NATURAL SUBSURFACE METHANE IN WASHINGTON, OREGON, CALIFORNIA, IDAHO, NEVADA, UTAH, AND ARIZONA
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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 (Livingston, 1958):
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 (Livingston, 1958). 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 (Swanson, 2003). 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 (Crabtree, 1995). 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 (Macdonald and others, 1983). 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 (U.S. Environmental Protection Agency, 2004). 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 (Livingston, 1958).
Test wells at a site on the Hanford Reservation yielded groundwater with dissolved methane values greater than 500 mg/L (Makhijani and Tucker, 1985). 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 (Lowenstern and Janik, 2002). 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 (MacFarland, 1979) 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 (Bruer, 1980). 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 (Armentrout and Suek, 1985), 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 (Armentrout and Suek, 1985) (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 (Whiticar, 1994). 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 (Golder Associates, 2005), and the Cowlitz Formation extends to depths greater than 8,000 feet near the Mist Gas Field (Armentrout and Suek, 1985). 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 (Armentrout and Suek, 1985), 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 (Penoyer and Niem, 1975). 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 (Heiss, 1978).
Water wells in Quaternary deposits south of Bend associated with woody organic material (Lite and Gannett, 2002).
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 (Lu, 2008).
Methane gas also bubbles from the ocean floor elsewhere off the Oregon Coast (Collier and Lilley, 2005). 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 (California State Mining Bureau, 1922)) 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 (Price, 1999), 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 (1994). 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 (U.S. Geological Survey, 2011) 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 (U.S. Geological Survey, 2011) and Lillis and others (2007), 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 (1994), 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 (Price and others, 1999). 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 (Lillis and others, 2007). Upper Cretaceous shale rests on bedrock in the northern and central parts of the basin (Scheirer and Magoon, 2007) and is at depths exceeding 16,000 feet along the western edge (Myer and others, 20005; Wentworth and Zoback 1989). Paleocene shale rests on bedrock in the southern part of the basin at depths as great as 20,000 feet (Scheirer and Magoon, 2007). Significant quantities of thermogenic methane can be yielded from marine source rocks at temperatures greater than 70oC (Whiticar, 1994). Upper Cretaceous and Paleocene shale that are possible source rocks occur below this depth (Scheirer and Magoon, 2007) (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 (Lillis and others, 2007). 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 (U.S. Geological Survey, 2011), 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 (Bartley and others, 2010). 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 (Stoffer and Gordon, 2001). 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 (Ritzius and others, 1993). 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 (Bowen, 1962).
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 (Kulongoske and others, 2006). 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 (Anders and Schroeder, 2003). 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 (U.S. Geological Survey, 2011)." 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 (Press, 2004). 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 (Press, 2004). Temperatures greater than 70 degrees centigrade may generate methane from kerogen in shale and mudstone (Doyle, 2001). 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 (Larue and Underweed, 1986). 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 (Lorenson and others, 1999). These younger units contain mudstone (Gordon, 2009). 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 (1994), although the offshore seeps may contain admixed bacterial methane (Lorenson and others, 1999).
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 (1994). 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 (Lowenstern and Janik, 2002) 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 (Quigley and others, 1999). 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 (Doyle, 2001), which would occur at a depth of about 16,000 feet (Price, 1999). Eocene through middle Miocene mudstone and shale is present below this depth (Fisher and others, 2005), 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 (Wilkinson, 1972). 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 (Wilkinson, 1972). 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 (Sever, 2007; Kim and Crowley, 2007). The USGS (U.S. Geological Survey, 2011) 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 (1994). 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 (Schumacher and Abrams, 1996).
Gas seeps in the Central Valley Trough
The USGS (U.S. Geolgical Survey, 2011) 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 (U.S. Geolgical Survey, 2011). 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 (Scheirer and Magoon, 2007).
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.