DEVONIAN-MISSISSIPPIAN SHALE GAS PLAYS IN THE SOUTHERN MIDCONTINENT AND SOUTHERN APPALACHIANS: A REVIEW

Devonian-Mississippian shale units produce natural gas at numerous locations in the southern Midcontinent and the southern Appalachians. Givetian-Tournasian 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. All these units were deposited in marine environments under stratified water columns in foreland basins related to major orogenic episodes. Therefore, a distinctive set of detrital, biologic, diagenetic, and structural properties influence gas production in each unit. These properties are ultimately related to the mechanical and chemical stability of the shale, organic content, and tectonic history.

Within the last 10 years, it has been established that shale units perform best as unconventional reservoirs if they are brittle and have limited reactivity with formation and completion fluid. Biogenic silica contributes greatly to brittleness, and formations containing intervals with high amounts of biogenic silica, such as the Barnett and Woodford, contain very good unconventional plays. In many regions, brittleness also is the overriding consideration for the selection of landing zones and hydrolic fracturing targets. Where argillaceous strata occur in these reservoirs, drilling and completion fluids need to be selected carefully to minimize clay expansion and reaction with carbonate and other soluble minerals.

In unconventional shale reservoirs, gas is stored in free and adsorbed states, with free storage occurring in a broad range of interparticle and intraparticle very small pores. Adsorption occurs primarily at the molecular scale within the polymeric net of organic matter. Water saturation in unconventional shale reservoirs commonly exceeds 20%. Therefore, hydrolysis is another storage mechanism that needs to be considered, especially in CO₂-rich petroleum systems.