San juan basin
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- 1 Introduction
- 2 History - Discovery and Hydrocarbon Plays
- 3 Geology and Petroleum Elements
- 4 Geologic Risks
- 5 Future Petroleum Potential
- 6 Petroleum & Facility Engineering Aspects
- 7 Reference List
The San Juan Basin is an approximately circular, asymmetric structural depression located located near the Four Corners of the United States in predominantly northwest New Mexico and southwest Colorado, with small remaining areas of the basin in Utah and Arizona. It covers an area of about 21,600 square miles and is home to the second largest gas basin in the United States. It is second to the larger Hugoton Field of Oklahoma, Kansas, and Texas.
The San Juan Basin is predominantly a natural gas production area from both conventional and unconventional tight sands, coal bed methane, and shale formations, although crude oil production in the region has been gaining momentum. Until the 1970s, most of the gas produced in the basin came from the three major fractured sandstone reservoirs: the Dakota Sandstone, the Mesaverde Group, and the Pictured Cliffs Sandstone Overall, there are currently more than 20,000 oil and gas wells in the San Juan Basin (including in the now popular reservoirs: Fruitland Formation and Mancos Shale)  .
History - Discovery and Hydrocarbon Plays
Although this basin can be found in four different states, the site of the first oil and gas discovery of the San Juan Basin was in New Mexico. The unexpected find happened in 1911 on the south flank of the basin in the Chaco Slope. While drilling a water well, Henry F. Brock accidently hit an oil reservoir in McKinley County, New Mexico. 
This discovery is credited as the location of the state of New Mexico’s first oil and gas discovery. Consequently, the state focused in on this area to explore and produce its petroleum resources. Unfortunately, none of the subsequent wells in this area produced more than a few barrels of oil and within twenty years, activity in this area had practically ceased. It has picked up again over time, but the history of this basin’s exploration and development has been a story of periodic stagnation due to unfavorable marketing and transportation conditions .
Until the 1970s, most of the gas produced in the basin came from the three major fractured sandstone reservoirs: the Dakota Sandstone, the Mesaverde Group, and the Pictured Cliffs Sandstone, all of Late Cretaceous age .
The San Juan Basin has also produced a total of 281 million barrels of oil (MBO) from a large number of relatively small oil fields. Nearly 175 MBO have come from the Tocito Sandstone Lentil of the Mancos Shale (sometimes called “Gallup Sandstone”). The two largest Tocito oil fields are the Bisti and Horseshoe fields that have produced about 42 MBO and 40 MBO, respectively .
Coal-bed Methane Plays
Starting in the late 1970s, and accelerating in the 1980s to the present, production from coal-bed methane reservoirs in the basin has gone from virtually none to around one trillion cubic feet of gas (TCFG) per year, making the San Juan Basin’s Fruitland field the largest coal bed methane field in the world. Fruitland Formation coal bed methane production is 15.7 TCFG from 7,700 wells .
Geology and Petroleum Elements
The reservoir rock mentioned in the gas, oil, and coal bed methane plays above consist of the following:
The Dakota Sandstone contains several sandstone beds separated by shales. It contains three principal lithologies: a sequence of buff to brown, cross bedded, poorly sorted, coarse-grained conglomeratic sandstone and moderately sorted, medium-grained sandstone in the lower part; dark-gray carbonaceous shale with brown siltstone and lenticular sandstone beds in the middle; and yellowish-tan, fine grained sandstone interbedded with gray shale in the upper part. The thickness of the Dakota Sandstone has an overall range of a few tens of feet to about 500 feet .
The Pictured Cliffs Sandstone is a regressive marine beach deposit. It generally consists of an upward-coarsening sequence of light-gray to yellowish-gray, thick- to very thick bedded, very fine to medium-grained, locally cross bedded and bioturbated sandstone. Thin interbeds of dark marine shale also are present, especially in the lower part of the formation. The thickness of this formation is variable .
In general, the Fruitland Formation consists of variable thicknesses of interbedded and repetitive sequences of non-marine channel sandstone, siltstone, shale, and claystone. Coal beds and carbonaceous shales are common .
The Mancos Shale comprises most of the Upper Cretaceous marine deposits. It was deposited in relatively shallow offshore marine environments, possibly no more than 400 feet deep. In general, the Mancos consists of gray to black shale and claystone with discontinuous yellowish-gray, calcareous siltstone and sandstone. Thin bentonite beds are common in the lower part of the formation. The Mancos also contains zones of calcareous concretions, some thin limestone beds, and offshore sandstone bar deposits .
Source Rock & Migration
Cretaceous rocks, beginning with deposition of the Dakota Sandstone, consist of wedges of marine-to-continental transgressive and regressive strata that occupy the broader San Juan Basin. The depositional history of the San Juan Basin is focused during the Late Cretaceous - an epoch in which at least 6,500 feet of sediments were deposited. The Cretaceous rocks were formed by transgressive and regressive marine shore-zone sediments that were deposited in shallow seas that encroached into the San Juan Basin area from the northeast, and also non-marine clastic deposits from source areas southwest of the basin .
Two different oils, “marine” and “non-marine,” are produced from Cretaceous age reservoirs. Most of the produced oils in the San Juan Basin belong to the “marine Cretaceous” group. These “marine Cretaceous” oils have been produced from reservoirs ranging from the Dakota Sandstone near the base of the Cretaceous section through reservoirs in the Farmington Sandstone Member of the Kirtland Shale above the Fruitland Formation. The “non-marine Cretaceous” oil is limited to the Dakota Sandstone in the extreme west side of the Mancos-Menefee Shales .
In the southern part of the San Juan Basin, traps in the Dakota, Mesaverde, Mancos and other formations are combined structural and stratigraphic traps. Discontinuous sandstones of marginal marine origin are juxtaposed with faults that not only compartmentalize the fields, but also may have served as pathways for local vertical migration of oil. Local porosity and permeability variations in the Dakota, were also considered to be important controls on trapping and retaining oil. Oil in Dakota reservoirs on the Four Corners platform and Chama platform is in anticlines, some of which may have associated faults. In these areas, the trapping mechanism is combined structural and stratigraphic .
Gas is trapped within the marginal marine sandstones and within fluvial sandstones where those units pinch out into paludal or marine shales, or overbank mudrocks, respectively .
Regional seals consist of marine shales (Mancos Shale below and Lewis Shale above). Locally, seals in Dakota reservoir may be a combination of shale facies into which marine and marginal marine sandstone facies pinch out and changes in permeability within the sandstone units. Seals “Gallup” and other Dakota reservoirs are a combination of interbedded shales and low permeability in the reservoirs. Additional seals are nearshore paludal shales and coal-bearing rocks in the Menefee and fluvial overbank mudrocks of the Menefee Formation .
In 2014, NASA and University of Michigan researchers reported the discovery of a methane cloud hovering over the Four Corners region of the United States. This cloud covered an area of about 2,500 square miles . Satellite data researched over a period of seven years (between 2003 and 2009) revealed the area released about 0.59 million metric tons of methane into the atmosphere . Eric Kort, the study's lead author explained that because the research study period was before the extensive use of fracture stimulation, known as fracking, he could almost confidently say the methane emissions should be attributed to leaks in natural gas production and processing equipment rather than the use of fracking in the area .
Future Petroleum Potential
In 2016, ConocoPhillips sold its stake in the region for $3 billion to Hilcorp San Juan LP. Two years later, Williams Partners LP sold its stake to Harvest Midstream for $1.123 billion. Possibly the biggest shift happened in August 2018 when BP American Production Co. announced its intentions to also pull out of the region . Big companies pulling out of older, more mature basins is not uncommon. In fact, it is a normal occurrence when newer more profitable regions appear. However, there are still companies producing in that area. This is in large part because in 2016, the United States Geological Survey (USGS) reported they increased the estimate of technically recoverable natural gas in the Mancos Shale deposit from 1.6 trillion cubic feet of natural gas to 66.3 trillion cubic feet . Experts also believe in the oil potential of Mancos Shale portion located in the southern end of the basin .
Petroleum & Facility Engineering Aspects
As mentioned above, this area has had multiple periods of low activity due to the age of discovery. With advances in horizontal drilling and hydraulic fracturing, companies started showing renewed interest in the basin. Fracking is the key to development and production in the San Juan Basin. The new technology made it possible to access resources that had in the past been uneconomical, or even impossible, to reach. In 2014 the federal agency started planning for how to develop roughly 2 million acres in the region using hydraulic fracturing .
- By NicholasGuiffre - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=53188768
- Whaley, J., 2017, Oil in the Heart of South America, https://www.geoexpro.com/articles/2017/10/oil-in-the-heart-of-south-america], accessed November 15, 2021.
- Wiens, F., 1995, Phanerozoic Tectonics and Sedimentation of The Chaco Basin, Paraguay. Its Hydrocarbon Potential: Geoconsultores, 2-27, accessed November 15, 2021; https://www.researchgate.net/publication/281348744_Phanerozoic_tectonics_and_sedimentation_in_the_Chaco_Basin_of_Paraguay_with_comments_on_hydrocarbon_potential
- Alfredo, Carlos, and Clebsch Kuhn. “The Geological Evolution of the Paraguayan Chaco.” TTU DSpace Home. Texas Tech University, August 1, 1991. https://ttu-ir.tdl.org/handle/2346/9214?show=full.