Stacks, Limbs, and Hinges: Shale Gas Plays of the Southern Appalachian Thrust Belt

Multiple shale gas plays are emerging in Cambrian through Carboniferous strata of the southern Appalachian thrust belt in Alabama, and geologic structure plays a key role in the distribution of proven and prospective reservoirs. These reservoirs are in mechanically weak lithotectonic units, and reservoir development is influenced by basement structure and thrust kinematics.

The Conasauga Formation (Middle Cambrian) contains the world's oldest shale gas reservoirs in an outer ramp facies consisting of thinly interbedded shale and micrite. The productive facies was deposited in an Iapetan basement graben, and during Alleghanian thrusting, the shale was deformed into giant antiformal stacks that rest on the basal detachment of the thrust belt. These antiformal stacks approach a thickness of 2500 m, and internally the stacks are chaotically deformed. The reservoirs have characteristics of both fractured and microporous gas reservoirs and are notable for intense gas pressure in fracture networks. The shale bodies are within the thermogenic gas window, and thermogenic charge from deep within the shale bodies appears to be the source of high gas pressure.

Devonian-Mississippian black shale is being developed at several locations within the southern Appalachians in Alabama, and production has been established in the Chattanooga Shale in the frontal part of the thrust belt. The Devonian-Mississippian shale section is characterized by net pay thinner than 30 m and forms a mechanically weak unit that was carried above a stiff Cambrian-Ordovician carbonate section. Thrust faults have a ramp-flat to listric geometry that determines the location and geometry of fold limbs and hinges. Shale in fold limbs can contain transported orthogonal joint networks and can contain folds and faults formed by layer-parallel shear. Fracturing in fold hinges may increase permeability. Breached anticlines are major zones of fresh-water recharge, which can facilitate mixing of thermogenic and late-stage biogenic gases in unconventional reservoirs.