with oil fields and petroleum production.

 Petroleum Production 

Shell first discovered oil in Railroad Valley in 1954 when they drilled on a seismic anomaly reportedly mapped in Miocene valley-fill deposits.

 Oil fields: 

Nine oil fields have been discovered throughout Railroad Valley, totaling 47,139,880 barrels of oil (BO) as of 2016. Although some wells are still active, most have reduced production. Some of the most notable wells include:

Eagles Springs- First discovered oil field in Railroad Valley by Shell in 1954. Approximately 5,615,504 BO cumulative from 20 produced wells. Production from Garrett Ranch, Sheep Pass Fm., and some production from Ely Limestone (Pennsylvanian).

Trap Springs- Discovered in 1976 as the second commercial oil field in Railroad Valley. Approximately 15,425,759 BO cumulative from 41 produced wells. Produced from Garrett Ranch.

 Grant Canyon-Upper reservoir discovered in 1983. Lower reservoir discovered in 1993. Approximately 21,407,314 BO cumulative from 6 produced wells. Production from Guilmette.

Bacon Flat- Discovered in 1981. Approximately 1,055,833 BO cumulative from 2 produced wells. Production from Guilmette.