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The Smackover Gray Sand is the target of intense exploration activity in the north Louisiana area. The gas-producing Gray Sand, a dark gray to black, very fine-grained sand, occurs as three sand tongues in the lower member of the Smackover Formation in the subsurface of Bossier, Webster, Claiborne, and Lincoln Parishes, Louisiana. The majority of Gray Sand wells have been drilled in Bossier and Webster Parishes; however, the most active exploration presently is to the east in Claiborne and Lincoln Parishes.
Samples of the Gray Sand are classified as sub-chertarenites because of their high percentage of quartz and the dominance of chert fragments over plagioclase. Additional mineral constituents include muscovite and biotite; oolites are also present. A flaser-bedded silty shale facies indicates deposition on a mid-tidal flat environment.
Smackover deposition during the Jurassic in the study area was located on the gently dipping slope between a broad coastal shelf on the north and a basin on the south. The Gray Sand was deposited over the Norphlet Formation and Louann Salt before flowage and swelling of the Louann Salt began. Uplift and swelling of the Louann Salt later in the Jurassic created growing anticlines; sediment slumped off the structural highs of the growing salt anticlines into basinal muds and silts. By superimposing the isopachous map of the Gray Sand interval over the structure map of the Gray Sand, it can be seen that the thickest Gray Sand intervals lie on the flanks of the anticlinal structures in South Sarepta, Ivan, and Cotton Valley fields. Absence of the Gray Sand between Ivan and Cotton Valley f elds indicates a facies pinch-out due to localized deposition of sand tongues on the structural highs.
The Gray Sand, because of its low porosity (7 to 10%) and permeability (0.5 md), must be stimulated through hydraulic fracturing to be productive. Extreme bottom-hole pressures and temperatures require the use of tailor-made high viscosity gels and high-strength proppants.
In Lincoln Parish fields, favorable structures for Gray Sand production are located by seismic exploration. The Smackover Gray Sand however continues to challenge exploration geologists because of the lateral pinch-out of its sand tongues.
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