DEVONIAN-MISSISSIPPIAN GAS SHALE PLAYS IN SOUTHERN USA

Devonian-Mississippian shale plays have become important unconventional gas reservoirs since the discovery of the unconventional Newark East gas field in the Fort Worth basin of north-central Texas in the mid-1990s. The field produces from the Mississippian Barnett Shale, which serves as source, seal, and reservoir to a world-class unconventional natural-gas accumulation. Within the past two decades, many unconventional gas fields have been discovered in Devonian-Mississippian shale units at numerous locations in the southern Midcontinent and the southern Appalachians, USA. Givetian-Tournaisian units include the Woodford Shale of the Anadarko and Arkoma basins and the Chattanooga Shale of the Appalachian and Black Warrior basins. Visean-Serpukhovian units include the Barnett Shale in the Fort Worth Basin, Fayetteville Shale in the Arkoma Basin, and Floyd (Neal) Shale in the Black Warrior Basin. These units were deposited in marine environments under stratified water columns related to major orogenic episodes. Accordingly, a distinctive set of detrital, biologic, diagenetic, and structural properties influenced their reservoir qualities. These properties are ultimately related to the mechanical and chemical stability of the shale, organic content, and tectonic history.

The Devonian-Mississippian units are excellent unconventional reservoirs when they are brittle and have limited reactivity with formation and completion fluids. Biogenic silica contributes greatly to brittleness, and intervals with high amounts of biogenic silica and the resulting chert in the Barnett and Woodford plays are considered pay zones and targets for drilling and hydraulic fracturing. Therefore, brittleness should be considered as an important aspect of unconventional gas shale reservoirs. In addition to source rock capabilities, determined based on TOC, T_max, HI, OI and R_f values, a complete evaluation of potential unconventional shale reservoirs in basins globally should include brittleness studies to ascertain the potential of the reservoir rock to contain natural fractures and propagate artificial fractures during hydraulic stimulation.