Pashin, Jack C.1, Dorothy E. Raymond1
(1) Geological Survey of Alabama, Tuscaloosa, AL
ABSTRACT: Glacial-Eustatic Control of Coalbed Methane Reservoir Distribution (Pottsville Formation; Lower Pennsylvanian) in the Black Warrior Basin of Alabama
New subsurface maps and stratigraphic cross sections of Pennsylvanian strata in the eastern Black Warrior basin are based on data from 5,052 well logs and cores. These data provide new evidence for the origin of Pennsylvanian depositional cycles and the stratigraphic controls on the distribution of coalbed methane reservoirs. Coal beds are concentrated in a series of 4th-order parasequences, or cyclothems, which are bounded by regionally extensive marine flooding surfaces. These cyclothems represent a maximum duration of 0.4 my, which is equivalent to the long orbital eccentricity period.
Each cyclothem contains subordinate marine flooding surfaces defining four 5th-order parasequences, which apparently are products of relative sea-level changes associated with the short eccentricity period (0.1 my). The first 5th-order parasequence in each cyclothem is dominated by progradational deltaic deposits. The second and third 5th-order parasequences contain a higher proportion of aggradational deposits and include incised valley fills as deep as 30 m overlain by widespread reservoir coal beds as thick as 3 m. The fourth parasequence is typically dominated by transgressive tidal deposits.
Although cyclothems provide an obvious basis for stratigraphic subdivision of Pennsylvanian strata in the Black Warrior basin, internal facies relationships suggest that major sea-level change was most effective in the short eccentricity band, as was the case during Pleistocene glaciation. Accordingly, rising base level in the second and third 5th-order parasequences of most cyclothems set the stage for widespread peat accumulation and preservation of the thickest and most widespread coalbed methane reservoirs in the Black Warrior basin.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.