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Gas Field Redevelopment (MS)
THE MOST PROLIFIC - YET LEAST RESPECTED - FORMATION
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The continental Pennsylvanian Pottsville Formation is the most prolific formation in the Black Warrior Basin, having produced (as of December 2009) over 2.2 trillion cubic feet of methane from the numerous thin coal seams of the Pottsville, in the extreme eastern portion of the Basin. It is interesting to note the wells that have produced that 2.2 TCF have also pumped over 1.4 billion barrels of water. Many of the most prolific coal seam wells were coal degasification wells drilled to "de-gas" the old coal mines that has been established in the western Tuscaloosa, Alabama area some 50-100 years previously (primarily in support of northwest Alabama's steel industry). Various Pennsylvanian sandstone reservoirs have also contributed approximately one third of the 159 billion cubic feet of gas produced from Corinne Field, the largest "conventional" gas field in the Black Warrior Basin, and are minor gas reservoirs in a number of other Basin gas fields. Generally, however, the Pennsylvanian has been neglected as a conventional resource target.
During the late Pennsylvanian, structural movement and one or more associated lowstands resulted in the erosion and redeposition of significant volumes of Pennsylvanian sediments in the eastern side of the Basin, while in the western province, syndepositional "stair-step" down-to-the-southwest normal faulting led to the deposition of dramatically thicker deposits of Pennsylvanian sediments along the depo-trough developing northeast of, and parallel to, the northwest/southeast-oriented Central Mississippi Oachita overthrust. Similar deposits of thick Pennsylvanian sediments (as much as 8,000 feet) accumulated within the deep north-south oriented grabens that were forming along the southerly extension of the Reelfoot Rift wrench-fault transtensional and transpressional zone (just south and west of Memphis, Tennessee). In southern Monroe County, a large late-Pennsylvanian canyon eroded all the way through the Pennsylvanian and into the top of the underlying Upper Mississippian Parkwood Formation, backfilled with late Pottsville shales and silts.
The entire Basin area was subsequently subjected to downwarping and further erosion in post-Pennsylvanian time. As a result, over much of the central portion of the Basin Upper Cretaceous strata unconformably overlie much older Paleozoic rocks, most notably the Pennsylvanian Pottsville Formation.
Petrophysically, the multiple sandstones of the Pottsville exhibit a porosity range of nil to 20% and a permeability range of nil to 200 millidarcies. In the past, the industry has typically utilized an 8% to 9% "productivity" cutoff for log crossplot porosity; it should be noted, however, that most density/neutron porosity logs that have been run in BWB wells used a limestone - not sandstone - lithology matrix when converting density to porosity, which in sandstones yields a porosity calculation that is generally 2% to 3% too optimistic (too high). Thus it appears the actual "productivity" cutoff for log crossplot porosity in Pennsylvanian sandstones (as well as the Mississippian sands in the Basin) is actually 6% to 7%. Induction log resistivities for producing reservoirs typically range from 40 ohm-meters to 200 ohm-meters or higher; however, for the Lower Pottsville "Box" and "Nevins" Sands of northwest Alabama and northeast Mississippi, the presence of chlorite and other interstitial clays dramatically impacts producing resistivity, such that commercial water-free gas production can be obtained from such sandstones despite a deep induction resistivity of less than ten ohm-meters. The Microlog is a valuable tool in evaluating the prospectivity of a Pennsylvanian sandstone reservoir; commercial completions typically require at least two feet of Microlog-implied permeability (separation). Small fracture stimulations (utilizing from 25,000 to 50,000 pounds of proppant) dramatically improve producibility, often resulting in multi-fold increases in daily rates of production. Oil shows are very common in Pottsville gas reservoirs, even those that produce only dry gas. There are many tight gas sands within the Pottsville that have never been effectively fracture-stimulated, leaving many to speculate that in the future innovative stimulation technology will lead to the recovery of a signifcant volume of methane from such tight gas reservoirs.
To view an enlarged version of Vision's Type Log for the Pennsylvanian Pottsville Formation in Northwest Mississippi (shown at left), click here.