Haynesville Shale

The Haynesville Shale is an informal, popular name for a Jurassic Period rock formation that underlies large parts of southwestern Arkansas, northwest Louisiana, and East Texas. It lies at depths of 10,500 to 13,000 feet below the land’s surface. It is part of a large rock formation which is known by geologists as the Haynesville Formation. The Haynesville Shale underlies an area of about 9,000 square miles and averages about 200 to 300 feet thick. The Haynesville Shale is overlain by sandstone of the Cotton Valley Group and underlain by limestone of the Smackover Formation.

It contains vast quantities of recoverable natural gas. This natural gas is known as "shale gas" because the wells produce from low permeability mudstones that are also the source for the natural gas. It was known to contain large quantities of natural gas prior to 2008. However, prior to that time, it was uneconomic to extract the natural gas. As a result of rising gas prices and improved technology in hydraulic fracturing and directional drilling, it became possible to extract the gas from the Haynesville Shale in an economic and cost-effective manner.

The Haynesville Formation, from which the Haynesville Shale gets its name, was named after the town of Haynesville, Claiborne Parish, Louisiana. The type location for Haynesville Formation, is the Hunt Oil Company No. 1 well in Haynesville oil field, sec. 3, T. 23 N., R. 7 W., Claiborne Parish, Louisiana. In many geological reports and journal papers, the Haynesville Shale was classified as part of the Bossier Shale, although they are now classified as separate formations.

The Haynesville Shale is a lithological heterogeneous, often organic-rich, mudstone. The composition of these mudstones varies greatly depending on position on geographic location and stratigraphic position. They vary from calcareous mudstone near the carbonate platforms and islands to argillaceous mudstone in areas where submarine fans prograded into the basin and diluted organic matter For example, the Haynesville Shale has been observed to vary in composition from 25 to 35 percent clay and 5 to 30 percent calcite in samples recovered from one oil and gas well. In that well, the Haynesville Shale consists of silty, argillaceous mudstones, silty, calcareous mudstones, and dolomitic mudstones and dolostones. The silty, argillaceous mudstones contain more than 30% silt-sized siliceous grains. The silt often occurs as laminations within these mudstones. In addition, the argillaceous matrix of such mudstones frequently contains numerous calcareous particles and stringers. The calcareous particles include coccoliths, bivalve, and gastropod fragments, and calcispheres. Organic matter in the form of amorphous kerogen coats the argillaceous material. Silty, calcareous mudstones contain more than 20% calcite. In these mudstones, the calcite occurs as silt-sized microfossil hash composed of fragmented fossils and carbonate mud. Where organic matter is low and silts are rare, the calcareous microfossil and carbonate mud has recrystallized. Possibly, further alteration of silty, calcareous mudstones has resulted in the formation of the dolomitic mudstones and dolostones. Both the silty, argillaceous mudstones and silty, calcareous mudstones often exhibit sparse to abundant, laminated pelletal fabrics.

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