Subsurface Methane - New Mexico (NT)
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NATURAL SUBSURFACE METHANE IN NEW MEXICO
By Darrel Dunn, Ph.D., PG, Consulting hydrogeologist-geologist (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 and gas seeps have been described in northwestern New Mexico both before and after commercial gas development in the area.
Natural gas fields
New Mexico has many commercial gas fields and fields that produce commercial quantities of gas associated with oil. Most of these gas producing fields are in two distinct geological areas - the San Juan Basin and the Permian Basin.
Gas fields in the San Juan Basin
The San Juan Basin is in northwestern New Mexico and extends a short distance into Utah and about 40 miles into southwestern 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. The gas is produced from sandstone strata that are associated with layers of dark shale, which is the probable source of the hydrocarbons.
Gas fields in the Permian Basin.
The Permian Basin is located in southeastern New Mexico and adjacent western Texas. 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. Sources of the oil and gas that has migrated into the reservoirs probably include the Woodford Shale. The Woodford Shale is an organic-rich black shale that was deposited in a sea that surrounded three sides of a broad land area that extended through the middle of what is now the United States about 340 to 400 million years ago. Other names applied to organic-rich black shale deposited in this sea include Marcellus Shale (New York, Pennsylvania, Ohio, West Virginia), Antrim Shale (Michigan), New Albany Shale (Illinois), Chattanooga Shale (Kentucky, Tennessee), Fayetteville (Arkansas), Caney (Oklahoma) Barnett Shale (Texas), Bakken Shale (Montana, North Dakota), and Exshaw Formation (Alberta, Saskatchewan). These shale formations are not continuous one with the other, but they were all deposited when the ancestral North American continent was near the equator, so surface water of the sea tended to be warm. The warm surface water may have promoted growth of organisms that died and settled to a bottom where there was little or no oxygen, which resulted in the preservation of the organic matter in bottom mud that later became deeply buried and compressed to black shale.
A prolific coalbed methane (CBM) field is located in the San Juan Basin northeast and south of Farmington. Most of the CBM production is in San Juan County. Some of this gas is extracted by horizontal wells. Coalbed methane is also produced in northeastern New Mexico west of the town of Raton.
Methane Associated with Fresh Groundwater
Even prior to commercial gas operations, methane gas was encountered at shallow depths in the San Juan Basin. The first recorded gas well in this area reached depth of 200 feet near Farmington. 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. 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 produced water with methane bubbles and bubbles were noted in the Animas River. Several pump houses in this area exploded. The 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 the area along the Animas River between Durango, Colorado, and Aztec, New Mexico, and published the results in 1994. In the New Mexico part of the area, they analyzed for methane in water samples from 132 water wells and one spring. 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 anomalous, being much larger that all of the other values. All three of these water wells are within 2000 feet of a gas well, as are many of the other wells with much less methane or no methane. Early oil and gas well construction practice in northwestern New Mexico allowed communication between subsurface strata containing methane and potable water aquifers.
Potable groundwater has also been sampled and analyzed for dissolved methane along a 100-mile stretch of the Rio Grande River in the Albuquerque area. The methane analyses are for 255 wells and seven springs. Sixty-five percent (65%) of these sampled sites yielded no dissolved methane above the minimum reported value of 0.0001 mg/L. All of the dissolved methane values were less than 0.1 mg/L except values from two monitoring wells at the same location in western Albuquerque, which sampled dissolved methane at 4 and 5 mg/L from depths ranging from 710 to 1050 feet. These wells are in an area where the sulfate concentration in the groundwater at this depth is low. Low sulfate promotes microbial generation of methane from organic material.
Methane in gas seeps
As mentioned above, settlers noticed gas bubbles in 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, 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 measuring the methane concentration in gas extracted from the soil in bare spots in the grass that were 5 to 10 feet across.
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