by Darrel Dunn, Ph.D., PG, Hydrogeologist - Geologist

(View Professional Synopsis 🔳)

This is a technical page on this subject. To go to a non-technical page, press here.

Subsurface Methane

Natural 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. Natural 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. The occurrences described below illustrate the ubiquitousness of methane, natural sources of methane, subsurface movement, and chemical and biological reactions.

Washington Subsurface Methane

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 vesicular basalt.

Natural gas fields in Washington

Methane is the main component of combustible gas. In the state of Washington, methane has been produced commercially from three small (currently inactive) gas fields (7):

  • 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 lenticular glacial sand deposits beneath clay beds on the flank of an anticline. The glacial material is underlain by a truncated Cretaceous (?) to lower Eocene sandstone and shale formation that contains some coal. The glacial deposits may contain organic material. The shale, coal, and organic material are all possible sources of the natural gas.

  • Commercial gas has been produced from wells along the beach of the Pacific Ocean west of Aberdeen (7). These wells may have been completed in severely sheared shale. The shale may be the source of the gas.

  • Small amounts of gas have been produced from vesicular basalt north of Richland at depths of 700 to 1,200 feet (Rattlesnake Hills Gas Field). The total thickness of the basalt here is probably about 12,000 feet (8). It is faulted and may be underlain by Tertiary sediments 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 interbasalt sediments. 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 magma (1,000 to 1,200oC) when the pressure is near atmospheric (46). 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 (47). Baked soil horizons in the interflow zones 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 subjacent vesicular basalt. I suspect it either explodes or escapes to the atmosphere.

Shows of gas have been encountered in petroleum test wells elsewhere in the state.

Coalbed methane in Washington

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 (4). Methane is also associated with coal beds in the Central Coal Region buried beneath the basalts of the Columbia Plateau. I do not know of any producing coalbed methane fields in Washington. Testing in the Pacific 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 fresh groundwater in Washington

Some water supply wells and irrigation wells in the Columbia River Basalt Group contain methane gas. Some water supply wells in various lithologies in the Pacific Coal Region also contain methane gas. A man digging a water well in the northern part of the Pacific Coal Region in 1893 caused an explosion (7).

Test wells at a site on the Hanford Reservation yielded groundwater with dissolved methane values greater than 500 mg/L (50). This water was from basalt (Grande Ronde Basalt) at a depth of about 3000 feet. The total dissolved solids in this aquifer in this area is about 800 mg/L (51). The dissolved methane is in a plume that extends downgradient from a fault. This occurrence is another indication of transport of methane upward along a fault from source beds in or below the thick Columbia River Basalt Group.

Methane in gas seeps in Washington

Natural gas seeps occur on the west side of the Olympic Peninsula (5) where there are outcrops of sandy shales.

Oregon Subsurface Methane

The occurrence of subsurface methane in Oregon is similar to that in Washington because the same geologic provinces are present.

Natural gas fields in Oregon

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 Eocene Clark and Wilson (C and W) Sandstone of the Cowlitz Formation, which consists of marine sandstone, siltstone and mudstone. The C and W Sandstone is overlain by a thick shale unit. The field consists of individual gas pools ranging in size from 20 acres to 120 acres. The pools are located in discrete fault blocks in a faulted anticline (6). They are at depths ranging from 1,200 to 2,700 feet. Only a few wells were still producing as of 2003. The gas field is on the east flank of the Nehalem Arch (53), which is a northern extension of the Coast Range Uplift. The d13C values given for methane from three of the pools in the field are -43.8, -43.6, and -42.5 per mil (parts per thousand). These values are greater than -55 per mil, which is often given for guidance in separating microbial methane from thermogenic methane (thermogenic being greater than -55 per mil). Eocene mudstone and shale could be the source of thermogenic methane. The geothermal gradient calculated from borehole temperature is 1.5oF per 100 feet (53) (0.83oC per 100 feet). Thermogenic methane may be produced from kerogen in shale and mudstone at temperatures greater than 70oC and methane generation is thought to peak at abut 150oC (44). With a ground surface temperature of 51.9oF (11.1oC), the depth to 70oC would be 7068 feet and the depth to 150oC would be 16,668 feet. The Yamhill Formation underlies the Cowlitz Formation (54), and the Cowlitz Formation extends to depths greater than 8,000 feet near the Mist Gas Field (53). The Yamhill Formation contains shale that may have potential as a hydrocarbon source rock. So it is possible that gas migrated upward from the Yamhill Formation. It is also possible that gas migrated updip from shale and mudstone in the Cowlitz and Yamhill formations that occur at greater depth in the Willamette Basin to the east, or even the Astoria Basin to the west. Opinion has varied on the origin of the gas in the Mist Field. Some have thought that the variability of the composition of the gas and the isolated nature of the pools indicates a local origin of the gas. On the other hand, Eocene deposits in southwestern Washington that have been buried greater than 10,000 feet have vitrinite reflectance values less than 0.5 suggesting that they are thermally immature, and that 7068 feet is not sufficient burial to generate thermogenic gas (53), One should keep in mind that factors affecting the generation of methane are variable and guidelines for temperature, vitrinite reflectance, and stable isotope ratios should not be applied too rigorously.

Coalbed methane in Oregon

Coalbed methane is being developed in the Coos Basin 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 in the Eocene Coaledo Formation.

Methane associated with fresh groundwater

Naturally occurring methane gas has been reported in water wells in Oregon:

  • A water well located in the Coast Range about 10 miles northwest of Corvallis (11). This well was completed in marine siltstone containing finely disseminated carbonaceous debris that could be the source of the methane.

  • Water wells on the Columbia Plateau in the Pine area of northeastern Oregon (12).

  • Water wells in Quaternary deposits south of Bend associated with woody organic material (10).

Methane gas seeps in Oregon

Methane gas flows from the sea floor at the submarine Hydrate Ridge about 60 miles off the central coast of Oregon. This gas is associated with a wedge of sediment being scraped off of the Juan de Fuca oceanic plate as it is subducted beneath the northwestern margin of the North American plate. It has been suggested that the source of this methane is organic rich Eocene rocks accreted to the continental plate. This suggestion is based on iodine-129 radiometric age dating of pore water associated with the methane. It requires transport of methane and the associated pore water over 30 miles laterally through the highly deformed accretionary wedge that contains porous volcanic ash (14).

Methane gas also bubbles from the ocean floor elsewhere off the Oregon Coast (15). 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 d13C = -28.7 per mil, consistent with a thermogenic origin. A shallow gas well located on land in a gas vent area near the shore abut 13 miles north-northeast of the seep produces gas with a 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 offshore geology along the coast of Oregon to assess the origin of the methane in the offshore seeps.

California Subsurface Methane

The geology of California is complex, and considerable information on methane occurrence is available.

Natural gas fields in California

California has many commercial gas fields. Most of this gas occurs in Upper Cretaceous and Tertiary sandstone associated with dark colored mudstone and shale. Some gas has been recovered along with oil from fractured siliceous shale in the organic-rich Miocene Monterey Formation. Gas is also produced from porcelanite (recrystallized diatomite) layers in this formation.

Gas fields in the California Central Valley Trough

Much of the commercial gas has been produced from the Central Valley Trough, a geologic basin between the Sierra Nevada Mountains and the Coast Ranges. This trough is comprised of the San Joaquin Basin in the south and the Sacramento Basin in the north separated by the transverse Stockton Arch. Within the Central Valley Trough, gas wells have ranged from less than 500 feet (supplied gas to a ranch cook-house in the early 1900's (16)) to greater than 12,500 feet deep.

Most of the gas fields are in the Sacramento Basin, 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 Sacramento Basin is likely to be mostly Upper Cretaceous shale (e.g. Winters Formation and Forbes Formation). Of 58 pairs of isotope values given by the USGS (34), d13C ranges from about -53 per mil to -24 per mil, and dD (deuterium ratio) ranges from about -201 to -100 per mil. Most of the values plot in the thermogenic range of Whiticar (44). However, three pairs plot as "geothermal/hydrothermal/crystalline" and six plot as "mix & transition." (These isotope ratios and subsequent California ones are read from the USGS graphs (34) unless another source is cited, so they may deviate slightly from the original data.)

The San Joaquin Basin 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 (34) and Lillis and others (37), 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 (44), two plot as "bacterial carbonate reduction," and six plot as "mix & transition." The remaining ten plot as thermogenic. Microbial gas can be formed at temperatures less than about 70oC, which would correspond to depths of 4700 feet in the northern San Joaquin Basin and about 5200 feet in the southern San Joaquin Basin (42). Some of the methane produced in the San Joaquin Basin is from depths less than 4700 feet (e.g. San Joaquin Formation in the Trico Gas Field). Microbial gas may also be produced from greater depths if the gas was generated by microbes and trapped at shallow depth before subsiding and being buried at greater depth. Methane produced by "bacterial carbonate reduction" is formed from the reduction of carbon in molecules and ions dissolved in the formation water. Hydrogen in the water is the reducing agent. The source for thermogenic gas in the northern and central San Joaquin Basin is likely to be Upper Cretaceous shale (37). Upper Cretaceous shale rests on bedrock in the northern and central parts of the basin (40) and is at depths exceeding 16,000 feet along the western edge (38,39). Paleocene shale rests on bedrock in the southern part of the basin at depths as great as 20,000 feet (43). Significant quantities of thermogenic methane can be yielded from marine source rocks at temperatures greater than 70oC (44). Upper Cretaceous and Paleocene shale that are possible source rocks occur below this depth (41) (e.g. Moreno Formation and Kreyenhagen Formation). Yield of methane peaks at about 150oC, which would occur at about 12,600 feet. Upper Cretaceous shale that might be source rock occurs below this depth. In addition, the deeper shales and mudstones of the Great Valley Group (Jurassic and Cretaceous) could generate gas. An additional seventy d13C values are available from Lillis and others (37). 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 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. These basins include the Santa Maria, Santa Barbara-Ventura, and Los Angeles basins. 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 (34), eleven are outside the thermogenic range of Whiticar. Of these eleven, three plot as "geothermal/hydrothermal/crystalline," one as "bacterial carbonate reduction," and seven as "mix and transition."

Coalbed methane in California

California has no coalbed methane production, although coal is present. Coal was produced from mines on the eastern slope of the Coast Range about 12 miles southeast of Livermore in the last half of the 1800s and early 1900s 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 (23). The age of the California coal beds are Tertiary, primarily Eocene. I found no information on methane associated with California coal.

Methane associated with fresh groundwater in California

Naturally occurring methane gas has been reported in water wells in California:

  • Water wells located near the coast about 10 miles northwest of Santa Cruz produce substantial amounts of methane (17). One well produces as much as 200 Mcf (thousand cubic feet) per day. This gas may originate in the organic rich shales of the Neogene Monterey Formation. Calcium carbonate structures in this formation near Santa Cruz have been interpreted as deposits of gas vents on the sea floor during the Miocene.

  • 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 (18). About this time many water wells were furnishing gas to farmhouses as far north as Tehama County and to several communities in the lower reaches of the Sacramento River valley (22).

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 (19). 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 (20). There are oil fields in Los Angeles County.

Methane in California gas seeps

There are many gas seeps in California. Much information on some of these seeps can be fund on a USGS website titled "Natural Oil and Gas Seeps in California (34)." 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 Sacramento and San Joaquin basins, and much gas seeps to the surface in the southern California coastal basins - Santa Maria, Santa Barbara-Ventura, and Los Angeles basins.

Gas seeps in the Eel River Basin

The gas seeps in the Eel River Basin are located both offshore and onshore. The bedrock in the Eel River Basin is conglomerate, sandstone, and claystone in the upper part of the Pliocene-Pleistocene Wildcat Group (29). Potential methane sources include the Mesozoic Franciscan Formation. The Franciscan Formation is an accretionary melange that extends to great depth. It has experienced low-grade metamorphism and contains platy dark-gray shale and mudstone although the dominant lithology is graywacke. Low-grade metamorphism implies depths of burial of 20,000 to 33,000 feet where temperatures range between 100 and 200 degrees centigrade (29). Temperatures greater than 70 degrees centigrade may generate methane from kerogen in shale and mudstone (30). Vitrinite reflectance in the Franciscan ranges up to 1.5 percent, which is in the petroleum generation range, and turbiditic mudstones in the coastal belt Franciscan of northern California contain Total Organic Carbon values averaging about 1% in Type III kerogens (32). It has also been proposed that the gas might originate from the Miocene Bar River Formation and/or the Pliocene Wildcat Group attaining gas catagenesis depth by underthrusting the Franciscan Formation (31). These younger units contain mudstone (28). Two offshore gas seeps produce methane with d13C ratios of -49 and -43 per mil and dD ratios of -199 and -180 per mil, respectively. An onshore gas seep near the Tompkins Hill gas field produces gas with a d13C ratio of about -33 per mil and a dD ratio of -140, and the Thompkins Hill wells produce gas with d13C ratios ranging from -31 to -35 and dD ratios from -155 to -127 per mil. All of these d13C ratios are in the thermogenic range according to the isotope signatures diagram of Whiticar (44), although the offshore seeps may contain admixed bacterial methane (31).

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 Formation bedrock, and the Franciscan Formation is a likely source for the reasons given above for the Eel River Basin. "Petroleum springs" along the Mattole River were mentioned in local newspapers as early as 1859. I found 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 reservoirs less than 1700 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 Formation and the Great Valley Sequence. The Great Valley Sequence is Upper Jurassic to Upper Cretaceous interbedded 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 Formation terrane by the Coast Range Fault. The shale is dark gray to black. The Franciscan Formation 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 range 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 "thermogenic" ranges of Whiticar (44). 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 (52) cite d13C ratios 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 Formation. The steam in the field is likely dominantly superheated meteoric groundwater that has circulated down through the Franciscan rocks and been affected by heat from igneous intrusions that rise to abut 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 isotope values that fall within the thermogenic range.

Gas seeps in the southern California coastal basins

The gas seeps in the southern coastal geologic basins occur both offshore and onshore. These basins contain thick sequences of Cretaceous and Tertiary organic rich mudstone, shale, siltstone, sandstone, and conglomerate. These consolidated strata are mostly covered with unconsolidated Quaternary 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 the Santa Barbara-Ventura Basin. It is in the Santa Barbara Channel offshore from the city of Santa Barbara (24). 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 by weight, and the remaining third is ethane, propane, butane and higher hydrocarbons. The large proportion of methane suggests a source rock temperature greater than 140 degrees Centigrade (30), which would occur at a depth of about 16,000 feet (35). Eocene through middle Miocene mudstone and shale is present below this depth (36), including the very organic Monterey Formation. Gas in the Los Angeles Basin likely has a similar origin. Gas is associated with oil in seeps offshore of Redondo Beach at Los Angeles (25). Several oil and gas seeps are aligned along a fault that extends into the ocean from onshore. They are about 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 (25). The bubbling continued for about six days after the earthquake. The famous La Brea Tar Pts in Los Angeles emit some methane which has been reported to be produced by microbes that consume petroleum and release methane (26,27). The USGS (34) 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 (44). I have not determined whether microbes acting on tar in a seep 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. It has been proposed that the source of the gas is sedimentary rocks present below the crystalline and metamorphic rocks of the San Bernardino Mountains (45).

Gas seeps in the Central Valley Trough

The USGS (34) does not report any gas seeps in the Sacramento Basin. The USGS reports two gas seeps on the Stockton Arch that separates the San Joaquin and Sacramento basins, and three near the south end of the San Joaquin Basin. No isotope data is given for any of these five seeps. The nearest oil and gas field to the two seeps on the Stockton Arch is the Brentwood East gas field, which is about eight miles north of the northernmost of the two seeps. The carbon isotope values for a sample of gas from this field plot in the thermogenic range. The source of this gas is likely to be the same as described above for commercial gas fields in the San Joaquin Basin. The three seeps in the southern San Joaquin Basin are co-located with oil fields near the western, southern, and eastern edges of the basin (34). These are shallow oil fields that do not contain commercial quantities of gas. Potential source rocks for the gas from these southern seeps include Pliocene kerogen rich mudstone of the San Joaquin and Etchegoin Formations and deeper sequences of Tertiary shale and mudstone (43).

California Methane Stable Isotope Data 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 Whiticar).

  • 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.

  • There is a slight positive correlation between dD values and d13C values.

Figure 1. Plot of California Methane d13C and dD pairs.

Statistical Analysis of Methane Isotopes XY Graph

Correlation of dD and d13C

Figure 2 shows the least squares linear regression line for the dD and d13C data from California. This line and the associated statistics were calculated using PAST(56). The coefficient of determination (r squared) is 0.40. The probability that d13C and dD are not correlated is 1.9E-18 (virtually zero). The test for the probability of correlation was a two-tailed t-test described by Spatz (2010)(57) and in other statistics books.

Gaussian kernel distribution of California methane stable isotope data

Figure 3 is a Gaussian kernel distribution for the dD and d13C XY plot calculated with PAST. It maps the relative density of points on the XY plot. This relative density map shows the sum of the Gaussian distances to each point on the graph from any location of the graph. The equation used to calculate the value that is contoured is:

where r is a damping factor that is arbitrarily chosen to best illustrate the density distribution of the points. Although the equation has the form of the Gauss equation for the normal frequency distribution, the map has no quantitative probability interpretation.

Figure 3. Gaussian density of points on California methane stable isotope plot.

The plot illustrates the following:

  • The density of the points is greatest in the Thermogenic range of Whiticar (compare to Figure 1).

  • There are no secondary density highs corresponding to "Geothermal" or "Microbial Reduction" although there are points in these ranges that distort the distribution slightly.

  • The two minor secondary highs at dD values less than -250 per mil are based on six values from South California Basins Wells and one value from the San Juaquin Basin Wells. They fall in the Thermogenic and Geothermal ranges of Whiticar.

Since the Gaussian density smooths the results, I checked for clustering using k-means cluster analysis (58), which does not not smooth results. It generates the specified number of clusters of points based on minimizing the sum of Euclidean distances between points and cluster means. I tried three clusters to test for clusters corresponding to the "Thermogenic," "Microbial Reduction," and "Geothermal" regions and found no correspondence between the clusters and the regions. I tried nine clusters to test for clusters corresponding to the California Provenances and found no correspondence between the clusters and the provenances.

Statistical tests for difference in stable isotope values associated with California provenances

The plot of stable isotope values for the various California provenances in Figure 1 suggests that some provenances may have significantly different distributions of values even though the distributions overlap. I performed statistical tests on the data from three provenances with relatively large sample sizes. These are the Sacramento Basin Wells (56 samples), San Joaquin Basin Wells (21 samples), and South California Coastal Basins Wells (53 samples). The other provenances have 6 samples or less, so that the power of statistical tests would probably be low. I used non-parametric tests because the d13C values from the Sacramento Basin Wells provenance failed the Shapiro-Wilk test for normality at the 0.05 level of significance.

I used the Two-sample Kolmogorov-Smirnov test to determine whether the dD samples from the three provenances are likely to come from different underlying probability distributions. This test looks at the maximum difference between two empirical (observed) cumulative frequency distributions. I did the calculations with PAST (58). The results indicated that all three of the sets of samples are from different underlying probability distributions when a level of significance of 0.05 is used. The calculated probabilities that the sample sets were from the same underlying distribution are as follows:

  • Sacramento Basin Wells versus San Joaquin Basin Wells dD: p(same) = 0.000016.

  • San Joaquin Basin Wells versus South California Coastal Basins Wells dD: p(same) = 0.0038.

  • Sacramento Basin Wells versus South California Coastal Basins Wells dD: p(same) = 5.1E-15.

When d13C sample sets are subjected to the same test, the calculated probabilities that the sample sets were from the same underlying distribution are as follows:

  • Sacramento Basin Wells versus San Joaquin Basin Wells d13C: p(same) = 0.0668

  • San Joaquin Basin Wells versus South California Coastal Basins Wells d13C: p(same) = 0.0524.

  • Sacramento Basin Wells versus South California Coastal Basins Wells d13C: p(same) = 1.7E-7.

Only one of these probabilities is less than 0.05. The hypothesis that the samples from the two other provenances are from the same underlying distribution cannot be rejected at the 0.05 level of significance, although the probably that the sample sets are from the same underlying distributions are low.

To summarize the results of the Two-sample Kolmogorov-Smirnov tests, the dD results show that the three provenances tested are probably all from different underlying distributions; but if only the d13C results were known, the difference would be accepted only for Sacramento Basin Wells versus South California Coastal Basins Wells. It appears that dD values and large sample sizes increase the likelihood of identifying differences in sets of methane stable isotope data in California.

Idaho Subsurface Methane

Natural gas fields in Idaho

In the state of Idaho, methane natural gas 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 an interstate pipeline, located about 10 miles away, before the gas can be sold. The gas is from sand layers in the Payette Formation (Miocene). The gas sands are from 1800 to 2500 feet below the land surface. The Payette formation consists of semi-consolidated clay, silt, arkosic sand, volcanic ash, diatomite, freshwater limestone, conglomerate, and intercalated basalt flows of the Columbia River Basalt Group. The sediments in the Payette Formation are of fluviatile and lacustrine origin deposited in a rift-like valley caused by Miocene down-faulting and folding. The formation contains swamp deposits including beds of vegetation in various stages of decomposition, which yield methane. The Payette Formation is underlain by volcanics, and granite is probably beneath the volcanics (59), neither of these lithologies is likely to be a major source of methane.

Coalbed methane in Idaho

I know of no producing coalbed methane fields in Idaho. There are steeply dipping sub-bituminous coal beds in the Upper Cretaceous Frontier Formation in the Overthrust Belt in southeastern Idaho. There are also lignite beds interbedded with Miocene lacustrine, fluvial, and volcanic ash deposits near Oakley southeast of Idaho Falls (61); and lignite in similar Miocene sedimentary deposits interbedded with basalt of the Columbia River Basalt Group near Orofinio, northeast of Lewiston (62).

Methane associated with fresh groundwater in Idaho

Methane is found in water wells throughout the western Snake River Plain (60). Organic material (such as woody material) in the aquifer is probably the source.

Methane in Idaho 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 (63), 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 of more than 85% Archaea which seemed to be dominantly methanogenic. The implication is that the methane is produced by microbial reduction of carbon. There was no isotope analysis to support a microbial origin. An alternative source of the methane might be dark shales in Paleozoic rocks thought to be present at a depth of about 5000 feet (64). Dark shales have been described cropping out in the Mississippian Railroad Canyon Formation, and dark mudstone has been described in the Devonian Three Forks Formation in the Beaverhead Mountains about 50 miles northwest of the Lidy Hot Springs (65). The Lidy Hot Springs are in a faulted area (63).

Nevada Subsurface Methane

Natural gas fields in Nevada

The only natural gas production in Nevada has been at the Kate Spring Field about 12 miles south southwest of Currant on the east edge of the Railroad Valley Basin. The gas is not actively marketed and is used to operate oil production equipment on the site. The gas is associated with oil produced from the Tertiary Horse Camp Formation (landslide breccia) and the Devonian Guilmette Formation between about 4,450 and 4,480 feet (77). The source of the petroleum and gas is likely the Mississippian Chainman Formation (79) or the Tertiary Sheep Pass Formation (78). Both formations contain dark shale and are present downdip in the central part of the Railroad Valley Basin at depths greater than 9,000 feet (79), which would put them in the thermogenic oil and gas generation temperature range (85).

Shows of gas have been encountered in petroleum exploration wells in intermontane basins throughout the state (96).

Coalbed methane in Nevada

Nevada has no commercial production of coalbed methane. However, coal beds are present in the state (97). Paleozoic (probably Mississippian) coal has been mined at Pancake Summit about 20 miles southeast of Eureka. A seam about two feet thick was mined for coal shipped to lead smelters in Eureka prior to 1905. Paleozoic coal is also reported (97) in a railroad cut about 4 1/2 miles east of Carlin; it has not been commercially mined. Low grade coal and lignite are present in Miocene and Pliocene deposits. Lignite is present in thin beds in the Humboldt Formation near Elko, and the remaining Neogene low grade coal and lignite exposures described are 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 in Nevada

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 (96,98). These are along a northeast trend from Carson Lake to Stillwater. Their gas is thought to come from organic material in the Pleistocene Wyemaha Formation of the Lahontan Valley Group. If so, the gas is microbial because the sediments of Lake Lahontan have not been deeply buried.

  • 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 probably be microbial (98).

Methane Nevada 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 (98).

  • 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 Quaternary sediments capable of generating microbial methane (98).

Utah Subsurface 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 produce methane.

Natural gas fields in Utah

Utah has many fields that produce commercial quantities of gas. These gas-producing fields are in three areas - Thrust Belt, Uinta Basin, and Paradox Basin. Good sources for information on these fields are Oil and Gas Fields Map of Utah (99) and Major Oil Plays in Utah and Vicinity (100). Shows of gas have also been reported in oil and gas exploration wells in the Basin and Range province of western Utah (108).

Gas fields in the Utah part of Thrust Belt

A tectonic element, termed the Cordilleran Orogenic Belt extends from northern Alaska to Central America. The Wyoming-Utah-Idaho part of this orogenic belt is characterized by a linear band of very thick sedimentary rocks that have undergone intensive folding and thrust faulting (101). The Utah part of this thrust belt extends from Bear Lake on the northern border of Utah to the southwestern corner of Utah and is about 140 miles wide (102). In most of this Utah thrust belt area, high angle Basin and Range faults cut the older thrust faults. However, the Utah part of the thrust belt that contains commercial oil and gas fields is located northeast of Coalville; and it is outside (east) of the Basin and Range faulting. Gas fields in this area produce from Jurassic Twin Creek Limestone and Nugget/Navajo Sandstone, Permian Phosphoria Formation, Pennsylvanian Weber Sandstone, and the Mississippian Madison Group. The hydrocarbons in these reservoirs are believed to have been generated from subthrust Cretaceous source rocks. These include organic-rich units in the Bear River, Aspen, and Frontier Formations. The source rocks began to mature after being overridden by thrust plates. Hydrocarbons were then generated, expelled, and subsequently migrated (primarily along fault planes) into overlying traps (100).

Gas fields in the Uinta Basin

The Uinta Basin is a large structural basin in northeastern Utah. It extends east-west from the Colorado line (Douglas Arch) to the Wasatch Mountains and north-south from the Uinta Mountains to the Uncompaghre and San Rafael Uplifts (roughly to Interstate Highway 70 and U.S. Highway 6). The gas and oil in the basin is mostly produced from Paleocene and Eocene Green River and Wasatch Formations, which contain lacustrine and alluvial beds deposited in and around ancestral Lake Uinta (100). The largest gas field is the Natural Buttes field which occupies about 400 square miles in the eastern part of the basin. It contains about 3950 wells with a cumulative production of over 2.1 trillion cubic feet of gas. The field produces natural gas primarily from tight sandstone in the Cretaceous Mesaverde Group and the Tertiary Wasatch Formation. Production in these tight-gas-sand reservoirs is achieved through massive hydraulic fracture treatments. The source of the gas is possibly organic-rich shales in the more deeply buried parts of the Mesaverde Group (104). Migration would be through the extensive fracture system. The methane in this nonassoccited gas of the Natural Buttes field is characterized by d13C of -42 to -34 per mil and dD of -220 to -165 per mil (104).

The methane in associated gases from the Green River Formation (Altamont-Bluebell and Redwash fields) is characterized by d13C of -60 to -45 per mil and dD of -280 to -225 per mil. These associated gases are interpreted to be thermogenic and mixed thermogenic and microbial (104), with the source being organic-rich shale in the Green River Formation. Evidence for microbial methane formation is observed only in the upper Green River Formation in the central and northern parts of the basin at depths less than 9000 feet. Bottom-hole temperature data indicates that the current temperature at 9000 feet is about 85oC (105).

Gas fields in the Paradox Basin

The Paradox Basin is a structural and sedimentary basin located in southeastern Utah and southwestern Colorado. It extends a short distance into Arizona and New Mexico in the four corners area (Utah, Colorado, Arizona, and New Mexico). The basin formed during the Pennsylvanian-Permian ancestral Rocky Mountain orogenic event. It received sediment from the Uncompahgre Uplift and developed a restricted connection with the open ocean to the south and west. The restricted connection promoted deposition of a thick evaporite sequence that contains black organic-rich shale (Paradox Formation). This black shale is probably the source of the oil and gas in the basin. Hydrocarbon generation occurred during maximum burial in the late Cretaceous and early Tertiary (100). Hydrocarbons migrated along fault planes. The gas in the basin is mostly produced from the Pennsylvanian Honaker Trail and Paradox formations, and from the deeper pre-basin Mississippian Leadville Limestone. Three small fields in the Lisbon field area have produced condensate and nonassociated gas from the Leadville Limestone. The gas in the Leadville Limestone occurs where it is juxtaposed directly against Pennsylvanian source rocks.

Coalbed Methane in Utah

Coalbed methane is produced from the Ferron Sandstone member of the Mancos Shale and from the Blackhawk Formation of the Mesaverde Group on the northwest flank of the San Rafael Uplift. This methane production is located in an area commonly referred to as the Ferron coal trend, which extends underneath the towns of Price, Huntington, Castle Dale, Ferron, and Emery (106). More than 700 coalbed methane wells have been drilled in this area.

Upper Cretaceous coal beds are present in several areas in eastern and southern Utah. Some of these coal areas have coalbed methane potential, notably the Wasatch Plateau west of Ferron coal trend, the Book Cliffs area of northern Grand and Emery counties, the Kaiparowits Plateau area in Garfield County, an area southwest of the Kaiparowits Plateau around Alton, and a small area north of the Uinta Mountains near the southwest corner of Wyoming.

Methane Associated with Fresh Groundwater in Utah

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 depth of about 400 to 700 feet near Farmington Bay in Davis County. This location is on the east shore of the Great Salt Lake about half way between Salt Lake City and Ogden (107). Gas from several wells constructed nearby was collected and shipped to Salt Lake City between 1895 and 1897 (108). 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 (109). The Jordan Valley is located west and south of Salt Lake City. These areas are in easternmost graben valleys of the Basin and Range province. The valleys contain thick Tertiary and Quaternary lake deposits. 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 is another graben at he eastern edge of the Basin and Range province. It also contains thick Tertiary and Quaternary Lake deposits. Shows of gas in water wells in this valley are common and the Tertiary and Quaternary 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 and a small crater developed around the casing (110). An analysis of the gas showed it to be 94.32 percent methane with small quantities of CO2, nitrogen and oxygen.

Methane in Utah Gas Seeps

Seeps of flammable gas and oil along the San Juan River near Mexican Hat were noticed prior to 1882 (111). 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 at the west edge of the Aneth Platform, which later became a prolific oil and gas production area. The gas has probably migrated to the surface along faults from organic-rich black shale in the Pennsylvanian Paradox Formation. Gas seeps in the Paradox Basin have also been described issuing from alluvium along the Colorado River near is confluence with the San Juan River and upstream near Moab (111).

Arizona Subsurface Methane

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 in Arizona

Arizona's natural gas fields are located on the Aneth Platform of the Paradox Basin, which is mainly in Utah, but extends a short distance into northeastern Arizona. Only a few gas wells remain in service.

Coalbed methane in Arizona

No coalbed methane is currently produced in Arizona. However, high quality coal is present in the Upper Cretaceous Mesaverde Group in the Black Mesa Basin. Coal is being mined on Black Mesa, which is an erosional remnant in the center of the basin. A large strip-mine complex is located near Kayenta. The coal is used for electrical power generation at a large plant near Page. More deeply buried coal in the Black Mesa Basin that cannot be economically mined has potential for coalbed methane extraction (112).

There are other smaller coal fields in eastern Arizona on the Colorado Plateau and in the Transition Zone between the Colorado Plateau and the Basin and Range province. The coal in these fields is Upper Cretaceous. The largest of these is near the town of Pinedale near the south edge of the Colorado Plateau.

Methane associated with fresh groundwater in Arizona

I found no reports of methane associated with fresh groundwater in Arizona.

Methane in Arizona gas seeps

I found no reports of gas seeps in Arizona.

Revised: December 6, 2018