Subsurface Methane West US (NT)
NATURAL SUBSURFACE METHANE IN WASHINGTON, OREGON, CALIFORNIA, IDAHO, NEVADA, UTAH, AND ARIZONA
(Non-Technical -- For technical version with references click here)
(To see a web page on Subsurface Methane in New Mexico, click here)
By Darrel Dunn, Ph.D., PG (Consulting hydrogeologist. Professional Synopsis)
Methane is widespread beneath the Earth's Surface. Methane gas is found in commercial quantities in oil and gas fields, and in sub-commercial quantities in oil and gas exploration test wells. It is found in coal beds and produced commercially in some places as coalbed methane. It is found in the subsurface associated with other organic rich rocks and sediments, especially gray and black shale and mudstone. Methane gas has been encountered in wells being constructed for water supply, and methane is dissolved in the groundwater from some water supply wells. It is found seeping from the ground into the atmosphere, streams, lakes, and oceans. In the following sections, I will describe natural occurrences in selected contiguous western states. These occurrences illustrate the ubiquitousness of methane, natural sources of methane, subsurface movement, and chemical and biological reactions.
Background -- Methane, Stable Isotopes
Methane is the simplest molecule that contains only carbon and hydrogen (CH4). It is usually generated by the breakdown of more complex molecules containing carbon and hydrogen (organic compounds). The organic compounds may be plant or animal debris, or organic debris that has been altered by heat during deep burial (kerogen, coal). Kerogen is organic material that gives the gray or black color to some shale and mudstone. Therefore, gray or black shale and coal may yield methane, which can migrate in the subsurface as a gas or dissolved in water that moves through the pores and fractures of rocks and sediments. The gas may accumulate where it is trapped beneath dense material (like clay or shale) that it cannot enter, or it may migrate to the Earth's surface and be discharged to the atmosphere. Dissolved methane may also migrate with the water that contains it and eventually be released from the water and trapped in the subsurface or discharged to the atmosphere.
Both the carbon and the hydrogen in methane contain stable isotopes that yield information on the source of the methane. Ordinary carbon has twelve neutrons in its nucleus (12C) but some of the carbon atoms in methane contain thirteen neutrons (13C) and are therefore heavier. Likewise, ordinary hydrogen has no neutron in its nucleus (just a proton), but some of the hydrogen atoms in methane contain one neutron. This heavier hydrogen is called deuterium (D). The ratio of 13C to 12C is expressed with respect to a standard and the result is symbolized in this document as d13C. The deuterium ratio is also related to a standard and the result is symbolized in this document as dD. For methane originating in the subsurface, these two stable isotope ratios are always negative.
The significance of these stable isotopes is that the molecules of methane that contain the heavier isotopes behave slightly differently in chemical and biological reactions than ordinary molecules that do not contain the isotopes. When the complex organic substances in kerogen and coal are modified by heat to less complex substances (still forming kerogen and coal) methane is emitted. Methane is formed in this way primarily at temperatures exceeding 70oC. It is called thermogenic methane. Typically d13C for thermogenic methane is greater than about -55 per mil (parts per thousand), although some investigators have offered slightly different guidelines. Methane originating in the subsurface at temperatures less than about 70oC is produced primarily by microbes. There are two types of microbial methane generation. One type is fermentation where microbes break down complex organic substances, with one product being methane. The other type is microbes causing carbon in substances dissolved in water (such as carbon dioxide, CO2) to react with hydrogen in the water to form methane. These two types of microbial methane tend to have different dD ratios. The fermentation methane tends to have dD less than about -260 per mil, and the methane produced by reaction of hydrogen with CO2 and other simple carbon substances tends to have dD greater than about -260. Consequently, if d13C is plotted against dD for methane samples derived from the subsurface, there are three fields on the graph corresponding to the different types of methane generation. Furthermore, methane associated with geothermal sources is thought to tend to have d13C ratios greater than thermogenic methane. The graph of these stable isotope tendencies that seems to be cited most oftern is one by Michael J. Whiticar. In the following sections I refer methane isotope results to Whiticar's graph as an indication of possible origin. Fields on this graph are labeled "Thermogenic," "Bacterial Methyl-type Fermentation," "Bacterial Carbonate Reduction," and "Geothermal, Hydrothermal, Crystalline," The area between these fields is labeled "mix and transition." Whiticar points out that this diagram does not unambiguously define the origin of a methane sample. However, there are statements in the scientific literature such as : "the isotopic composition of methane provides a reliable 'fingerprint' for distinguishing gases from different sources." The use of the term "fingerprint" can be misleading, because it is associated with human fingerprints which, unlike methane "fingerprints," are unique to a particular source (person).
In the State of Washington, as elsewhere, methane is widespread in the subsurface. However, it has not been found in large commercial accumulations of natural gas. One small abandoned gas field is noteworthy because the gas was produced from basalt, which is an unusual occurrence.
Natural gas fields
Methane is the main component of combustible natural gas. In the state of Washington, methane has been produced commercially from three small gas fields. None were active in 2010.
- The Bellingham gas field is located about five miles northwest of the town of Bellingham near the Canadian border north of Seattle. The gas was produced at depths of less than 500 feet in sand that was deposited by a glacier during the ice age. The glacial deposits are underlain by sandstone, shale, and coal. The glacial deposits may contain organic material. The shale, coal, and organic material are all possible sources of the methane in the natural gas.
- Commercial gas has been produced from porous basalt north of Richland at depths of 700 to 1,200 feet. The basalt is composed of layers formed by extensive prehistoric lava flows, and is about 12,000 feet thick in this area. The thick basalt is cut by nearly vertical faults (continuous breaks in the rocks where the rocks on one side have moved relative to the other side) and may be underlain by sandstone and shale containing coal beds that could be a source for the gas via migration upward along the faults. Another possible source is organic material in thick layers of sediments that occur some places between the layers of basalt. It is unlikely that the methane was present in the molten lava when it was deposited, because methane decomposes and reacts to produce other substances at the internal temperature of basaltic lava (1,000oC to 1,200oC) when the pressure is near atmospheric. It is noteworthy that methane gas occurs near the edges of active lava flows. It is generated when vegetation is covered and heated by the molten lava, and methane gas explosions have occurred near the leading edges of flows. Baked soil horizons that are present between the basalt layers probably contained some organic matter that might have yielded methane through pyrolysis (thermal decomposition of organic matter in the absence of oxygen). However, I have not found any report of methane formed by lava flowing over vegetation and soils being preserved in underlying porous basalt. I suspect it either explodes or escapes to the atmosphere.
Minor amounts of gas have been encountered in petroleum test wells elsewhere in the state.
Methane is associated with coal beds located in the Pacific Coal Region west of the Cascade Range from the Canadian border to the Columbia River and in a small area east of the Cascade drainage divide near Roslyn. Methane is also associated with coal beds in the Central Coal Region buried beneath the thick basalt in eastern Washington. I do not know of any producing coalbed methane fields in Washington. Testing in the Pacific Coal Region has shown coalbed gas content ranging up to at least 86 standard cubic feet per ton of coal. In the United States, the preferred amount of methane is 300 cubic feet per ton or more; but if production costs are low and the resource is great, 30 to 70 cubic feet per ton may be commercial.
Methane associated with groundwater
Some water supply wells and irrigation wells in the aforementioned basalt contain methane gas. Some water supply wells in various types of rock in the Pacific Coal Region also contain methane gas. A man digging a water well in the northern part of the Pacific Coal region caused an explosion while digging a water well in 1893.
Test wells at a site on the Hanford Reservation near Richland yielded groundwater with dissolved methane values greater than 500 mg/L (milligrams per liter). This water was from basalt (like that at Rattlesnake Hills) at a depth of about 3000 feet. The dissolved methane is in a plume that extends in the direction of groundwater movement from a fault. This occurrence is another indication of transport of methane upward along a fault from source beds in or below the thick basalts.
Methane in gas seeps
Natural gas seeps occur on the west side of the Olympic Peninsula (west of Seattle) where there are outcrops of sandy shale.
The occurrence of subsurface methane in Oregon is similar to that in Washington because similar geologic conditions are present.
Natural gas fields
Oregon has one active commercial gas field. It is located about 60 miles northwest of Portland near the town of Mist. The gas is found in the sandstone in subsurface layers of sandstone, siltstone and mudstone. These layers are overlain by thick shale. The field consists of individual gas pools ranging in size from 20 acres to 120 acres at depths of 1,200 to 2,700 feet. Only a few wells were still producing as of 2003. The d13C values given for methane from three of the pools in the field are -43.8. -43.6, and -42.5 per mil. These values are within the thermogenic range. Shale that may have potential as a hydrocarbon source rock occurs at depths exceeding 8,000 feet in the region. At this depth, the temperature would exceed 70oC. So it is possible that thermogenic gas migrated from this shale.
Coalbed methane is being developed in coastal southwestern Oregon. The status in 2010 appeared to be exploration, development, and permitting, but no commercial production. The coal beds are 2,000 to 4,000 feet below the ground surface.
Methane associated with groundwater
Naturally occurring methane gas has been reported in water wells in Oregon:
- A water well located in the Coast Range about ten miles northwest of Corvallis. This well was completed in siltstone containing organic debris that could be the source of the methane.
- Water wells in the Pine area of northeastern Oregon.
- Water wells south of Bend in sediments containing woody organic material.
Methane gas seeps
Methane gas flows from the sea floor at the submarine Hydrate Ridge about sixty miles off the central coast of Oregon. It has been suggested that this methane has been transported as dissolved methane in groundwater from a source over thirty miles east of the submarine seeps.
Methane gas also bubbles from the ocean floor elsewhere off the Oregon Coast. One such seep is at Coquille Bank located about 17 miles southwest of Coos Bay. The carbon isotope ratio of the methane from this seep is d13D = -28.7 per mil, consistent with a thermogenic origin. A shallow gas well located on land in a gas vent area near the shore about 13 miles north-northeast of the seep produces gas with nearly identical carbon isotope ratio, d13C = -28.4 per mil. Other submarine methane seeps in this area also have heavy carbon isotopic compositions. I do not have sufficient information on off-shore geology along the coast of Oregon to assess the origin of the methane in the off-shore seeps.
The geology of California is complex, and considerable information on methane occurrence is available.
Natural gas fields
California has many commercial gas fields. Most of this gas occurs in sandstone associated with dark-colored mudstone and shale.
Gas fields in the Central Valley Trough. Much of the commercial gas has been produced from the Central Valley Trough, a geologic province between the Sierra Nevada Mountains and the Coast Ranges. Within the Central Valley Trough, gas wells have ranged from less than 500 feet deep (supplied gas to a ranch cook-house in the early 1900's) to greater than 12,500 feet deep.
Most of the gas fields are in the northern part of the trough, and the largest is the Rio Vista Gas Field. It produces gas from depths of 1,250 feet to 11,000 feet, and is now largely depleted. The source of methane in the northern Central Valley Trough is probably deeply buried shale. Of 58 pairs of isotope values given by the USGS (United States Geological Survey) for the northern part of the trough, d13C ranges from about -53 per mil to -24 per mil, and dD ranges from about -201 to -100 per mil. Most of the values plot in the thermogenic range of Whiticar. However, three pairs plot as "geothermal/hydrothermal/crystalline" and six plot as "mix & transition."
The southern part of the Central Valley Trough contains many gas fields and fields that have produced gas associated with oil. Some isotope data is available for methane from the fields. Of 23 pairs of carbon and hydrogen isotope values given by the USGS and by others, d13C ranges from -71 per mil to -24 per mil, and dD ranges from -255 to -130 per mil. Five of these data pairs plot as "geothermal/hydrothermal/crystalline" on the graph of Whiticar; two plot as "bacterial carbonate reduction," and six plot as "mix & transition." The remaining 10 plot as thermogenic. Microbial gas can be formed at temperatures less than about 70oC, which would correspond to depths of about 4700 feet to 5200 feet in this southern part of the trough. Some of the methane produced in this area is from depths less than 4700 feet (e.g. Trico Gas Field). The source for the thermogenic gas is likely to be deeply buried shale. Shale occurs at depths as great as 20,000 feet in this southern part of the trough. An additional 70 d13 values are available (without associated dD values). They range from -67.99 to -24.19 per mil. Thirteen of these d13C values are less than -55 per mil, which suggests a microbial origin.
Gas fields in the Southern California Coastal Basins. In addition to the Central Valley Trough, some commercial gas is produced from folded and faulted sedimentary basins located along the southern California coast and extending beneath the Pacific Ocean (rock layers are deeper in the central part of a basin than near the edges). Methane from oil and gas wells in the Southern California Coastal Basins has d13C values ranging from -62 to -34 per mil and dD values ranging from -295 to -140 per mil. Out of 154 pairs of values given by the USGS, only eleven are outside the thermogenic range of Whiticar. Of these eleven, three plot as "geothermal/hydrothermal/crystalline," one as "bacterial methyl-type fermentation," and seven as "mix & transition."
California has no coalbed methane production, although coal is present. Coal was produced from mines on the eastern slope of the Coast Ranges about 12 miles southeast of Livermore in the last half of the 1800's and early 1900's and also about 20 miles north of Livermore near Nortonville. Much farther south, there was some meager output near Coalinga. Non-commercial coal seams have been reported in Shasta County at the north end of the Sacramento Valley, and in the Eel River area near the northeast coast of California. I found no information on the methane associated with California coal.
Methane associated with fresh groundwater
Naturally occurring methane gas has been reported in water wells in California:
- Water wells located near the coast about ten miles northwest of Santa Cruz produce substantial amounts of methane. One well produces as much as 200 Mcf (thousand cubic feet) per day. This gas may originate in the organic rich shale that is present in the area.
- A water well was drilled in the City of Stockton between 1854 and 1858 to a depth of 1,002 feet, and natural gas was produced with the water. The gas was burned at the Stockton courthouse for many years. About this time many water wells were furnishing gas to farm houses as far north as Tehama County and to several communities in the lower reaches of the Sacramento River valley.
Naturally occurring dissolved methane has been reported in California groundwater:
- Dissolved methane was measured by the USGS for water wells in valleys north of San Francisco Bay in 2004. Three wells in the Sonoma Valley yielded values of 0.02, 0.03, and 0.35 mg/L. One well in the Napa Valley yielded a value of 0.57 mg/L. There is no commercial oil or gas production in this area.
- The USGS has measured dissolved methane in groundwater monitoring wells in Los Angeles County in the vicinity of large groundwater recharge basins for tertiary-treated municipal wastewater. They found concentrations from 0.3 micrograms/L to 19.3 micrograms/L in groundwater outside of the recycled water plume. Dissolved methane concentrations within the plume were below the analytical detection limit. There are oil fields in Los Angeles County.
Methane in gas seeps
There are many gas seeps in California. Much information on some of these seeps can be found in the USGS website titled "Natural Oil and Gas Seeps in California." A map on this web site shows more than seventy gas seeps. Most of the gas seeps on this map are not in an oil or gas field or immediately adjacent to one. The seeps are mostly in the northern Coast Ranges. A cluster of gas seeps is present in the Eel River Geologic Basin that extends onshore southeast of Eureka near the northwestern corner of California and offshore to the northwest of Eureka. Another cluster is just south of the Eel River basin extending from the town of Petrolia southeastward up the Mattole River. A few seeps are present in the Central Valley Trough, and much gas seeps to the surface in the Southern California Coastal Basins.
Gas seeps in the Eel River Basin. The gas seeps in the Eel River Basin are located both offshore and onshore. Rock near the land surface in the Eel River basin is conglomerate, sandstone, and claystone. Potential methane sources include complexly faulted and folded, clayey, silty and sandy rocks that extend to great depth and are sometimes called the Franciscan Complex. The Franciscan Complex contains some dark-gray shale and mudstone which has probably been at depths greater than 20,000 feet where heat was sufficient to produce thermogenic methane. Two offshore gas seeps produce methane with d13C values of -49 and -43 per mil and dD values of -199 and -180 per mil, respectively. An onshore gas seep near the Tompkins Hill gas field produces gas with d13C values ranging from -31 to -35 and dD values from -155 to -127 per mil. All of these stable isotope values are in the thermogenic range according to the isotope signatures diagram of Whiticar, although the offshore seeps may contain admixed microbial methane.
Gas seeps in the Mattole River area. The gas seeps in the Mattole River area just south of the Eel River Basin discharge from Franciscan Complex bedrock, and the Franciscan Complex is a likely source. "Petroleum springs" along the Mattole River were mentioned in local newspapers as early as 1859. I found stable isotope values for only one seep. These are d13C of -39 and dD of -155 per mil and plot in the thermogenic range. This seep is located about five miles north of the abandoned Petrolia Oil Field. Gas from the Petrolia Oil Field has d13C values ranging from about -55 to -35 per mil and dD values from -175 to -151 per mil; the values are in the main thermogenic range except -55 for d13C, which is marginal. One pair of isotope values from the Petrolia Field is virtually the same as the pair from the seep (d13C = -38, dD = -155). The seep is five miles from the Petrolia Field, which produces from rocks less than 1,700 feet deep.
Gas seeps in the Northern Coast Ranges. The gas seeps in the Northern Coast Ranges (north of San Francisco) discharge from bedrock composed of Franciscan Complex and the Great Valley Sequence. The Great Valley Sequence is composed of layers of sandstone and shale up to 43,000 feet thick. It is located along the eastern edge of the Coast Ranges and extends beneath the Central Valley. It is separated from the Franciscan Complex by a fault that follows the eastern edge of the Coast Range. The shale is dark gray to black. The Franciscan Complex and the Great Valley Sequence have both been buried deeply enough for their dark shale and mudstone to generate methane. The six sets of methane isotope values reported for the Northern Coast Ranges are from springs and a fumerole. The d13C values range from -41 to -26 per mil, and the dD values rage from -180 to -140 per mil. Two seeps in the Wilbur Springs hot springs area have methane isotope values that fall near the boundary between the "geothermal/hydrothermal/crystalline" and the "thermogenic" ranges of Whiticar. Wilbur Springs is on a fault near the eastern edge of the Coast Ranges. Geysers Fumerole is in the Geysers Steam Field about 15 miles north of Healdsburg. The d13C (-33 per mil) and dD (-170 per mil) values shown on the aforementioned USGS website plot in the "geothermal/hydrothermal/crystalline" range of Whiticar. However, Lowenstern and Janik (49) cite d13C values as low as -40 per mil and provide evidence that most of the methane from wells in the steam field is likely derived from complex hydrocarbons. From information in this paper, one might infer that the complex hydrocarbons are organic material in the Franciscan Complex. The steam in the field is likely dominantly superheated groundwater that has circulated down through the Franciscan Complex and been affected by heat from igneous intrusions that rise to about 23,000 feet below the ground surface. There are faults in the field that might be associated with fumeroles that predate the commercial development of the steam field. The remaining three seeps that have been sampled for methane isotopes in the Northern Coast Ranges are all hot springs. They have methane stable isotope values that fall within the thermogenic range.
Gas seeps in the Southern California Coastal Basins. The gas seeps in the Southern California Coastal Basins occur both offshore and onshore. These basins contain thick sequences of organic rich mudstone, shale, siltstone, sandstone, and conglomerate. These rocks are mostly covered with sandy and muddy sediment in the marine areas, and partly covered with sand and gravel onshore. One of the world's largest natural oil seep areas is in one of these basins. It is in the Santa Barbara Channel offshore from the city of Santa Barbara. Reports of tar originating from this seep area date back to 1542. A very large flow of gas is associated with these seeps. It is about two-thirds methane, and the remaining third is other combustible hydrocarbons. The large proportion of methane suggests a source rock temperature greater than 140oC, which would occur at a depth of about 16,000 feet. Mudstone and shale present below this depth contains much organic material. Gas is associated with oil in seeps offshore of Redondo Beach at Los Angeles. Several oil and gas seeps are aligned along a fault that extends into the ocean from onshore. They are two to eight miles from the shore, and produce streams of gas bubbles extending to the ocean surface. Submarine seepage is affected by earthquake activity. Shortly after an earthquake in 1979, gas bubbles began appearing at the ocean surface off Malibu Pier west of Los Angeles. The bubbling continued for about six days after the earthquake. The famous La Brea Tar Pits in Los Angeles emit some methane which has been reported to be produced by microbes that cosume petroleum and release methane. The USGS provided carbon isotope data on four methane samples from the La Brea Tar Pits: d13C values range from -45 to -41 and dD ranges from -205 to -175. These values are within the thermogenic range of Whiticar. I have not determined whether microbes acting on tar can produce methane with stable isotope ratios in the thermogenic range.
Gas seeps in the Transverse Ranges. Arrowhead Hot Springs in the San Bernardino Mountains, six miles northeast of San Bernardino, emit bubbles of methane gas. The springs are located on a splay of the San Andreas Fault. They issue from granite and other rocks that contain no organic material. It has been proposed that these rocks have been thrust a long distance over rocks containing shale or other possible sources of methane, so that an organic source is present at depth.
Gas seeps in the Central Valley Trough. The USGS does not report any gas seeps in the northern part of the Central Valley Trough, even though many gas fields are present there. The USGS reports five gas seeps in the southern part of the trough, three near the south end. No isotope data is given for any of these five seeps. The three seeps near the south end of the trough are co-located with oil fields. These are shallow oil fields that do not contain commercial quantities of gas. Potential source rocks for the gas from these southern seeps include mudstone and shale that are deep enough to provide thermogenic methane.
California Methane Stable Isotope Data Analysis
Sufficient data in stable carbon and hydrogen isotopes are available to allow statistical analysis.
Synoptic Analysis of XY Graph
Figure 1 is a graph of all of the pairs of d13C and dD values from California methane samples reported above. Values from wells and seeps in the various geological provenances are shown in relation to the ranges of vales in the isotope signatures diagram of Whiticar (44). This XY plot illustrates the following:
- A large proportion of the values are in the thermogenic range.
- Many of the values are in the Geothermal range ("geothermal/hydrothermal/crystalline" of Whiticar).
- Very few values (3) are in the Microbial Reduction range ("bacterial carbonate reduction" of Whiticar).
- No values plot in the Microbial Fermentation range ("bacterial methyl-type fermentation" of Whiticar).
- Several values plot in the area between Thermogenic and Microbial Reduction ("Mix & Transition" of Whticar).
- The ranges of the values from the geologic provenances overlap.
- Some pairs of values from distinct and different provenances are virtually the same. (There are seven pairs from different provenances that I read as exactly the same from the USGS graphs.)
- The dD values for samples from the South California Coastal Basin wells and seeps tend to be less than those from Sacramento Basin wells, with dD values from the other provenances being medial. (Statistical calculations described in technical page show that the stable isotope values from Sacramento Basin Wells, San Joaquin Basin Wells, and South California Coastal Basins Wells probably all come from different underlying distributions of the stable isotope values.)
- There is a positive correlation between dD values and d13C values.
Figure 1. Plot of California Methane d13C and dD pairs.
Idaho has a couple of recently discovered natural gas fields, no coalbed methane production, and relatively little data on methane in potable groundwater and gas seeps.
Natural gas fields
In the state of Idaho, methane had not been produced commercially as of November, 2011. However, two gas fields (Willow and Hamilton fields) have been discovered in the western Snake River Plain near New Plymouth, about 30 miles northwest of Boise. These fields must be connected to a pipeline that is located about 10 miles away before the gas can be sold. The gas is from sand layers at depths of 1800 to 2500 feet below the land surface. The sand layers are associated with swamp deposits including beds of vegetation in various stages of decomposition which yield methane. These sedimentary deposits are underlain by volcanics, and granite is probably beneath the volcanics. Neither volcanic rock or granite is likely to be a major source of methane.
I know of no producing coalbed methane fields in Idaho. There are steeply dipping sub-bituminous coal beds in southeastern Idaho. There are also lignite beds interbedded with lacustrne, fluvial, and volcanic ash deposits near Oakley southeast of Idaho Falls; and lignite in similar sedimentary deposts interbedded with basalt near Orofino, northeast of Lewiston.
Methane associated with fresh groundwater
Methane is found in water wells throughout the western Snake River Plain, which is centered on Boise. Organic material (such as woody material) in the aquifer is probably the source.
Methane in gas seeps
I found no reports of gas seeps in Idaho. However, dissolved methane is reported in water sampled at a depth of 656 feet at Lidy Hot Springs, which are located on the north edge of the Snake River Plain about 15 miles west of Dubois. The methane concentration was 2.0 mg/L in water at 137 oF. The microbial population of this water consisted largely of a type of microbe that might be able to produce methane from hydrogen and carbon compounds in the water. No isotopic analysis was performed to support a microbial origin. An alternative source of the methane might be thermal decomposition of organic material in dark shale thought to be present at a depth of about 5000 feet. The Lidy Hot Springs are in a faulted area, and the hot water is moving from depth to the surface along faults.
Natural Gas Fields
The only natural gas production in Nevada has been the Kate Spring Field about 12 miles south-southwest of Currant on the east edge of Railroad Valley. The gas is associated with oil produced there from rocks about 4,500 feet below the land surface. The gas is not actively marketed and is used to operate the oil production equipment on the site. The gas probably comes from dark colored shale that is present in the central part of the valley at depths greater than 9,000 feet, which is deep enough for the formation of thermogenic methane.
Nevada has no commercial production of coalbed methane. However, coal beds are present in the state. Coal has been mined at Pancake Summit about 20 miles southeast of Eureka. A coal bed about two feet thick was mined and coal was shipped to lead smelters in Eureka prior to 1905. Coal is also reported in a railroad cut about 4 1/2 miles east of Carlin; it has not been commercially mined. Lignite (soft coal) is present in thin beds near Elko, and other lignite is exposed at the land surface in the western part of the state. A couple of these beds were mined in the 1800's and early 1900's.
Methane associated with fresh groundwater
Naturally occurring methane gas has been reported in water wells in Nevada:
- Some water wells located in the Carson Desert south and east of Fallon produce gas. These are along a northeast trend from Carson Lake to Stillwater. Their gas is thought to come from the decomposition of organic material in sediments of a prehistoric lake that was present in this area during the last ice age. If so, the gas is microbial because the sediments have never been buried deep enough to have experienced temperatures needed for thermogenic production of methane.
- In 1920, flammable gas was reported from a shallow water well about 12 miles west of the north end of Pyramid Lake. It is thought to be microbial.
Methane in gas seeps
Seeps of flammable gas have been reported in Nevada:
- Gas seeps have been reported in the Carson Desert in the same area mentioned above where water wells have produced gas.
- A gas seep has been reported at the eastern edge of Pine Valley about 36 miles south of Carlin.
- Flammable gas has been reported emitted from a warm spring pool called Diana's Punch Bowl in the central part of Monitor Valley. The spring is in sediments capable of generating microbial methane.
Utah has highly productive oil and gas fields that yield methane, and the state has commercial coalbed methane. Water wells have yielded methane gas, and there are gas seeps that contain methane.
Natural gas fields
Utah has many fields that produce commercial quantities of gas. These gas-producing fields are in three geologically distinct areas in the eastern half of Utah - Thrust Belt, Uinta Basin, and Paradox Basin. Shows of oil and gas have also been reported in oil and gas exploration wells in the Basin and Range province in the western half of Utah.
Gas fields in the Utah part of the Thrust Belt
The Thrust Belt in Utah is a small part of a belt of intensively folded sandstone, shale, and limestone layers that have been broken and thrust eastward over one another. The entire belt extends from Alaska to Central America. The Utah part of this belt extends from Bear Lake on the northern border of Utah to the southwestern corner of Utah and is about 140 miles wide. The Utah part that contains oil and gas fields is located northeast of Coalville. Gas fields in this area produce from layers of porous sandstone and limestone at various depths. The oil and gas in these rocks are believed to have been thermally generated from organic rich shales that they have been thrust over.
Gas fields in the Uinta Basin
The Uinta Basin is a large area in northeastern Utah located roughly south of the Uinta Mountains (near the Wyoming border) and north of Interstate Highway 70 and U.S. Highway 6. The gas and oil in this area is produced from sandstones with low permeability that must be treated by hydraulic fracturing (fracking) to release the gas. These gases are interpreted to be thermogenic or mixed thermogenic and microbial.
Gas fields in the Paradox Basin
The Paradox Basin is a geologic province located in southeastern Utah and adjacent southwestern Colorado. It extends a short distance into Arizona and New Mexico in the four corners area (Utah, Colorado, Arizona, and New Mexico). This basin contains much black organic-rich shale that is the source of gas that has migrated through faults and fractures in the rock to porous sandstone and limestone. Commercial gas is produced from the sandstone and limestone.
Much methane has been produced from coal beds in the so-called Ferron coal trend that extends underneath the towns of Price, Huntington, Castle Dale, Ferron, and Emery. More than 700 coalbed methane wells have been constructed in this area. Also, coal beds are present in several areas of eastern and southeastern Utah. Some of these coal areas have coalbed methane potential.
Methane associated with fresh groundwater
Naturally occurring methane gas has been reported in water wells in Utah. Natural gas was discovered during the drilling of a water well in 1891 at a depth of about 400 to 700 feet near Farmington Bay in Davis County on the east shore of the Great Salt Lake. This location is about half way between Salt Lake City and Ogden. Gas from several wells constructed nearby was collected and shipped to Salt Lake City between 1895 and 1897. Similarly, a well drilled for water in the Bear River Migratory Bird Refuge west of Brigham City in the early 1930's yielded methane gas, which was utilized at the refuge headquarters for several years. Numerous shallow water wells east and south of the refuge have yielded some methane, and water wells in the Jordan Valley (a.k.a. Salt Lake Valley) are reported to yield methane gas. The Jordan Valley is located west and south of Salt Lake City. These wells are in valleys that contain thick lake deposits, and the source of the gas is probably mostly the organic material in these lake sediments.
The Cache Valley in the Logan area of northern Utah also contains thick lake deposits. Shows of gas in water wells in this valley are common and the lake deposits have been prospected as a local source of gas for fuel. In 1975, a water well being drilled three miles west of Richmond blew out and flowed gas in a spray of muddy water, pebbles and small cobbles. The gas flow continued erratically for about a month. An analysis of the gas showed it to be 94.32 percent methane with small quantities of carbon dioxide, nitrogen and oxygen.
Methane in gas seeps
Seeps of flammable gas and oil along the San Juan River near Mexican Hat were noticed prior to 1882. The seeps led to the discovery of the Mexican Hat oil field in 1907. This area is near the south edge of the Paradox Basin, which later became a prolific oil and gas production area. The gas in the seeps probably migrated to the surface along faults in the rock from organic-rich black shale. Gas seeps in the Paradox Basin have also been described issuing from alluvium along the Colorado River near its confluence with the San Juan River and upstream near Moab.
Arizona has a few oil and gas fields that yield methane, no commercial coalbed methane, and I found no reports of methane associated with fresh groundwater or methane gas seeps.
Natural gas fields
Arizona's natural gas fields are located in the Paradox Basin; which is mainly in Utah, but extends a short distance into northeastern Arizona. Only a few gas wells remain in service.
No coalbed methane is currently produced in Arizona. However, high quality coal is being mined on Black Mesa near Kayenta. The coal is used for electrical power generation at a large plant near Page. More deeply buried coal under Black Mesa that cannot be economically mined has potential for coalbed methane extraction.
There are other smaller coal fields in eastern Arizona. The largest of these is near the town of Pinedale near the south edge of the Colorado Plateau.
Methane Associated with Fresh Groundwater
I found no reports of methane associated with fresh groundwater in Arizona. This result does not prove an absence of methane in Arizona groundwater.
Methane in Gas Seeps
I found no reports of gas seeps in Arizona. This result does not prove an absence of methane gas seeps in Arizona.
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