AN IMPORTANT LOWER MISSISSIPPIAN RESERVOIR
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The Lewis Sand is a significant hydrocarbon reservoir in the Black Warrior Basin. Stratigraphically, the Lewis Sand is bracketed by the Lewis Limestone (above) and Tuscumbia (aka Iowa) Limestone (below) in the lower half of the Lower Mississippian Floyd Shale. The Lewis Limestone and Lewis Sand represent a classic transgressional/regressional sequence. The predominantly shale section that extends from the base of the Bangor Lime equivalent (base Parkwood / top Floyd Shale) becomes more organic-rich and petroliferous as one moves deeper in the interval towards the Lewis Limestone, such that the Floyd Shale immediately overlying the Lewis Limestone becomes microfractured in northwest Alabama and northeast Mississippi, a consequence of the generation and expulsion of hydrocarbons from the Floyd source rock. There is some speculation about the Upper Devonian Chattanooga Shale also serving as a source rock in the Basin, but the Chattanooga Shale is very thin and not considered to be a significant contributor of hydrocarbons. The Lewis Sand typically averages 10-20 feet in thickness but in certain areas several lobes or lenses of the sand are encountered in a locally thicker Lewis Sand interval. The overlying Lewis Limestone "cap" is often found deposited directly atop the Lewis Sand but in some areas a thin calcareous shale separates the limestone from the underlying sandstone. Oil shows are very common in Lewis Sand gas reservoirs, even those that produce only dry gas.
Petrophysically, the Lewis Sand exhibits a porosity range of nil to 16% and a permeability range of nil to 100 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 the Lewis Sand (as well as the other 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. The Microlog is a valuable tool in evaluating the prospectivity of a Lewis 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 ten-fold increases in daily rates of production. Although nearly all of the hydrocarbon production from Lewis Sand reservoirs is methane, some oil reservoirs have also been discovered and developed; however, in most cases the porosity and permeability of the Lewis Sand is not conducive to efficient reservoir drainage, and recoveries in most Lewis Sand oil reservoirs has been poor and generally non-commercial.
The SP (Spontaneous Potential) Curve is one log curve that should not be used to condemn a sandstone logged in the Lewis Sand interval (or for that matter, any sandstone interval within the Mississippian in the BWB). Many prolific Lewis Sand wells exhibit little or no SP Curve deflection on the correlation/induction log, suggesting a very silty or shaley composition; yet the Gamma Ray Curve will exhibit a clean sand profile across the same interval, as will the porosity log and the Microlog. Thus it is important to examine a complete log suite when evaluating a Mississippian sandstone in the BWB; relying on one-inch correlation logs can result in one walking away from a good commercial completion.
To view a larger version of the type log for the Lower Paleozoic of the Black Warrior Basin (including the Lewis Lime and Lewis Sand interval) shown at left, click here.