SALINIC-ACADIAN FORELAND BASIN ON THE ALABAMA PROMONTORY: CHALLENGES FOR NATURAL GAS DEVELOPMENT IN STRUCTURALLY COMPLEX SHALE FORMATIONS

The Alabama Promontory is a protuberance of the Laurussian craton that was characterized mainly by low-accommodation shelf and shoreline sedimentation during the Salinic and Acadian orogenies. Exploration for natural gas in the southeastern Black Warrior Basin and the adjacent Appalachian thrust belt, however, has revealed a significant foreland basin in which Silurian-Devonian strata thicken basinward from 65 to 600 m. The preserved depocenter is in the Greene-Hale Synclinorium, which is immediately northwest of the Talladega Belt and is separated from the Black Warrior Basin by two dipping structural panels of Cambrian-Ordovician carbonate.

In the Silurian System, the classic ironstone-bearing Red Mountain section (Llandoverian-Wenlockian) passes basinward into organic-rich shale containing radioactive zones. The Red Mountain is overlain by a series of ramp carbonate units ranging in age from Pridolian to Eifelian that also pass basinward into shale. The carbonate succession is in places thicker than 200 m, and the coeval shale section is condensed to 30 m in the depocenter. Above this condensed section is a synorogenic Middle Devonian-Lower Mississippian clastic wedge that is up to 425 m thick and is composed of interbedded shale, siltstone, and limestone. Numerous radioactive shale units onlap the Devonian carbonate ramp, and the section thins cratonward to < 5 m of radioactive shale.

Within the basin fill, TOC locally exceeds 9%, and the shale is in the dry gas window. Rock-Eval pyrolysis demonstrates that the shale has exhausted its generative potential, and adsorption isotherm and tight rock analysis indicate high gas storage capacity, with ~63% of the gas stored in an adsorbed state. Original gas-in-place is estimated to be 10.5 Tcf in the southeastern Black Warrior Basin and 34 Tcf in the Greene-Hale Synclinorium. Exploration efforts have been challenged by shear zones in which gas shows make tempting completion targets. Sustained low-level gas shows in some shale units, however, may provide superior development opportunities.