Subsurface Methane - New Mexico (T)
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NATURAL SUBSURFACE METHANE IN NEW MEXICO
By Darrel Dunn Ph.D., PG (Consulting hydrogeologist. Professional Synopsis)
New Mexico has large oil and gas fields that yield methane, and the state has produced a great amount of coalbed methane. Water wells have been reported to contain methane in the commercial gas producing San Juan Basin in northwestern New Mexico, both before and after extensive commercial gas development. Likewise, gas seeps have been reported in this area both before and after extensive development. Early oil and gas well construction practice in this area allowed communication between strata containing methane and potable water aquifers.
Natural gas fields
New Mexico has many commercial gas fields and fields that produce commercial quantities of associated gas. Most of these gas producing fields are in the San Juan Basin or the Permian Basin. Relatively small amounts of gas are produced outside of these basins.
Gas fields in the San Juan Basin. The San Juan Basin is a structural basin that began to form in the Late Cretaceous. It includes northwestern New Mexico and extends a short distance into Utah and about 40 miles into Colorado. Its southeastern corner is near Albuquerque. The major gas fields are in the "central basin area" east and southeast of Farmington. Gas has been produced in this area since 1921 from thousands of wells. This is primarily a gas producing area. Late Cretaceous sandstone units are the oil and gas reservoirs, primarily in stratigraphic traps. In descending order they are the Farmington Sandstone, Fruitland Formation, Pictured Cliffs Sandstone, Cliff House Sandstone, Point Lookout Sandstone, and the Dakota Formation. The Pictured Cliffs, Cliff House and Point Lookout sandstones are in the Mesaverde Group, which is mentioned above in connection with gas and coal in Utah. These units are interbedded with dark marine shale, which is the probable source of the hydrocarbons. The Menefee Formation, which is terrestrial mudstone, shale, sandstone, and coal between the Cliff House and Point Lookout sandstones has produced some gas from the sandstone and coal layers. A small amount of associated gas has been produced from fractured Mancos shale and limestone; and from the Gallup Sandstone, which interfingers with the Mancos Shale. Some gas has been produced from Tertiary sandstone west of Dulce, and from the Pennsylvanian Hermosa Group in the Barker Creek Field just south of the Colorado line north of Farmington. The Hermosa Group was deposited in the Pennsylvanian Paradox Basin. The southern part of the Paradox Basin extends into the area of the San Juan Basin that was formed much later.
Gas fields in the Permian Basin. The Permian Basin is a structural and sedimentary basin located in southeastern New Mexico and adjacent western Texas. The basin formed during the Pennsylvanian-Permian Ancestral Rocky Mountain orogenic event at the same time the Paradox Basin of Utah and Colorado was forming. The basin contains shallow-marine limestone layers deposited on shelves and platforms that were separated by intervening sub-basins that contain organic-rich black shales. Pennsylvanian and Permian reservoirs include clastics from the surrounding highlands and biohermal limestone deposits. The part of the Permian Basin that extends into New Mexico includes the northwestern part of the Central Basin Platform, the northern part of the Delaware Basin that is located west of the Central Basin Platform, and part of the Northwest Shelf that is north and west of the Central Basin Platform and the Delaware Basin. The Permian Basin is one of the most prolific petroleum provinces of North America, and an enormous quantity of gas and oil has been produced in the New Mexico part of it. One of the largest fields is located near Hobbs, New Mexico. The Permian Basin fields contain both associated and nonassociated gas. Although gas production is dominantly Permian (121), gas is also produced from Pennsylvanian formations. Source rocks probably include the Upper Devonian Woodford Shale and organic-rich Pennsylvanian shales (122), with migration paths that include faults. Some of the Permian Basin associated gas is from horizontal wells completed in the Leonard Shale and in carbonate reservoirs of the Abo Formation. The Leonard and Abo are both Permian in age.
The Woodford Shale is an organic-rich black shale that is one of several black shale formations that were deposited in Middle Devonian through Mississippian time in a broad epeiric sea that invaded the North American cratonic area between the transcontinental arch and mountainous areas near the craton margin (123). The Woodford Shale was deposited at the toe of the arch. Other black shale formations deposited around the flanks of the arch include Marcellus (Appalachian Basin, New York, Pennsylvania, Ohio, West Virginia), Antrim (Michigan Basin), New Albany (Illinois Basin), Chattanooga (Kentucky, Tennessee), Fayetteville and Caney (Arkoma Basin, Arkansas and Oklahoma), Barnett (Fort Worth Basin, Texas), Bakken (Williston Basin, Montana and North Dakota), and Exshaw (Western Canada Basin, Alberta and Saskatchewan). There is also a late Mississippian black shale unit in the southeast corner of New Mexico that is correlated with the Barnett. These shales are not all synchronous or interconnected, but they were all deposited during a Paleozoic interval when the sea was transgressing the edges of the craton and the ancestral North American continent was near the equator so surface water tended to be warm. Local conditions were complex. In general, warm surface water may have promoted organic-rich surface water and density stratification. Stratification in the sea produced anaerobic and dysaerobic (low oxygen) bottom conditions, which resulted in preservation of the organic matter settling from the surface.
A prolific coalbed methane (CBM) field is located in the San Juan Basin northeast and south of Farmington. Most of this CBM production is in San Juan County, but some is in Rio Arriba County. The methane is produced from coal in the Late Cretaceous Fruitland Formation. Some of the gas is extracted by horizontal wells.
Coal bed methane is also produced in the Raton Basin of northeastern New Mexico, which is located west of the town of Raton. This methane comes from coal beds in the Vermejo and Raton Formations (Cretaceous to Paleocene).
Methane Associated with Fresh Groundwater
Even prior to commercial gas drilling operations, methane gas was encountered in the shallow subsurface in the San Juan Basin. The first recorded gas well in the San Juan Basin reached a depth of 200 feet penetrating the Kirtland Shale near Farmington, New Mexico. This well began as a search for water, but produced only gas. It was constructed prior to completion of the first commercial gas well which was constructed near Aztec, New Mexico in 1921 (124). Settlers noticed gas bubbles along the Animas River prior to 1900.
More recently, water wells in the area where the Animas River crosses the New Mexico - Colorado line have been reported to have produced water with methane bubbles and bubbles were noted in the Animas River (124). Several pump houses in this area exploded. This area is one where a large number of commercial gas wells are located close to a large number of residences that rely on water wells. The U.S. Geological Survey studied the occurrence of methane in this area between Durango, Colorado, and Aztec, New Mexico, and published the results in 1994 (125). In the New Mexico part of the area, they analyzed for methane in water samples from 132 water wells and one spring. A histogram with a class interval of one mg/L is shown in Figure 4. Seventy percent (70%) of these sampled sites yielded dissolved methane values less than the minimum reported value of 0.005 mg/L. The three largest values (15, 33, and 39 mg/L) are clearly anomalous. Values for d13C and dD were obtained for these three high values. They are shown in Table 1. The stable isotope values for the water sample with dissolved methane of 39 mg/L in Table 1 fall in the Microbial Reduction range of the Whiticar graph. This sample is from a water well in Valley Fill. The stable isotope values for the sample with 33 mg/L in Table 1 fall in the thermogenic range of Whiticar. This sample is from a water well in the Nacimiento Formation. The stable isotope values for the sample with 15 mg/L mathane in Table 1 fall outside of the Whiticar graph due to the high dD value (-42). High dD values may result from micorbial oxidation. All three of these water wells are within 2000 feet of a gas well, as are many of the other wells sampled for dissolved methane. The Paleocene Nacimiento Formation is composed of shale, siltstone, and sandstone deposited in lake and fluvial envoronments. It does not produce commercial quantities of gas in this area.
Figure 4. Histogram of dissolved methane in well and spring water near the Animus River in New Mexico.
Table 1. Stable isotope values in well water near the Animus River in New Mexico.
Potable groundwater has also been sampled and analyzed for dissolved methane in the Middle Rio Grande Basin (also known as the Albuquerque Basin) located along a 100-mile stretch of the Rio Grande River in the Albuquerque area (126). This basin is the eastern-most down-faulted block of the Basin and Range Province. It borders the southeastern side of the Colorado Plateau. The methane analyses are for 255 wells (including test holes and geoprobes) and seven springs. Sixty-five percent (65%) of these sampled sites yielded dissolved methane values less than the minimum reported value of 0.0001 mg/L. All of the methane values were less than 0.1 mg/L except values from two monitoring wells at the same location in western Albuquerque, which sampled from depths ranging from 710 to 1050 feet. Figure 5 shows dissolved methane plotted against dissolved sulfate. Where a monitoring site had two values of either constituent, the average is plotted. The two high methane values correspond to the two monitoring wells in western Albuquerque. These two wells appear to be sampling part of the basin aquifer system where sulfate is at sufficiently low concentrations so that significant methanogenesis can occur. That is, sulfate reducing microbes are unable to out-compete methanogens due to lack of sulfate in the parts of the aquifer sampled by these two wells.
Figure 5. Graph of dissolved sulfate versus dissolved methane in Albuquerque Basin monitoring wells.
Methane in gas seeps
As mentioned above, settlers noticed gas bubbles along the Animas River prior to 1900. Since shallow commercial oil and gas was later discovered in the area (1921) and fractures and faults have been mapped in the area (125), these bubbles probably contained methane. Later, after extensive oil and gas development in the area along the Animas River, gas seeps were observed in pastures in the Animas River Valley near the town of Cedar Hill. These seeps were identified by soil-gas methane measurements in bare spots in the grass that were 5 to 10 feet across (125).
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