Coalbed methane is produced from naturally fractured strata in the lower Pennsylvanian Pottsville Formation in the eastern part of the Black Warrior basin, Alabama. Major fracture systems include orthogonal fractures, which consist of systematic joints in siliciclastic strata and face cleats in coal that strike northeast throughout the basin. Calcite and minor amounts of pyrite commonly fill joints in sandstone and shale and, less commonly, cleats in coal. Joint-fill calcite postdates most pyrite and is a weakly ferroan, coarse-crystalline variety that formed during a period of uplift and erosion late in the burial history. Pyrite forms fine to coarse euhedral crystals that line joint walls or are complexly intergrown with calcite.
Stable-isotope data reveal large variations in the carbon isotope composition of joint- and cleat-fill calcite (−10.3 to +24.3‰ Peedee belemnite [PDB]) but only a relatively narrow range in the oxygen-isotope composition of this calcite (−16.2 to −4.1‰ PDB). Negative carbon values can be attributed to 13C-depleted CO2 derived from the oxidation of organic matter, and moderately to highly positive carbon values can be attributed to bacterial methanogenesis. Assuming crystallization temperatures of 20–50°C, most joint- and cleat-fill calcite precipitated from fluids with δ18O ratios ranging from about −11 to +2‰ standard mean ocean water (SMOW). Uplift and unroofing since the Mesozoic led to meteoric recharge of Pottsville strata and development of freshwater plumes that were fed by meteoric recharge along the structurally upturned, southeastern margin of the basin. Influxes of fresh water into the basin via faults and coalbeds facilitated late-stage bacterial methanogenesis, which accounts for the high gas content in coal and the carbonate cementation of joints and cleats.
Diagenetic and epigenetic minerals can affect the transmissivity and storage capacity of joints and cleats, and they appear to contribute significantly to interwell heterogeneity in the Pottsville Formation. In highly productive coalbed methane fields, joint- and cleat-fill calcite have strongly positive δ13C values, whereas calcite fill has lower δ13C values in fields that are shut in or abandoned. Petrographic analysis and stable-isotope geochemistry of joint- and cleat-fill cements provide insight into coalbed methane reservoir quality and the nature and extent of reservoir compartmentalization, which are important factors governing methane production.
Janet Pitman is a senior research geologist with the Central Energy Resources Team at the U.S. Geological Survey. She attended the University of Colorado and the Colorado School of Mines. Her research at the U.S. Geological Survey focuses on sandstone diagenesis and reservoir-quality analysis in domestic and international energy-rich basins. She is currently conducting petroleum-generation and migration-modeling studies in the Gulf Coast and Middle East. Janet is a member of the AAPG and the Society for Sedimentary Geology.Jack Pashin is head of the Energy and Minerals Section at the Geological Survey of Alabama, where he has been conducting research on coalbed methane, coal geology, and petroleum geology since 1988. He received a Ph.D. in geology from the University of Kentucky in 1990 and a B.S. degree in geology from Bradley University in 1982. Jack currently serves as the president of the Alabama Geological Society and chairs the Coal Committee of AAPG's Energy Minerals Division.
Joseph R. Hatch has been a research geochemist with the U.S. Geological Survey since 1974. His areas of expertise are the Illinois basin and mid-continent regions where he has studied the geochemistry of Paleozoic oil and hydrocarbon source rocks, made quality and quantity assessments of Pennsylvanian coal, and studied the organic and element geochemistry of Pennsylvanian marine black shale.
Martin B. Goldhaber received his bachelor's degree in chemistry in 1968, and Ph.D. in geochemistry in 1973, both from University of California-Los Angeles. He was a postdoctoral fellow at Yale and joined the U.S. Geological Survey in 1975 where he is a research geochemist. His work at the U.S. Geological Survey has involved mineral resource and mineral environmental studies. He is presently associated with the Geochemical Backgrounds and Baselines Project. His research focuses on migration of fluids in the Earth's crust, with emphasis on potentially toxic elements such as arsenic. He is currently an associate editor of the journal Geochimica et Cosmochimica Acta.