THERMAL AND GEOCHEMICAL EVOLUTION OF GAS-CHARGED MUSHWADS IN THE CONASAUGA FORMATION (CAMBRIAN), SOUTHERN APPALACHIAN THRUST BELT, ALABAMA

Mushwads are giant, disharmonically folded and faulted shale duplexes that are integral components of the the southern Appalachian thrust belt in Alabama. Mushwads in Cambrian Conasauga shale constitute an antiformal complex that extends along regional strike for more than 140 km, is as much as to 32 km wide, and is locally thicker than 4.25 km. The mushwads are gas-pressured, and original gas-in-place is estimated to be 625 Tcf. Integrated petrologic and geochemical analysis of organic matter, produced gas, and calcite provide insight into the thermal and geochemical processes that operated during mushwad formation.

Reflectance analysis of vitreous herbaceous kerogen in Conasauga shale indicates that the mushwads are within the thermogenic gas window. Limestone near the roof of the duplex system contains residual oil, whereas shale in the heart of the shale masses is deep within the dry gas window. Stable isotopic analysis verifies that the produced gas is of thermogenic origin. Vitrinite reflectance increases uniformly with depth, indicating that deformation largely pre-dated maximum burial and thermal maturation. Maturation patterns in roof strata above the leading margin of the mushwad corroborate the dominance of post-kinematic maturation.

Vein-filling calcite is abundant in the Conasauga mushwads. Most of the calcite is fibrous synkinematic cement that fills opening-mode fractures and shear gashes. Bed-parallel veins near the roof of the mushwad complex have kinematic aperture exceeding 3 cm, demonstrating that hydrostatic pressure exceeded lithostatic pressure. Stable isotopic analysis of the vein-filling calcite indicates a dominant contribution from normal marine carbonate. Many stylolites are strongly discordant with bedding, indicating that pressure solution was an important process during mushwad formation. Overpressuring of the shale masses facilitated widespread pressure solution and mobilization of marine carbonate that precipitated in vein networks during mushwad emplacement.