--> Abstract: Coalbed Methane Exploration in Thrust Belts: Experience from the Southern Appalachians, U.S.A., by Jack Pashin; #90039 (2005)

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Coalbed Methane Exploration in Thrust Belts: Experience from the Southern Appalachians, U.S.A.

Jack Pashin
Geological Survey of Alabama, Tuscaloosa, AL

More than 1,100 coalbed methane wells have been drilled in the southern Appalachian thrust belt of Alabama, and geologic and fluid production data from this area provides important insight into coalbed methane exploration strategies for thrust belts. In the southern Appalachian coalbed methane fields, reservoir performance reflects a complex interplay among stratigraphic, structural, and hydrologic variables. Productive coal beds are of Lower Pennsylvanian age, are distributed through 700 to 1,200 m of section, and are of high volatile A bituminous to low volatile bituminous rank. Completed coal beds range in thickness from 0.3 to 4 m. Lower Pennsylvanian sedimentation in the southern Appalachian coalbed methane fields was regulated by glacial eustasy in the Milankovitch band, and the thick, continuous coal beds constituting primary completion targets accumulated in response to 5th-order rises of base level.

Thrust-related folds in the southern Appalachian coalbed methane fields have steep forelimbs, gently dipping backlimbs, and contain multiple hinge zones. Fresh-water recharge is most effective in steep fold limbs, and fresh-water plumes extend from the outcrop to reservoir depth in thick, continuous coal beds. Carbon isotopic data from produced gas and fracture-filling carbonate indicates that late-stage bacterial methanogenesis within the fresh-water plumes facilitates high gas saturation in coal. Linear productivity sweet spots occur along fold hinges. Hinges in proximity to fresh-water recharge areas are water-productive, whereas those that are distal to recharge are gas-productive. Within broad, gently dipping fold limbs, linear gas- and water-production sweet spots appear to mark the positions of vertical conjugate shear zones.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005