Coalbed Methane in the Fruitland Formation, San Juan Basin, Western United States

The Fruitland Formation (Upper Cretaceous) in the San Juan Basin, although an “unconventional” source of natural gas, surpasses many conventional reservoirs in production, reserves, and original resources. Production and reserve values confirm the San Juan Basin as the world's leading producer of coalbed gas, and they establish the Fruitland fairwayas a giant gas field within the basin. Original coalbed gas in place in the Fruitland Formation was approximately 50 tcf (1.4 Tm 3), and coalbed-gas reserves were 7.8 tcf (223 Bm 3) at the beginning of 1998. In 1999, the Fruitland Formation produced 1 tcf (28.6 Bm 3) of coalbed gas, and production reached a peak or plateau. That same year, cumulative coalbed-gas production from the basin surpassed 7 tcf (200 Bm 3) . The San Juan Basin can be divided into three regions with markedly different coalbed-gas occurrence, resources, and production characteristics that result from variable depositional, structural, and hydrologic settings. Depositional setting controlled Fruitland coal occurrence, thickness, and geometry. The thickest coal deposits, with net coal thickness between 50 and 70 ft (15 and 21 m), occur in the northern part of the basin in northwest-trending, coastal-plain sediments that were deposited landward of the Pictured Cliffs wave-dominated deltaic and barrier shoreline sandstones. In the southwest part of the basin, back-barrier coal deposits are present, but equally important are northeast-trending coals, with net coal thicknesses of 30-60 ft (9-18 m), which were deposited as floodplain facies between Fruitland channel-fill sandstone belts. Laramide tectonism indirectly controlled the occurrence of thick coal deposits by affecting shoreline stillstands, resulting in aggradation in the northern part of the basin, northeast of a structural hinge line. Moreover, Laramide tectonism formed the San Juan Basin, which influenced thermal maturation patterns and hydrocarbon generation and set the structural framework for the present hydrologic system. Significantly, the structural hinge line is a no-flow boundary that divides the basin into provinces or trends with notably different hydrologic regimes, coalbed-gas compositions, and production performance. Three coalbed-gas production trends and several subtrends are recognized in the San Juan Basin. In Trend 1, north of the hinge line, the Fruitland Formation is a regionally overpressured aquifer. Most coalbed wells produce significant quantities (as much as 2000 bbl/day [318 m 3/day] of low-chloride, high-alkalinity water, and coalbed gas is chemically dry (C1/C1-C5 >0.97) and contains 3-13% CO2. The most prolific coalbedgas wells in the world are in Trend 1 A, the fairway. Typically, peak production rates of fairway wells are l-6mmcf/day (28 ;000-l 70,000 m 3/day]. The most effective fairway completion is the open-hole cavity. In Trends IB and 1C, peak gas production rates are generally less than 300 mcf/day (8500 m 3), and wells perform better when completed with cased holes and fracture stimulation treatments than when completed with open-hole cavities. In Trends 2 and 3, which cover much of the rest of the basin, the Fruitland Formation is underpressured, with local exceptions. Most coalbed-gas wells in Trend 2 produce no water or small volumes of NaCl-type waters. Coalbed gas in Trend 2 is chemically variable (wet to dry; C1/C1-C5 = 0.86-0.98) and contains less than 1.5% CO2. Typically, peak gas production rates in Trend 2 are less than 300 mcf/day (8500 m 3), but production may be as great as 700 mcf/day (20,000 m 3). Most wells are completed with cased holes and fracture stimulation. Trend 3 is a little-explored part of the eastern San Juan Basin that has less Fruitland coal and is inferred, from limited testing, to be an underpressured area having low-permeability coal beds. Understanding the unique reservoir characteristics of each region or trend, including the highly productive fairway, is essential to optimize coalbed-well completions, field development, and facilities design in the San Juan Basin.