Abstract

The fractured Devonian shale gas play of the Appalachian Basin is fundamentally controlled by its temperature and pressure history. Of primary importance is the character of the Devonian shale as a hydrocarbon source rock. At maximum burial depth in the latest Paleozoic, part of the Devonian shale section reached a temperature adequate for the generation of oil and gas. Some of these shales contained organic material in high enough concentrations to result in primary migration. Primary migration was facilitated by the opening of horizontal extension fractures, as the pore pressure in the shales accompanying hydrocarbon generation exceeded the lithostatic gradient. The elevation of pore pressure as a result of oil and gas generation was also significant in the development of a variety of fracture systems in the Devonian shales. The Alleghenian Orogeny resulted in an increased level of regional stress and a variety of structural movements, including thrusting along the Alleghenian Front and reactivation of basement normal and strike-slip faults. The high degree of overpressuring in the Devonian shale section at that time made it particularly susceptible to fracturing in response to even small increments of tectonic stress, leading to concentrations of fractures at local structures.

Removal of overburden by erosion throughout the Mesozoic and Cenozoic Eras gradually resulted in cooling of the Devonian shale section, and a consequent pressure decline to its current value, which is underpressured with respect to the hydrostatic gradient. Reservoir pressure drawdown during production results in release of abundant gas adsorbed on kerogen and clay surfaces in the shale. Compositional segregation during migration through the low-permeability fractured shale reservoir results in the production of methane and lesser amounts of heavier hydrocarbons, while the depleted shale reservoir remains largely saturated with oil. The Lower Huron Shale has been by far the most productive part of the Devonian shale gas play, and is well defined between specific limits of low organic carbon content and high vitrinite reflectance. The Rhinestreet and Marcellus Shales have been less productive to date, but have also been less consistently explored.