Hydrocarbon Generation and Brine Migration in the Central Appalachian Basin

EVANS, MARK A., Georgia Southern University, Statesboro, GA

Fluid inclusions in mineralized natural fractures from six Devonian shale cores were used to document hydrocarbon generation and brine migration in the central Appalachian basin. The sequence of formation of four regional fracture sets containing the inclusions was used to constrain the relative timing of fluid evolution.

The earliest formed fluid inclusions are single-phase liquid inclusions containing a complex mixture of methane, ethane, higher hydrocarbons, and nitrogen. These inclusions formed during burial of the Devonian shales and early hydrocarbon generation in the oil window. As burial proceeded to a maximum and hydrocarbon generation entered the gas phase, later formed fluid inclusions record the presence of a more methane-rich fluid with minor ethane and nitrogen.

Either during maximum burial or early uplift of the Devonian shale section, regional stress relaxation was accompanied by regional brine migration. Fluid inclusions record the influx of a methane-saturated, sodium chloride-rich brine and subsequent mixing with a presumably in-situ calcium-rich brine. The migration pathway is presumed to be the Devonian shale detachment zone and underlying Devonian Oriskany Sandstone. This migration may be related to the fluids forming Mississippi Valley-type ore deposits.

Present-day brine compositions reflect this ancient mixing. Brines from deep Cambrian through Silurian rocks are more calcium-chloride rich than brines from shallower Devonian and younger rocks. The sodium chloride-rich brines from Upper Devonian through Pennsylvanian rocks become more dilute as a result of mixing with meteoric water.

AAPG Search and Discovery Article #91005 © 1991 Eastern Section Meeting, Pittsburgh, Pennsylvania, September 8-10, 1991 (2009)