Eastern great basin

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Figure 1. Location of Eastern Great Basin in western Utah and eastern Nevada.

Introduction

[1]The Great Basin covers much of the western portion of the United States of America including most of Nevada, Utah, and Oregon, and parts of California, Idaho, Wyoming, and Baja California, Mexico. The Great Basin, also called the Great Basin Desert, is a very large area of land that gets very little rainfall and has its own internal draining system where no water within it reaches an ocean.

The Eastern Great Basin is the portion of the Great Basin that covers land in eastern Nevada, western Utah, and southeastern Idaho (Figure 1). The Eastern portion of the basin is unique because it has a more moist climate due to the rain shadow of the Sierra Nevada in the western portion of the basin.[1] Much of the Eastern Great Basin’s production comes from the Upper Devonian- Lower Mississippian Pilot Shale and the Mississippian Chainman Formation[2] (Figure 2).

Geologic History

[3]The Great Basin is composed of quartzite, shale, limestone, and a small portion of granite. Most of the rock in this region was formed during the Cambrian period. The Sevier Orogeny episode, an episode of mountain building, thrusted great layers of rock onto one another, doubling the thickness of the continental crust. (Figure 3)

Figure 2. Pilot Shale AU shown in red. Chainman Shale AU shown in blue.

[3]However, the current basin topography formed in the last thirty million years. During this time, there has been thinning and stretching to the crust, causing it to separate into blocks. During the Tertiary period, volcanic rocks formed, but have more recently been cut and displaced by the block faults. Fresh surfaces near the faults, earthquakes, and micro earthquakes indicate recent movement in faulting.

[1]The Great Basin is characterized by valleys, which are mostly desert areas, that have floors ranging from 1,000 to 6,000 feet above sea level, and mountains with peaks of 9,000 feet or greater. These features were formed by the large blocks of the Earth’s crust which have been uplifted, sunk, and tilted. The blocks of crust sliding along the cracks and faults turned into the valleys while the uplifted parts remained intact and constitute the mountains there today.

Petroleum History

[4]Preliminary exploration of the Eastern Great Basin began in the late 1940’s and 1950’s when various deep wells were drilled to test Paleozoic rocks. In 1954, the first producing well was drilled by Shell Oil Co. yielding high pour-point waxy oil from the Tertiary volcanics. This was named the Eagle Springs Discovery and provoked much more exploration in the Eastern Great Basin, but most of this activity was unsuccessful in producing petroleum.

Figure 3. Sevier Orogenic Belt forming. Image from Lucky Sci.

[4]Exploration in the area increased again in the 1970’s following the oil embargo. This led to numerous small field discoveries in Railroad Valley and Elko, NV. The Tertiary basins were the primary producer for the ten oil fields contained in the Eastern Great Basin in 1983.

[4]Common Characteristics of these Oil Fields:

→Traps associated with a Tertiary unconformity

→Thick oil column in reservoirs

→Reservoir quality enhanced by fractures

→Subsurface oil and gas and seeps

Major Plays

[4]The two predominant plays identified here are the unconformity play, the leading play, and the upper Paleozoic play. Later came the high potential pre-Devonian play.

Antler Orogenic.png

[4]Unconformity Play

  • Play Boundaries: The unconformity play contains land of approximately 35,000 square miles. This is located east of the Antler orogenic belt. (Figure 4) Part of the region contains conodont alteration index work that shows upper Paleozoic rocks may not have been subjected to excessive thermal effects.
  • Reservoirs: Beneath the unconformity are fractures Paleozoic reservoirs and lacustrine sandstone, siltstone, and carbonate beds. These reservoirs are enhanced by fracturing, but matrix porosity in the carbonate and sandstone beds can be high. Good porosity and permeability may also be present locally in Tertiary volcanic rocks.
  • Source Rocks: Common source rocks found here are lacustrine oil shale or bituminous lacustrine shale and carbonate, and middle to upper Paleozoic marine organic-rich shale in unconformity or fracture communication with overlying reservoirs. The tertiary potential source rocks are mature in areas of higher heat flow, but immature in some other areas.
  • Traps and Seals: Folds, faulted folds, and buried hills beneath valley fill, sealed by valley-fill or volcanic beds or against faults.
  • Migration: Devonian and Mississippian source rocks likely reached oil stage by Permian or Triassic time. Stratigraphic and structural traps were continuing to form after Devonian time. Regional uplift and erosion of the eastern Great Basin region destroyed many traps during the Mesozoic period, but enhanced some other traps. Regional volcanism created additional seals, and the increased heat flow possibly matured Tertiary source beds.

[4]Upper-Paleozoic Play

  • Play Boundaries: rocks The upper Paleozoic play contains land of approximately 55,000 square miles. This play is located in central and east-central Nevada and west-central Utah. Upper Paleozoic rocks are buried at shallower depths here. The play is defined as an intra-upper Paleozoic play where reservoirs may be confined by interbedded shaly seals independent of the Tertiary unconformity trapping mechanism. This play could be divided by area and stratigraphically into sub-plays. The rock types are almost all marine and have good potential reservoirs and source rocks. However, these rocks were exposed or removed in almost all of the basin due to erosion. Mesozoic and Tertiary tectonic activity as well as high thermal effects in the area destroyed or remigrated most of the early petroleum accumulations. Adequate seals should be present in most valleys, but the deterrent is the lack of evaporites as an effective seal.
  • Reservoirs: Good potential reservoirs are present in porous dolomite and dolomitized limestones in all parts of the section.  Porous marine quartz sandstones are present in other areas. Most reservoirs in the area are greatly enhanced by fracturing.
  • Source Rocks: Black to dark-gray, organic-rich shales are interbedded with carbonates and sandstones. Major units of good source rock quality include the Upper Devonian-Lower Mississippian Pilot Shale and the Mississippian Chainman Shale. Source rocks are probably thermally altered to the post-maturity stage in much of the region, although there are large areas where the rocks are still in the oil window.
  • Traps and Seals: Folds, fault blocks, and fault zones are potential traps. The folds are mostly faulted and the fault blocks are sealed by upper Paleozoic shales. Sandstone stratigraphic traps are common in the belt of facies change between the Diamond Peak Sandstone and Chainman Shale. Porosity Change and organic carbonate buildup traps are present, but may be poorly sealed and possibly damaged by tectonic activity.
  • Migration: Migration here is similar to that of the unconformity play.

[4]Pre-Devonian Play

  • Reservoirs: Porous dolomite and dolomitized limestones are present in many areas. In other highly fractured areas, fractured reservoirs exist on rare occurrences.
  • Source Rocks: Dark marine shales are interbedded with carbonate rocks, but these beds have been buried 20,000 feet or more deep.
  • Traps and Seals: Upper Paleozoic shale beds on fractured and faulted structures are potential seals.
  • Migration: Hydrocarbons were generated and trapped as early as Paleozoic time, but have been mostly destroyed by tectonism, igneous activity, or buried to great depths.

Geologic Uncertainties

[4]The Eastern Great Basin has many uncertainties and negative factors to consider. The rigid tectonic history of the area led to disturbance in the Paleozoic section, fracturing and faulting, and uplift, which all have unfavorable effects on the shale seals and petroleum accumulations. Many potential source rocks were over-cooked from the thermal history of the region. Excessive burial depths caused over-maturity of Paleozoic source rocks. Insufficient burial depths in other parts of the basin caused immaturity of the younger potential source rock. Lastly, there is a lack of evaporite seals in the reservoirs.

Future Assessment

[2]The United States Geological Survey estimates 534 MMBO, 156 BCFG, and 2 MMBNGL remain to be undiscovered in the Eastern Great Basin. Of the total 534 MMBO to be discovered, 46% is predicted to be located in the Pilot Shale Oil AU, and 54% is predicted to be located in the Chainman Shale Oil AU (Figure 2).

Sources

  1. Britannica, The Editors of Encyclopaedia. "Great Basin". Encyclopedia Britannica, 9 Dec. 2019, https://www.britannica.com/place/Great-Basin. Accessed 6 May 2021.[1]
  2. Schenk, C.J., Mercier, T.J., Woodall, C.A., Finn, T.M., Gaswirth, S.B., Marra, K.R., Le, P.A., Brownfield, M.E., Leathers-Miller, H.M., Drake, R.M., II, and Kinney, S.A., 2019, Assessment of continuous oil resources in the Eastern Great Basin Province of Nevada, Utah, and Idaho, 2018: U.S. Geological Survey Fact Sheet 2019–3002, 2 p., https://doi.org/10.3133/fs20193002.[2]
  3. Van Hoesen, John. "Great Basin." National Park Service, U.S. Department of the Interior. 2001[3], https://www.nps.gov/grba/planyourvisit/upload/Geology.pdf.
  4. Peterson, James A. "Eastern Great Basin and Snake River Downwarp, Geology and Petroleum Resources." Department of the Interior, U.S. Geological Survey. https://pubs.usgs.gov/of/1988/0450h/report.pdf.[4]
  5. Assessment of undiscovered oil and gas resources of the Eastern Great Basin Province, 2005; 2005; FS; 2005-3053; Geological Survey (U.S.),[5] https://pubs.usgs.gov/fs/2005/3053/.
  6. Cortney. "How the west was made: western North American orogenies." LuckySci, 2015, http://www.luckysci.com/2015/10/how-the-west-was-made-western-north-american-orogenies/.[6]
  7. Armstrong, R. (1972). Low-Angle (Denudation) Faults, Hinterland of the Sevier Orogenic Belt, Eastern Nevada and Western Utah. Geological Society of America Bulletin, 83, 1729-1754.[7]
  1. 1.0 1.1 1.2 1.3 Britannica, The Editors of Encyclopaedia. "Great Basin". Encyclopedia Britannica, 9 Dec. 2019, https://www.britannica.com/place/Great-Basin. Accessed 6 May 2021.
  2. 2.0 2.1 2.2 Schenk, C.J., Mercier, T.J., Woodall, C.A., Finn, T.M., Gaswirth, S.B., Marra, K.R., Le, P.A., Brownfield, M.E., Leathers-Miller, H.M., Drake, R.M., II, and Kinney, S.A., 2019, Assessment of continuous oil resources in the Eastern Great Basin Province of Nevada, Utah, and Idaho, 2018: U.S. Geological Survey Fact Sheet 2019–3002, 2 p., https://doi.org/10.3133/fs20193002.
  3. 3.0 3.1 3.2 Van Hoesen, John. "Great Basin." National Park Service, U.S. Department of the Interior. 2001, https://www.nps.gov/grba/planyourvisit/upload/Geology.pdf.
  4. 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 Peterson, James A. "Eastern Great Basin and Snake River Downwarp, Geology and Petroleum Resources." Department of the Interior, U.S. Geological Survey. https://pubs.usgs.gov/of/1988/0450h/report.pdf.
  5. Assessment of undiscovered oil and gas resources of the Eastern Great Basin Province, 2005; 2005; FS; 2005-3053; Geological Survey (U.S.), https://pubs.usgs.gov/fs/2005/3053/.
  6. Cortney. "How the west was made: western North American orogenies." LuckySci, 2015, http://www.luckysci.com/2015/10/how-the-west-was-made-western-north-american-orogenies/.
  7. Armstrong, R. (1972). Low-Angle (Denudation) Faults, Hinterland of the Sevier Orogenic Belt, Eastern Nevada and Western Utah. Geological Society of America Bulletin, 83, 1729-1754.